© 2000 Oxford University Press
Human Molecular Genetics, 2000, Vol. 9, No. 16 Review 2409–2419
Counting cross-overs: characterizing meiotic
recombination in mammals
Terry Hassold+, Stephanie Sherman1 and Patricia Hunt
Department of Genetics and the Center for Human Genetics, Case Western Reserve University and University
Hospitals of Cleveland, Cleveland, OH, USA and 1Department of Genetics, Emory University School of Medicine,
Atlanta, GA, USA
Received 7 July 2000; Revised and Accepted 13 July 2000
Until recently, most of our understanding of meiotic recombination has come from studies of lower eukaryotes. However, over the past few years several components of the mammalian meiotic recombination pathway
have been identified, and new molecular and cytological approaches to the analysis of mammalian meiosis
have been developed. In this review, we discuss recent advances in three areas: the application of new techniques to study genome-wide levels of recombination in individual meioses; studies analyzing temporal
aspects of the mammalian recombination pathway; and studies linking the genesis of human trisomies to
alterations in meiotic exchange patterns.
INTRODUCTION
In their classic 1980 paper, Botstein et al. (1) proposed using
DNA polymorphisms to construct a human linkage map and
thereby uncover the location and, ultimately, the identity of
human disease loci. In the two intervening decades, the wisdom
of this suggestion has been amply justified, as linkage analysis
has been the primary weapon in the disease gene hunter’s
arsenal. Curiously, however, during this time human geneticists
have devoted relatively little energy to studying the process that
makes linkage analysis work, i.e. meiotic recombination.
Instead, most of the advances in our understanding of meiosis
and recombination have come from studies of lower eukaryotes,
in particular Saccharomyces cerevisiae and Drosophila (2,3).
Fortunately, this situation is now changing. The identification
of mammalian homologs of recombination proteins of lower
organisms, the analysis of meiosis in appropriate knockout
mice and the introduction of novel cytological and molecular
technology to meiotic analysis have revitalized research into
mammalian and human meiosis. In this review we summarize
recent advances in three areas: the application of molecular and
cytological techniques to study the number and location of
meiotic exchanges in murine and human meioses; studies
analyzing temporal aspects of the mammalian recombination
pathway, making use of newly identified recombination
proteins; and studies that have linked human non-disjunctional
events with alterations in meiotic exchange patterns.
WHERE AND HOW MANY: FREQUENCY AND
DISTRIBUTION OF MEIOTIC EXCHANGES
The construction of different types of human genetic map has
helped to reveal several important properties of mammalian
meiotic recombination. For example, standard linkage
analyses of whole chromosomes or chromosome regions have
confirmed earlier cytogenetic reports of sex-specific differences, with females having higher recombination rates than
males, and with males recombining preferentially in distal
regions of the chromosome (4). These and other approaches,
such as analysis of linkage disequilibrium or single sperm
mapping, have been useful in defining recombination ‘hot
spots’ (5,6) and in uncovering significant inter-individual variation in recombination over short intervals (7).
However, the majority of these analyses have been restricted
to chromosome regions or single chromosomes, with no
attempt being made to examine exchange patterns over all
chromosomes. Recently, two different approaches—one
cytogenetic and the other molecular—have been introduced
that make it possible to analyze genome-wide exchange
patterns in individual meioses.
Cytological approaches to crossing over
In a series of elegant studies conducted over two decades,
Hulten and colleagues used conventional cytogenetics to
analyze diakinesis preparations from human and murine meiocytes (8,9). This approach enabled them to examine chiasma
patterns on individual chromosomes, as well as to estimate
genome-wide chiasma frequencies. A number of general
features of mammalian meiosis emerged from these studies,
including evidence for positive interference and evidence for
gender-specific differences in chiasma frequency and/or distribution, as well as the estimates of chiasma frequency. Whenever it has been possible to test these observations (e.g. by
linkage analysis), they have proven accurate.
+To whom correspondence should be addressed at: Department of Genetics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106,
USA. Tel: +1 216 368 3433; Fax: +1 216 368 0491; Email: [email protected]
2410 Human Molecular Genetics, 2000, Vol. 9, No. 16 Review
Figure 1. Moleular cytogenetic and immunofluorescent approaches to studying mammalian meiotic exchanges. (a) Spectral karyotyping of a mouse female diakinesis preparation (with conventionally stained image of same cell for comparison). (b) Analysis of a murine female pachytene preparation, using antibodies
against SCP3 (to identify the synaptonemal complex) and MLH1 (to identify points of meiotic exchanges). (c) Analysis of a human male pachytene preparation,
using antibodies against SCP3 and MLH1. The arrow identifies a desynapsing XY pair.
Conventional diakinesis analyses, however, are hindered by
two obstacles. First, in the human, the cells of interest are relatively inaccessible: analysis of males requires a testicular
biopsy and analysis of females is even more difficult, since
recombination occurs in the fetal ovary. Thus, availability of
material will always be limiting in cytological analyses of
human recombination. Secondly, diakinesis preparations are
difficult to analyze, even under ideal circumstances: chromosome morphology is suboptimal, interfering with banding and
thus with chromosome identification, and the chromosomes
are typically highly condensed, making localization of chiasmata to specific chromosome regions difficult. New multicolor
approaches now make it possible to perform chromosomespecific analyses on diakinesis preparations (Fig. 1). Furthermore, and of greater importance, the introduction of new
immunofluorescence techniques may make it possible to
replace, or at least supplement diakinesis analysis with an
alternative approach.
In 1996, Baker et al. (10) reported an important meiotic role
for the DNA mismatch repair protein, MLH1. In studies of male
mice with a targeted disruption of the Mlh1 gene, meiotic
crossing over was virtually eliminated. As a result, most
chromosomes were present as univalents during meiosis I,
causing the arrest of spermatocytes at this stage. Subsequently,
Woods et al. (11) demonstrated that Mlh1-null females are
similarly recombination-deficient. The basis for these recombination defects became clear when Baker et al. (10) immunostained surface-spread spermatocytes and oocytes from wild-type
mice with an antibody to MLH1, and observed discrete foci on
pachytene-stage synaptonemal complexes. The frequency and
distribution of foci was remarkable: at mid-pachytene, the
overall number and location of foci corresponded to previous
cytogenetic analyses of chiasmata; in the male, a single MLH1
focus was observed at the distal end of the XY pairing region,
consistent with previous genetic studies of the pseudoautosomal
region; and in the female, some foci persisted into diplotene, and
were observed at the sites of chiasmata. These results suggested
that MLH1 contributed to the formation and/or processing of
meiotic recombination events and that the meiotic abnormalities observed in null mice resulted from a breakdown in the
Human Molecular Genetics, 2000, Vol. 9, No. 16 Review 2411
recombination pathway. Importantly, the results implied that
analysis of MLH1 foci in meiosis could be used as a surrogate
for diakinesis studies in the analysis of meiotic exchange
patterns.
More recent studies indicate that this is indeed the case. In
1999, Anderson et al. (12) conducted immunostaining studies
of male mouse pachytene preparations using antibodies to
MLH1 and SCP3, a component of the lateral element of the
synaptonemal complex (Fig. 1 shows examples of this
approach, as applied to human male and mouse female meiocytes). Their observations fulfilled several of the criteria
predicted for a molecule that ‘marks’ the sites of exchanges:
for example, the overall number of foci per genome was ∼23,
similar to previous estimates of chiasmata number in male
mice; the number of foci varied depending on the length of the
bivalent, with those joined by shorter synaptonemal complexes
typically having only one focus and longer bivalents having
one or two foci; there was a bias toward distally located foci,
especially among bivalents with shorter synaptonemal
complexes; and both chromosome-specific and genome-wide
distributions of exchanges were non-random and consistent
with positive interference.
Similar results have been presented for humans. Using
comparable methodology, Barlow and Hulten (13) analyzed
MLH1 foci in spermatocytes of a fertile male and in ooctyes of
a female fetus. In the male, they observed ∼50 autosomal foci
per nucleus, consistent with estimates of an overall genetic
length of ∼2700 cM in the human male (14). Additionally,
there was a bias toward distally placed foci, consistent with
available data from linkage studies. Only a few analyzable
cells were obtained from the female. Nevertheless, consistent
with expectation the overall number of MLH1 foci was much
higher (mean = 95) than that observed in the male, and distal
foci were much less common than in the male, again as
expected.
Thus, preliminary results from two mammalian species
suggest that the frequency and distribution of MLH1 foci
correspond to the number and location of meiotic cross-over
events. If confirmed in further analyses, this will have major
implications for future studies of mammalian recombination.
For example, the ability to use immunostaining methodology
will make it possible to examine temporal and spatial relationships between chiasma formation and recombination pathway
proteins; indeed, several groups have already initiated these
analyses (15). Additionally, the enhanced resolution afforded
by the immunostaining approach will make it possible to
localize cross-overs to specific chromosome regions. This will
allow us to conduct experiments that are impossible or impractical using diakinesis preparations: for example, comparisons
of chiasma distributions among individuals (or, in the mouse,
between strains or in F1 hybrids); combined fluorescence
in situ hybridization/immunostaining studies to determine
whether MLH1 foci localize to specific chromosomal sites/
DNA sequences (e.g. putative recombination hot spots defined
by other approaches); and analyses of chiasmata in individuals
with possible meiotic defects (e.g. infertile males). Cytological
studies of crossing over, previously the provence of a small
number of laboratories, may well become the method of choice
in analyzing exchange patterns in mammalian meiosis.
Genome-wide recombination scans
In the late 1990s, genome scans became a staple of human gene
mapping studies. More recently, Broman and colleagues have
demonstrated the utility of this approach in characterizing
basic properties of meiotic recombination. In an initial study,
Broman et al. (14) used >8000 short tandem repeat polymorphisms (STRPs) to construct genetic maps for each of the 22
autosomes and the X chromosome, based on eight of the
Centre d’Etude du Polymorphisme Humain (CEPH) reference
families. Altogether, nearly 1 000 000 genotypes were scored,
with the average distance between markers being ∼0.5 cM. As
expected, each of the chromosomes displayed a female:male
length ratio of >1. Genome-wide, the ratio was ∼1.6 (4435 cM
in females and 2730 cM in males). Sex-specific differences in
the location of recombinational events were also observed,
with female:male ratios typically highest around centromeres
and lowest in telomeric regions.
In addition to analyses of individual chromosomes, Broman
et al. (14) also examined the overall number of detectable
recombinational events per meiosis. In females, the average
was ∼40 events and among males, ∼23. Interestingly, significant variation in the number of observable recombinants was
evident among the eight mothers in the families, but not among
fathers. No age effect on recombination was observed.
In other analyses, Broman and Weber (16) have used their
mapping database to examine cross-over interference patterns
in human meiosis. In both males and females, evidence for
strong positive interference was observed, with no obvious
variation between sexes or among chromosomes. Among
chromosomes with at least one exchange per arm, a negative
correlation was observed between the locations of the
exchanges on the two arms. Thus, the centromere did not
appear to be a barrier to interference.
Many of the general aims of these studies—construction of
chromosome-specific maps, examination of sex-specific
differences in exchange events and analysis of interference—
can also be addressed using cytological methodology, as
described above. However, Broman and colleagues have also
asked questions that cannot be approached cytologically, due
to the lack of suitable cytogenetic polymorphisms. For
example, they recently used their genome scan data to search
for segments of homozygosity among CEPH individuals (17).
Surprisingly, relatively long (∼10–20 cM) homozygous
stretches were identified in a number of the individuals.
Indeed, in one of the families, all individuals showed at least
one homozygous segment, with the results most likely attributable to inheritance of two copies of the same ancestral chromosome segment (autozygosity).
WHEN AND WHO DOES IT: TEMPORAL ASPECTS
OF MAMMALIAN RECOMBINATION
In 1989, Sun et al. (18) provided evidence that in the budding
yeast S.cerevisiae, meiotic recombination was mediated
through double-strand breaks (DSBs). In the intervening
decade, there has been an explosion of information from lower
eukaryotes, detailing the roles of specific proteins in the initiation of DSBs, the processing of recombination intermediates,
and the association of recombination with the synaptonemal
complex (19,20). Information on recombination in mammals
2412 Human Molecular Genetics, 2000, Vol. 9, No. 16 Review
has been slower in coming. However, over the past 2–3 years
the pace has quickened, with many of the major players now
being identified. In this section, we briefly review recent
studies of the mammalian recombination pathway, focusing on
immunolocalization studies and studies of knockout mice.
Because a number of recent studies have detailed meiotic
abnormalities in DNA mismatch repair-deficient mice, the role
of mismatch repair proteins in mammalian meiosis is presented
separately. The meiotic roles of other protein families, for
example cohesins and condensins, have been reviewed elsewhere (e.g. ref. 21) and are not considered here.
Overview of the process
During prophase of the first meiotic division, homologous
chromosomes condense, pair and synapse, and exchange
genetic material in preparation for segregation at metaphase I.
Stripped to its basics, this requires two processes that are
common to almost all organisms, including mammals. First,
the chromosomes are broken (by DSBs) and rejoined, with
some of the breakage–reunion events that occur between
homologs leading to recombinant products, visualized microscopically as chiasmata. Secondly, a meiosis-specific structure, the synaptonemal complex (SC), is formed that facilitates
synapsis and recombination between the homologs.
Recent studies indicate that the temporal relationship
between these two processes varies among different organisms. In S.cerevisiae, DSBs occur before synapsis; indeed,
DSBs are thought to be essential for formation of a normal SC
(3). However, in other organisms this appears not to be the
case, for example in Drosophila (22) and Caenorhabditis
elegans (23). SC formation is essentially normal in mutants
that abolish recombination. Perhaps surprisingly, recent
evidence suggests that mammals follow the yeast paradigm,
with DSBs occurring before synapsis (S. Keeney, F. Baudat
and M. Jasin, personal communication).
Initiation and processing of DSBs
In S.cerevisiae, an exhaustive search for the initiating event in
double-strand breakage led to the identification of the responsible protein, Spo11 (24). Spo11 has sequence similarity to a
family of type II topoisomerases originally identified in archebacteria (25), and presumably acts through a topoisomeraselike transesterification mechanism in which Spo11 is covalently bound to the target DNA. Homologs of yeast Spo11
have now been identified in several other organisms and
appear to have a similar role in initiation of DSBs, suggesting
that this component of the recombination pathway has been
conserved throughout evolution. The recent identification of
murine and human Spo11 homologs (26–28) was thus not
unexpected. Relatively little is yet known about the meiotic
activities of these mammalian SPO11 proteins. Nevertheless,
initial expression studies indicate that they are present in
gonadal tissue, and are expressed at time points consistent with
a role in DSB formation. Therefore, it seems likely that in
mammals, as well as other organisms, SPO11 is responsible
for initiation of DSBs.
Relatively little is known about the recombinogenic proteins
that act downstream of SPO11. However, recent studies by
Heyting and co-workers (29,30) suggest that a complex
containing RAD50 and MRE11 proteins functions early in the
process. In S.cerevisiae, null mutants of Rad50 and Mre11
prevent formation of DSBs, whereas certain non-null mutants
block the 5′→3′ resection of DSB ends (31). Thus, Rad50 and
Mre11 proteins appear to be important in both the formation
and in the early processing of DSBs. This may well be the case
in mammals as well. Immunostaining studies indicate that the
two proteins co-localize within mouse spermatocyte nuclei,
and that they are most abundant very early in meiosis I, before
the appearance of components of the SC (29,30).
Further processing of DSBs requires the activity of strand
invasion proteins. In mammals as in other organisms, this is
accomplished by bacterial RecA-like proteins, at least two of
which, RAD51 and DMC1, are known to function in mammalian meiosis (32). Recently, two-hybrid and co-immunoprecipitation studies have demonstrated that human DMC1 and
RAD51 interact directly with one another (33).
Functional studies of the meiotic role of mammalian Rad51
have been limited, due to the fact that targeted mutations of the
murine locus result in embryonic lethality. However, mice
deficient for Dmc1 have been generated (34,35) and their
meiotic phenotypes examined. As expected, meiosis is grossly
abnormal: chromosomes are unable to synapse and, in the most
mature spermatocytes, consist of 40 univalents rather than the
normal 20 bivalents. Consequently, germ cells arrest early in
prophase and both males and females are sterile.
The synaptonemal complex
In S.cerevisiae and possibly in mammals as well, the earliest
events in recombination occur prior to synapsis of homologous
chromosomes. However, the later stages, including the formation of functional chiasmata, require that the homologs be
brought into intimate association with one another. This
process is mediated by a meiosis-specific structure, the synaptonemal complex. The mature SC is a tripartite structure,
consisting of two lateral elements (to which the sister chromatids of an individual chromosome are tethered) and a central
element (linking the two lateral elements of a homologous
pair). Formation of the SC begins with the assembly of small
segments of the lateral elements, referred to at this stage as
axial elements. As synapsis proceeds, axial elements of a
homologous pair approach one another and eventually the
central element forms between them.
Three major protein components of the mammalian SC have
been identified: SCP1 (also called SYN1), SCP2 and SCP3
(also called COR1) (for a review see ref. 36). SCP1 localizes to
the central region of the SC (37,38). It contains an extended
coiled-coil region and is thought to be a major component of
the transverse filament, which links the two lateral elements.
SCP2 and SCP3 form components of the axial/lateral
elements, and are first detected on forming axial elements in
leptotene and persist through diplotene (39). SCP2 has weak
sequence similarity to Red1, an S.cerevisiae protein associated
with axial/lateral elements (3). SCP3 has no known yeast
homolog. It can form thick, cross-striated fibers and is thought
to be the core structure of the lateral element (40,41).
Our understanding of the function of mammalian SC
proteins has come largely from in vitro analyses or analyses of
testicular samples from wild-type animals. Recently however,
Yuan et al. (41) provided the first mutational analysis of the
mammalian SC, as they generated a null mutation for Scp3 and
Human Molecular Genetics, 2000, Vol. 9, No. 16 Review 2413
analyzed meiotic progression in Scp3-deficient male mice. The
animals were developmentally normal, consistent with a
meiosis-specific function for SPC3, but were sterile due to a
breakdown of spermatogenesis early in prophase. In immunostaining studies, axial elements were absent and the chromosomes were represented by 40 univalents rather than 20
bivalents, indicating that the homologs were unable to synapse
with one another. Additionally, abnormalities in the spatial
distribution of other recombination-associated proteins (e.g.
RAD51) were observed. Thus, the report of Yuan et al. (41)
provides the first genetic evidence that SCP3 is required for
normal meiotic progression in mammals. However, intriguingly, Scp3-deficient females are fertile, although they are less
fecund than wild-type controls. It will be extremely important
to determine the basis for this sex-specific difference.
In addition to the axial/lateral and central elements, studies
of lower eukaryotes have identified other meiosis-specific
structures that are associated with the SC. These are the recombination nodules, electron-dense spherical structures located at
discrete intervals along the SC and long thought to be protein
complexes acting at the sites of breakage/processing of recombinational events (e.g. ref. 42). Two types of nodule are
thought to exist: ‘early’ nodules, which are numerous and
present on unsynapsed axial elements and briefly on the newly
formed SC, and ‘late’ nodules, which are fewer in number and
distributed non-randomly on the fully synapsed SC, presumably representing the sites where chiasmata will form (15).
Due to the difficulty in visualizing recombination nodules in
mammals, relatively little effort has been made to characterize
them. However, with the development of immunolocalization
methodologies, efforts are now underway to determine
whether the distribution of any of the recombinogenic proteins
is consistent with that predicted for recombination nodules. In
initial studies, Ashley and co-workers (15,43) and Moens and
co-workers (36,44,45) have investigated several putative
recombination nodule proteins, using antibodies to
SCP3(COR1) to mark the axial/lateral elements of the SC, and
analyzing the spatial and temporal distribution of the different
proteins. RAD51 and DMC1 are likely components of early
nodules. Electron-microscopic studies of mouse spermatocyte
chromosomes indicate that they co-localize at the cores of
spermatocyte chromosomes, suggesting that they complex
with one another (44). On immunostaining, discrete RAD51
and/or DMC1 foci are observed in association with the axial
elements; foci are abundant early in prophase, at leptotene/
zygotene, but effectively disappear from the synapsed lateral
elements by mid pachytene (15). The number of foci far
exceeds that predicted for functional chiasmata in both mouse
and humans, consistent with a role in the formation of early,
but not late, recombination nodules (43,46).
Other proteins are also known to associate with RAD51/
DMC1 or with the forming axial elements. For example, Plug
et al. (43) have shown that the single-stranded DNA-binding
protein RPA localizes to axial elements at or near RAD51 foci
during zygotene. However, whereas RAD51 foci disappear
relatively quickly from the fully synapsed SC, some RPA foci
persist, and frequently co-localize with MLH1, thought to be a
component of late nodules (see below). This and other
evidence has led Plug et al. (43) to suggest that a subset of
early nodules are converted into late nodules, with RPA
contributing to both structures.
Other proteins that have been shown to interact with DMC1/
RAD51 or with RPA include the Bloom syndrome helicase
BLM and the ataxia telangiectia-related DNA damage checkpoint control protein ATR (43,45,47,48). However, information on the spatial and/or temporal distribution of these
proteins is equivocal and thus their positions in the recombination pathway are still uncertain.
One likely protein component of late nodules now has been
identified, namely MLH1. As discussed in a previous section,
the frequency and distribution of MLH1 foci on the fully
synapsed SC closely parallels the number and placement of
chiasmata. Thus, it seems likely that MLH1 foci mark the sites
of the late recombination nodules.
Mismatch repair proteins and mammalian recombination
The mismatch repair (MMR) system was first characterized in
Escherichia coli, where three proteins, MutS, MutL and MutH,
mediate the repair of DNA mismatches (reviewed in ref. 49).
The MMR system is highly conserved, but in higher eukaryotes the simple three-protein bacterial system has been
replaced by multiple MutS and MutL proteins, and a number of
additional proteins have been implicated (50).
In addition to their role in DNA repair, MMR proteins are
also required for meiotic recombination. The meiotic roles of
individual members of the MMR system have been best
defined in S.cerevisiae. Six MutS homologs (Msh1–6) and
four MutL homologs (Mlh1–3 and Pms1) have been identified
and, based on mutant analysis, nine of these genes are thought
to be involved in meiotic recombination (Table 1) (reviewed in
ref. 51). Two of the MutS homologs, Msh4 and Msh5, have a
meiosis-specific function and mutations in these genes result in
decreased levels of recombination and increases in postmeiotic segregation (indicative of a failure to correct
mismatches in heteroduplex DNA) (52–54). Recombination
levels are not decreased in mutants for the MutS homologs
Msh2 and Msh3, or for the MutL homolog Pms1; however, an
increase in post-meiotic segregation is a feature of all three
(55–57). In addition, these proteins are thought to play a role in
preventing recombination between homologous sequences
and, consistent with this, a slight increase in recombination is
observed when these mutations are present on a non-isogenic
genetic background (58–60). The MutL homolog, MLH1, is
thought to physically interact with the other three MutL
homologs, and mutations in this gene, as in the meiosisspecific genes, result in a decrease in recombination and an
increase in post-meiotic segregation (51).
In mammals, interest in MMR proteins was sparked by the
publication, in 1993, of two papers demonstrating an association between mutations in MMR genes and some forms of
hereditary cancer (61,62). Studies conducted over the intervening 7 years have considerably advanced our understanding
of the role that loss of mismatch repair plays in the genesis of
human cancers (reviewed in ref. 63). As one part of these
efforts, targeted disruptions of seven of the mouse MMR genes
have now been generated (53,64–73), making it possible to
examine the meiotic roles of the gene products. For several of
these, the meiotic phenotypes of the null animals suggest a
high degree of functional conservation between the mouse and
yeast homologs, for example (i) both male and female mice
homozygous for targeted disruptions of either Msh4 or Msh5
2414 Human Molecular Genetics, 2000, Vol. 9, No. 16 Review
Table 1. Mismatch repair genes with a meiotic role
Meiotic phenotype of null mutations for mismatch repair genes
Yeast
References
Mouse
Recombination
Rate of non-disjunction
Post-meiotic segregation
Meiotic abnormalities
Msh2
No effecta
No effect
Increased
No obvious abnormalities
55,65,66
Msh3
No
effecta
No effect
Slightly increased
No obvious abnormalities
56,73
Msh4
Reduced
Increased
No effect
Male/female sterile; block at early prophase
52,53
Msh5
Reduced
Increased
No effect
Male/female sterile; block at early prophase
54,70,71
Msh6
No effect
No effect
Slightly increased
No obvious abnormalities
69,86
Reduced
Increased
Increased
Male/female sterile; recombination virtually
abolished
67,68,86
Unknown
51
51
MutS homologs
MutL homologs
Mlh1
Mlh2
Mlh3
Reduced
Increased
Slightly increased
Unknown
Pms1b
No effecta
No effect
Increased
Male sterile with synaptic defects; female fertile 57
aRecombination
bPms1
levels are slightly elevated when these mutations are present on a non-isogenic background (58–60).
is the yeast homolog of human/mouse Pms2.
are sterile, with disruption early in meiotic prophase, consistent
with recombination failure (53,70,71); (ii) in the Mlh1-null
mouse, synapsis of homologous chromosomes occurs
normally, but recombination is virtually abolished, the result is
sterility of both male and female null mice, with arrest during
the meiotic divisions, presumably due to the presence of
multiple univalent chromosomes (67,68,74); and (iii) the Pms2
(the mouse homolog of yeast Pms1) mutant mouse has an
abnormal but poorly understood meiotic phenotyp; although the
null female is fertile, the Pms2 null male is sterile; analysis of
pachytene spermatocytes revealed evidence of synaptic disturbances (64); however, the level of meiotic disturbance is insufficient to explain the infertility, and a significant number of
cells complete meiosis, as evidenced by the production of
sperm.
Meiotic abnormalities have not yet been identified in the
other null mutants, including those deficient for Msh2 or Msh3
(65,66,73). However, it is important to note that none of the
studies were designed to address less obvious abnormalities, for
example subtle alterations in chiasma frequency or distribution,
or effects on the possible role of these proteins in preventing
recombination between evolutionarily diverged sequences.
Thus, more detailed meiotic analyses of these mutants may
reveal subtle meiotic defects. In addition, one potentially interesting player, Mlh3, has yet to be analyzed. Recent studies in
yeast have demonstrated a significant reduction in recombination in Mlh3 mutants (51), hence there is reason to suspect that,
like the Mlh1 knockout mouse, targeted disruption of the Mlh3
gene will result in both male and female sterility.
WHEN THINGS GO WRONG: RECOMBINATION AND
HUMAN NON-DISJUNCTION
In 1968, Henderson and Edwards (75) proposed a link between
recombination and human non-disjunction, suggesting that
declining levels of recombination were the cause of the
maternal age effect on trisomy. Specifically, they hypothesized
that meiotic chromosomes of older women were held together
by fewer chiasmata, and that consequently the chromosomes
were more likely to non-disjoin at meiosis I. Their model, the
so-called ‘production line’ hypothesis of maternal agedependent non-disjunction, sparked a series of cytogenetic
investigations into the relationship of chiasmate ‘disturbances’
and meiotic non-disjunction. However, because of the inherent
difficulties in obtaining material from human gametes, most
such studies involved analysis of mouse oocytes. Indeed, it
was not until the advent of DNA polymorphism analysis in the
mid 1980s that it became possible to test the production line
model directly, and more generally the association of aberrant
recombination and non-disjunction, in humans. The results of
these analyses are incompatible with predictions of the production line hypothesis and it is now clear that this model cannot
explain the maternal age effect on human trisomy. Nevertheless, the results provide ample evidence that Henderson and
Edwards’ (75) major premise was correct, i.e. that alterations
in recombination are an important determinant of human
trisomy.
Alterations in the level and/or location of exchanges are a
feature of most, if not all, human trisomies
In an analysis of 34 Down syndrome families, Warren et al.
(76) provided the first direct evidence of an association
between reduced recombination and human trisomy. Subsequently, their interpretations have been confirmed by Lamb et
al. (77) who used centromere mapping techniques or analyses
of meiotic exchange configurations to examine meiotic recombination in normal meioses and in meioses leading to trisomy
21. Results of studies of ∼200 maternal meiosis I-derived trisomies indicated two different recombination-associated avenues to chromosome 21 non-disjunction (Table 2). First, an
Human Molecular Genetics, 2000, Vol. 9, No. 16 Review 2415
Table 2. Summary of studies analyzing the effect of genetic recombination on the origin of human trisomies (data from refs 77–81 and 83, and T. Hassold and S.
Sherman, unpublished data)
Trisomy
Parent and stage of origin of trisomy
Association with number and/or location of exchanges
15
Maternal meiosis I
Overall reduction in exchanges, due to high proportion (20%) of achiasmate bivalents; no obvious effect
on location of exchanges
16
Maternal meiosis I
Overall reduction in exchanges, but most bivalents with at least one; striking alteration in placement of
exchanges, with most being distally located on the long or short arms
18
Maternal meiosis I
Overall reduction in exchanges, with estimated 30% of cases achiasmate; some alterations in location of
exchanges, with proximal 18p and medial 18q having fewest exchanges
Maternal meiosis II
No obvious effect
Maternal meiosis I
Overall reduction in exchanges, with estimated 40% of cases involving achiasmate bivalents; if exchanges
are present, typically in distal 21q
Maternal meiosis II
Overall increase in exchanges, especially in proximal 21q
Maternal meiosis I
Overall reduction in exchanges, with estimated 30% of cases involving achiasmate bivalents; possible
increase in centromeric or pericentromeric exchanges
Maternal meiosis II
No obvious effect
Paternal meiosis I
Overall reduction in exchanges, likely due to increase in achiasmate bivalents
21
XXY/XXX
21
XXY
Paternal meiosis II
No obvious effect on number of exchanges; possible increase in proximally located exchanges
Paternal meiosis I
Striking reduction in recombination, with estimated two-thirds of cases involving achiasmate XY pairs
estimated 35% of cases involved meioses in which the chromosomes 21 failed to pair and/or recombine, i.e. the chromosome
21 bivalent was achiasmate. This was not surprising since, from
studies of other organisms, it is clear that mutants abolishing
recombination invariably increase non-disjunction. However,
the second avenue was unexpected, since it involved chromosome 21 bivalents in which at least one exchange had occurred
(i.e. ‘chiasmate’ bivalents). Specifically, in the majority of chiasmate maternal meiosis I non-disjunctional events, a single
exchange was observed in distal 21q. This exchange configuration was also observed in normal female meioses, but it was
significantly enriched in the non-disjunctional events. Thus, it
seems that, at least for chromosome 21, ‘distal-only’ exchanges are less efficient at segregating chromosomes than are medially placed exchanges. Similar observations have been made
for spontaneous X chromosome non-disjunction in Drosophila
(78), suggesting that this is a feature of meiosis in other organisms as well.
Not surprisingly, much less is known about other human
trisomies. Nevertheless, initial studies of maternal meiosis Iderived trisomies 15 (and UPD 15), 16 and 18 and sex chromosome trisomies, paternal meiosis I trisomies 21 and sex
chromosome trisomies indicate that each is associated with a
reduction in recombination (Table 2). Thus, it appears that
alterations in recombination are a feature of all human trisomic
conditions. However, there is also mounting evidence that this
effect varies considerably among individual chromosomes. For
example, achiasmate bivalents are responsible for the majority
of cases of paternally derived cases of 47,XXY (79), and for an
estimated 20–40% of maternal meiosis I trisomies 15, 18 and
21, and sex chromosome trisomy (77,80–82); however, achiasmate events appear to be unimportant in the genesis of trisomy
16 (T. Hassold, unpublished data). Furthermore, distally
placed exchanges appear to be an important contributor to
trisomies 16 and 21, but have not been identified in any of the
other trisomies (83). Thus, it seems unlikely that any one
trisomy can be used as a model for all human non-disjunctional
events. Instead it appears that the effects of altered recombination on non-disjunction are mediated by chromosome-specific
properties.
Since meiotic recombination occurs at meiosis I, there was
no reason to believe that exchange patterns would be altered in
trisomies of meiosis II origin. Thus, it was somewhat
surprising when Lamb et al. (84) reported a significant
increase in recombination, especially in the proximal region of
21q, in trisomy 21 of maternal meiosis II origin. Lamb et al.
(84) offered two possible explanations for this unusual observation. First, the presence of proximal chiasmata could lead to
‘chromosome entanglement’ at meiosis I, with the two homologous chromosomes 21 travelling together to the same pole. If
at meiosis II the bivalent divided reductionally, the result
would be two disomic gametes, each with identical centromeres, thus leading to the case being scored as a meiosis II
error. Alternatively, the presence of proximal chiasmata might
interfere with centromeric sister chromatid cohesion, resulting
in premature separation of sister chromatids at meiosis I. If the
sisters travelled to the same pole at meiosis I, they would have
a 50% chance of migrating together again at meiosis II, leading
to an apparent meiosis II error. Regardless of the correctness of
these or other models, the implication of the observations was
clear: virtually all cases of maternal trisomy 21, even those
scored as arising at meiosis II, have their genesis at meiosis I,
so that efforts to identify determinants of human non-disjunction might well be restricted to an examination of events occurring during meiosis I.
However, subsequent studies of trisomy 18 indicate that
other chromosomes may behave differently. In studies of 88
maternal meiosis II-derived cases, Bugge et al. (81) were
unable to identify any significant effect of recombination,
suggesting that non-disjunction did indeed occur at meiosis II.
Therefore, similar to the situation for meiosis I trisomies, it
appears that observations for one trisomy may not apply to
other chromosomes.
2416 Human Molecular Genetics, 2000, Vol. 9, No. 16 Review
Maternal age, recombination and non-disjunction
The association between increasing maternal age and trisomy
is arguably the most important etiological factor in human
genetic disease. The risk of trisomy is at least 15 times greater
for women in their forties than for women in their twenties, and
appears to be exerted on all women, regardless of reproductive
history, race, geography or socioeconomic status.
Despite the clinical importance of the maternal age effect,
we know virtually nothing about its origin. Thus, the demonstration of an association between recombination and human
trisomy led to an obvious question: are abnormal levels or
locations of meiotic exchanges linked to maternal age? Initial
studies of trisomies 21 and 16 indicated that the answer was
yes. In both conditions, genetic maps based on maternal
meiosis I errors were similarly reduced in younger and older
women and, in trisomy 21, maps associated with meiosis II
errors were similarly increased in younger and older women.
From this it was suggested that there might be two ‘hits’ to
maternal age-related non-disjunction (84). The first hit would
involve the establishment of a suboptimal chiasmate configuration (e.g. a bivalent with a single, distally placed exchange) in
the fetal oocyte; this event would be age independent. The
second hit would involve abnormal processing of the susceptible bivalent at metaphase I, and would be the age-dependent
component of the process. If this model is correct, it implies
that the non-disjunctional process is similar in women of
different ages. The increase in trisomy with age simply derives
from the fact that the older ovary is less efficient than the
younger ovary in processing certain types of exchange configuration.
More recently, several groups have extended these initial
studies, asking whether the observations on trisomies 16 and
21 could be confirmed, and whether they extended to other
chromosomes as well. Results of these analyses for three
conditions, trisomies 15, 16 and 21 of maternal meiosis I
origin, are provided in Figure 2. The results for both trisomies
16 and 21 are consistent with the earlier observations, i.e. the
exchange distributions appear to be unaffected by maternal
age. However, the results clearly are different for trisomy 15,
as the proportion of events scored as achiasmate decreases
with age and the proportion of three to four chiasmate events
increases with age (80).
How can we account for these different results? One possibility is that there are at least two types of susceptible meiotic
configuration—one type involving an achiasmate bivalent and
the other a bivalent with a chiasma at an ‘unfortunate’ location
(i.e. either too distal or too proximal)—with the two categories
having different relationships with maternal age and varying in
their importance in different trisomies. For example, an achiasmate bivalent may impart an age-independent risk; after all,
failure to recombine should increase the risk of mal-segregation regardless of the age of the woman, and appears to be relatively more important in the genesis of trisomies 15 and 21
than that of trisomy 16 (Fig. 2). The apparent discrepancy
between trisomies 15 and 21 might be a technical artifact,
reflecting differences in the ability to detect real achiasmate
events. That is, in female meiosis the chromosome 21 bivalent is
typically held together by one to two chiasmata, whereas chromosome 15 is joined by two to three chiasmata; consequently, situa-
Figure 2. Number of detectable exchanges by maternal age in non-disjunctional meioses leading to trisomies 15, 16 and 21. Data are from Robinson
et al. (80) and T. Hassold and S. Sherman (unpublished data).
tions in which crossing over occurs but both recombinant or both
non-recombinant chromatids are recovered, leading to scoring of
the case as achiasmate, are more likely to involve trisomy 21.
Thus, there may be an age-related reduction in the proportion
of achiasmate cases in both trisomies 15 and 21, with the
power to detect the effect reduced for trisomy 21.
Human Molecular Genetics, 2000, Vol. 9, No. 16 Review 2417
The second category, involving a bivalent with an aberrantly
located exchange, appears to be the most important risk factor
for trisomy 16. Indeed, the majority of cases of trisomy 16
appear to involve meioses with distally located exchanges (T.
Hassold, unpublished data). As there is no obvious age-related
difference in the number of exchanges in trisomy 16 (Fig. 2),
the originally stated features of the two-hit model may still
apply to this condition.
The relationship between maternal age, recombination and
non-disjunction for other human chromosomes is not yet clear.
Analysis of these conditions, as well as analysis of additional
cases of trisomies 15, 16 and 21, will be important determining
whether the two-hit model of the maternal age effect applies to
any, several or all human trisomies.
Genome-wide effects on recombination in
nondisjunctional meioses
One of the most intriguing questions regarding the association
of altered recombination and non-disjunction involves the
extent of the effect: are the alterations in exchange patterns
limited to the non-disjoining bivalent, or are there are genomewide ‘disturbances’ in recombination in non-disjunctional
meioses? Preliminary evidence for trisomy 21 suggests that, at
least in some trisomy-generating meioses, the latter is the case
(85). Utilizing DNA samples from the trisomic individual and
his/her parents and grandparents, Savage Brown et al. (85)
conducted genome-wide recombination screens in 15 maternal
meiosis I-derived cases in which recombination between the
non-disjoining bivalents had not been detected. They found
evidence of a cell-wide reduction in recombination in the
trisomy-generating meiosis, as the mean number of detectable
exchanges was 35.5, significantly reduced from the value of
39.9 in CEPH controls. Centromeric and telomeric regions
were similarly affected, and there was no evidence that specific
chromosomes were driving the effect; thus, the reduction
appeared to be global in nature. Savage Brown et al. (85)
concluded that the effect was consistent with normal variation
in recombination rates among oocytes and suggested that when
the number of genome-wide recombination events falls below
a threshold, specific chromosomes become more likely to nondisjoin.
ACKNOWLEDGEMENTS
Research conducted in our laboratories and discussed in this
report was supported by NIH grants HD21341 (to T.H.),
HD32111 (to S.S.) and HD31866 (to P.H.). We gratefully
acknowledge Dr Terry Ashley for providing comments on the
manuscript.
REFERENCES
1. Botstein, D., White, R.L., Skolnick, M. and Davis, R.W. (1980) Construction of a genetic linkage map in man using restriction fragment length
polymorphisms. Am. J. Hum. Genet., 32, 314–331.
2. Moore, D.P. and Orr-Weaver, T.L. (1998) Chromosome segregation during meiosis: building an unambivalent bivalent. Curr. Top. Dev. Biol., 37,
263–299.
3. Roeder, G.S. (1997) Meiotic chromosomes: it takes two to tango. Genes
Dev., 11, 2600–2621.
4. Robinson, W.P. (1996) The extent, mechanism, and consequences of
genetic variation, for recombination rate. Am. J. Hum. Genet., 59, 1175–
1183.
5. Yip, S.P., Lovegrove, J.U., Rana, N.A., Hopkinson, D.A. and Whitehouse,
D.B. (1999) Mapping recombination hotspots in human phosphoglucomutase (PGM1). Hum. Mol. Genet., 8, 1699–1706.
6. Jeffreys, A.J., Murray, J. and Neumann, R. (1998) High-resolution mapping of crossovers in human sperm defines a minisatellite-associated
recombination hotspot. Mol. Cell, 2, 267–273.
7. Yu, J., Lazzeroni, L., Qin, J., Huang, M.M., Navidi, W., Erlich, H. and
Arnheim, N. (1996) Individual variation in recombination among human
males. Am. J. Hum. Genet., 59, 1186–1192.
8. Hulten, M. (1974) Chiasma distribution at diakinesis in the normal human
male. Hereditas, 76, 55–78.
9. Laurie, D.A. and Hulten, M.A. (1985) Further studies on bivalent chiasma
frequency in human males with normal karyotypes. Ann. Hum. Genet., 49,
189–201.
10. Baker, S.M. et al. (1996) Involvement of mouse Mlh1 in DNA mismatch
repair and meiotic crossing over. Nature Genet., 13, 336–342.
11. Woods, L.M., Hodges, C.A., Baart, E., Baker, S.M., Liskay, M. and Hunt,
P.A. (1999) Chromosomal influence on meiotic spindle assembly: abnormal meiosis I in female Mlh1 mutant mice. J. Cell Biol., 145, 1395–1406.
12. Anderson, L.K., Reeves, A., Webb, L.M. and Ashley, T. (1999) Distribution of crossing over on mouse synaptonemal complexes using immunofluorescent localization of MLH1 protein. Genetics, 151, 1569–1579.
13. Barlow, A.L. and Hulten, M.A. (1998) Crossing over analysis at pachytene in man. Eur. J. Hum. Genet., 6, 350–358.
14. Broman, K.W., Murray, J.C., Sheffield, V.C., White, R.L. and Weber, J.L.
(1998) Comprehensive human genetic maps: individual and sex-specific
variation in recombination. Am. J. Hum. Genet., 63, 861–869.
15. Ashley, T. and Plug, A. (1998) Caught in the act: deducing meiotic function from protein immunolocalization. Curr. Top. Dev. Biol., 37, 201–239.
16. Broman, K.W. and Weber, J.L. (2000) Characterization of human crossover interference. Am. J. Hum. Genet., 66, 1911–1926.
17. Broman, K.W. and Weber, J.L. (1999) Long homozygous chromosomal
segments in reference families from the centre d’Etude du polymorphisme
humain. Am. J. Hum. Genet., 65, 1493–1500.
18. Sun, H., Treco, D., Schultes, N.P. and Szostak, J.W. (1989) Double-strand
breaks at an initiation site for meiotic gene conversion. Nature, 338, 87–90.
19. Zickler, D. and Kleckner, N. (1999) Meiotic chromosomes: integrating
structure and function. Annu. Rev. Genet., 33, 603–754.
20. Zickler, D. and Kleckner, N. (1998) The leptotene–zygotene transition of
meiosis. Annu. Rev. Genet., 32, 619–697.
21. van Heemst, D. and Heyting, C. (2000) Sister chromatid cohesion and
recombination in meiosis. Chromosoma, 109, 10–26.
22. McKim, K.S., Green-Marroquin, B.L., Sekelsky, J.J., Chin, G., Steinberg,
C., Khodosh, R. and Hawley, R.S. (1998) Meiotic synapsis in the absence
of recombination. Science, 279, 876–878.
23. Dernburg, A.F., McDonald, K., Moulder, G., Barstead, R., Dresser, M.
and Villeneuve, A.M. (1998) Meiotic recombination in C. elegans initiates by a conserved mechanism and is dispensable for homologous
chromosome synapsis. Cell, 94, 387–398.
24. Keeney, S., Giroux, C.N. and Kleckner, N. (1997) Meiosis-specific DNA
double-strand breaks are catalyzed by Spo11, a member of a widely conserved protein family. Cell, 88, 375–384.
25. Bergerat, A., de Massy, B., Gadelle, D., Varoutas, P.C., Nicolas, A. and
Forterre, P. (1997) An atypical topoisomerase II from Archaea with implications for meiotic recombination. Nature, 386, 414–417.
26. Keeney, S., Baudat, F., Angeles, M., Zhou, Z.H., Copeland, N.G., Jenkins,
N.A., Manova, K. and Jasin, M. (1999) A mouse homolog of the Saccharomyces cerevisiae meiotic recombination DNA transesterase Spo11p.
Genomics, 61, 170–182.
27. Metzler-Guillemain, C. and de Massy, B. (2000) Identification and characterization of an SPO11 homolog in the mouse. Chromosoma, 109, 133–
138.
28. Romanienko, P.J. and Camerini-Otero, R.D. (1999) Cloning, characterization, and localization of mouse and human SPO11. Genomics, 61, 156–
169.
29. Eijpe, M., Offenberg, H., Goedecke, W. and Heyting, C. (2000) Localisation of RAD50 and MRE11 in spermatocyte nuclei of mouse and rat.
Chromosoma, 109, 123–132.
30. Goedecke, W., Eijpe, M., Offenberg, H.H., van Aalderen, M. and Heyting,
C. (1999) Mre11 and Ku70 interact in somatic cells, but are differentially
expressed in early meiosis. Nature Genet., 23, 194–198.
2418 Human Molecular Genetics, 2000, Vol. 9, No. 16 Review
31. Haber, J.E. (1998) The many interfaces of Mre11. Cell, 95, 583–586.
32. Pittman, D.L. and Schimenti, J.C. (1998) Recombination in the mammalian germ line. Curr. Top. Dev. Biol., 37, 1–35.
33. Masson, J.Y., Davies, A.A., Hajibagheri, N., Van Dyck, E., Benson, F.E.,
Stasiak, A.Z., Stasiak, A. and West, S.C. (1999) The meiosis-specific
recombinase hDmc1 forms ring structures and interacts with hRad51.
EMBO J., 18, 6552–6560.
34. Pittman, D.L., Cobb, J., Schimenti, K.J., Wilson, L.A., Cooper, D.M.,
Brignull, E., Handel, M.A. and Schimenti, J.C. (1998) Meiotic prophase
arrest with failure of chromosome synapsis in mice deficient for Dmc1, a
germline-specific RecA homolog. Mol. Cell, 1, 697–705.
35. Yoshida, K., Kondoh, G., Matsuda, Y., Habu, T., Nishimune, Y. and
Morita, T. (1998) The mouse RecA-like gene Dmc1 is required for homologous chromosome synapsis during meiosis. Mol. Cell, 1, 707–718.
36. Moens, P.B., Pearlman, R.E., Heng, H.H. and Traut, W. (1998) Chromosome cores and chromatin at meiotic prophase. Curr. Top. Dev. Biol., 37,
241–262.
37. Schmekel, K., Meuwissen, R.L., Dietrich, A.J., Vink, A.C., van Marle, J.,
van Veen, H. and Heyting, C. (1996) Organization of SCP1 protein
molecules within synaptonemal complexes of the rat. Exp. Cell Res., 226,
20–30.
38. Liu, J.G., Yuan, L., Brundell, E., Bjorkroth, B., Daneholt, B. and Hoog, C.
(1996) Localization of the N-terminus of SCP1 to the central element of
the synaptonemal complex and evidence for direct interactions between
the N-termini of SCP1 molecules organized head-to-head. Exp. Cell Res.,
226, 11–19.
39. Schalk, J.A., Dietrich, A.J., Vink, A.C., Offenberg, H.H., van Aalderen,
M. and Heyting, C. (1998) Localization of SCP2 and SCP3 protein
molecules within synaptonemal complexes of the rat. Chromosoma, 107,
540–548.
40. Yuan, L., Pelttari, J., Brundell, E., Bjorkroth, B., Zhao, J., Liu, J.G.,
Brismar, H., Daneholt, B. and Hoog, C. (1998) The synaptonemal complex protein SCP3 can form multistranded, cross-striated fibers in vivo. J.
Cell Biol., 142, 331–339.
41. Yuan, L., Liu, J.G., Zhao, J., Brundell, E., Daneholt, B. and Hoog, C.
(2000) The murine SCP3 gene is required for synaptonemal complex
assembly, chromosome synapsis, and male fertility. Mol. Cell, 5, 73–83.
42. Carpenter, A.T. (1987) Gene conversion, recombination nodules, and the
initiation of meiotic synapsis. BioEssays, 6, 232–236.
43. Plug, A.W., Peters, A.H., Keegan, K.S., Hoekstra, M.F., de Boer, P. and
Ashley, T. (1998) Changes in protein composition of meiotic nodules during mammalian meiosis. J. Cell Sci., 111, 413–423.
44. Tarsounas, M., Morita, T., Pearlman, R.E. and Moens, P.B. (1999)
RAD51 and DMC1 form mixed complexes associated with mouse meiotic
chromosome cores and synaptonemal complexes. J. Cell Biol., 147, 207–
220.
45. Moens, P.B., Tarsounas, M., Morita, T., Habu, T., Rottinghaus, S.T.,
Freire, R., Jackson, S.P., Barlow, C. and Wynshaw-Boris, A. (1999) The
association of ATR protein with mouse meiotic chromosome cores.
Chromosoma, 108, 95–102.
46. Barlow, A.L., Benson, F.E., West, S.C. and Hulten, M.A. (1997) Distribution of the Rad51 recombinase in human and mouse spermatocytes.
EMBO J., 16, 5207–5215.
47. Moens, P.B., Freire, R., Tarsounas, M., Spyropoulos, B. and Jackson, S.P.
(2000) Expression and nuclear localization of BLM, a chromosome stability protein mutated in Bloom’s syndrome, suggest a role in recombination
during meiotic prophase. J. Cell Sci., 113, 663–672.
48. Walpita, D., Plug, A.W., Neff, N.F., German, J. and Ashley, T. (1999)
Bloom’s syndrome protein, BLM, colocalizes with replication protein A
in meiotic prophase nuclei of mammalian spermatocytes. Proc. Natl Acad.
Sci. USA, 96, 5622–5627.
49. Modrich, P. and Lahue, R. (1996) Mismatch repair in replication fidelity,
genetic recombination, and cancer biology. Annu. Rev. Biochem., 65,
101–103.
50. Kolodner, R. (1996) Biochemistry and genetics of eukaryotic mismatch
repair. Genes Dev., 10, 1433–1442.
51. Wang, T.F., Kleckner, N. and Hunter, N. (1999) Functional specificity of
MutL homologs in yeast: evidence for three Mlh1-based heterocomplexes
with distinct roles during meiosis in recombination and mismatch correction. Proc. Natl Acad. Sci. USA, 96, 13914–13919.
52. Ross-Macdonald, P. and Roeder, G.S. (1994) Mutation of a meiosisspecific MutS homolog decreases crossing over but not mismatch correction. Cell, 79, 1069–1080.
53. Kneitz, B. et al. (2000) MutS homolog 4 localization to meiotic chromosomes is required for chromosome pairing during meiosis in male and
female mice. Genes Dev., 14, 1085–1097.
54. Hollingsworth, N.M., Ponte, L. and Halsey, C. (1995) MSH5, a novel
MutS homolog, facilitates meiotic reciprocal recombination between
homologs in Saccharomyces cerevisiae but not mismatch repair. Genes
Dev., 9, 1728–1739.
55. Reenan, R.A. and Kolodner, R.D. (1992) Characterization of insertion
mutations in the Saccharomyces cerevisiae MSH1 and MSH2 genes: evidence for separate mitochondrial and nuclear functions. Genetics, 132,
975–985.
56. New, L., Liu, K. and Crouse, G.F. (1993) The yeast gene MSH3 defines a
new class of eukaryotic MutS homologues. Mol. Gen. Genet., 239, 97–
108.
57. Prolla, T.A., Christie, D.M. and Liskay, R.M. (1994) Dual requirement in
yeast DNA mismatch repair for MLH1 and PMS1, two homologs of the
bacterial mutL gene. Mol. Cell. Biol., 14, 407–415.
58. Chambers, S.R., Hunter, N., Louis, E. and Borts, R.H. (1996) The mismatch repair system reduces meiotic homologous recombination and
stimulates recombination-dependent chromosome loss. Mol. Cell. Biol.,
16, 6110–6120.
59. Selva, E.M., Maderazo, A.B. and Lahue, R.S. (1997) Differential effects
of the mismatch repair genes MSH2 and MSH3 on homologous recombination in Saccharomyces cerevisiae. Mol. Gen. Genet., 257, 71–82.
60. Datta, A., Adjiri, A., New, L., Crouse, G.F. and Jinks Robertson, S. (1996)
Mitotic crossovers between diverged sequences are regulated by mismatch repair proteins in Saccaromyces cerevisiae. Mol. Cell. Biol., 16,
1085–1093.
61. Fishel, R., Lescoe, M.K., Rao, M.R., Copeland, N.G., Jenkins, N.A.,
Garber, J., Kane, M. and Kolodner, R. (1993) The human mutator gene
homolog MSH2 and its association with hereditary nonpolyposis colon
cancer. Cell, 75, 1027–1038.
62. Leach, F.S. et al. (1993) Mutations of a mutS homolog in hereditary nonpolyposis colorectal cancer. Cell, 75, 1215–1225.
63. Jiricny, J. and Nystrom-Lahti, M. (2000) Mismatch repair defects in cancer. Curr. Opin. Genet. Dev., 10, 157–161.
64. Baker, S.M. et al. (1995) Male mice defective in the DNA repair gene
PMS2 exhibit abnormal chromosome synapsis in meiosis. Cell, 82, 309–
319.
65. de Wind, N., Dekker, M., Berns, A., Radman, M. and te Riele, H. (1995)
Inactivation of the mouse Msh2 gene results in mismatch repair deficiency, methylation tolerance, hyperrecombination, and predisposition to
cancer. Cell, 82, 321–330.
66. Reitmair, A.H. et al. (1995) MSH2 deficient mice are viable and susceptible to lymphoid tumors. Nature Genet., 11, 64–70.
67. Baker, S. et al. (1996) Involvement of mouse MLH1 in DNA mismatch
repair and meiotic crossing over. Nature Genet., 13, 336–342.
68. Edelmann, W. et al. (1996) Meiotic pachytene arrest in MLH1-deficient
mice. Cell, 85, 1125–1134.
69. Edelmann, W. et al. (1997) Mutation in the mismatch repair gene Msh6
causes cancer susceptibility. Cell, 91, 467–477.
70. Edelmann, W., Cohen, P.E., Kneitz, B., Winand, N., Lia, M., Heyer, J.,
Kolodner, R., Pollard, J.W. and Kucherlapati, R. (1999) Mammalian
MutS homologue 5 is required for chromosome pairing in meiosis. Nature
Genet., 21, 123–127.
71. de Vries, S.S., Baart, S.B., Dekker, M., Siezen, A., de Rooij, D.G.,
de Boer, P. and te Riele, H. (1999) Mouse MutS-like protein Msh5 is
required for proper chromosome synapsis in male and female meiosis.
Genes Dev., 13, 523–531.
72. de Wind, N. et al. (1999) HNPCC-like cancer predisposition in mice
through simultaneous loss of Msh3 and Msh6 mismatch-repair protein
functions. Nature Genet., 23, 359–362.
73. Edelmann, W. et al. (2000) The DNA mismatch repair genes Msh3 and
Msh6 cooperate in intestinal tumor suppression. Cancer Res., 60, 803–
807.
74. Woods, L.M., Hodges, C., Baart, E., Baker, S.M., Liskay, M. and Hunt,
P.A. (1999) Chromosomal control of meiotic spindle assembly: abnormal
meiosis I in female Mlh1 mutant mice. J. Cell Biol., 145, 1395–1406.
75. Henderson, S.A. and Edwards, R.G. (1968) Chiasma frequency and
maternal age in mammals. Nature, 217, 22–28.
76. Warren, A.C., Chakravarti, A., Wong, C., Slaugenhaupt, S.A., Halloran,
S.L., Watkins, P.C., Metaxotou, C. and Antonarakis, S.E. (1987) Evidence
for reduced recombination on the nondisjoined chromosomes 21 in Down
syndrome. Science, 237, 652–654.
Human Molecular Genetics, 2000, Vol. 9, No. 16 Review 2419
77. Lamb, N.E. et al. (1997) Characterization of susceptible chiasma configurations that increase the risk for maternal nondisjunction of chromosome
21. Hum. Mol. Genet., 6, 1391–1399.
78. Koehler, K.E., Hawley, R.S., Sherman, S. and Hassold, T. (1996) Recombination and nondisjunction in humans and flies. Hum. Mol. Genet., 5,
1495–1504.
79. Thomas, T., Collins, A., Scambler, P., Hassold, T. and Jacobs, P. (2000)
A reinvestigation of nondisjunction resulting in 47,XXY males of paternal
origin. Eur. J. Hum. Genet., in press.
80. Robinson, W.P. et al. (1998) Maternal meiosis I non-disjunction of chromosome 15: dependence of the maternal age effect on level of recombination. Hum. Mol. Genet., 7, 1011–1019.
81. Bugge, M. et al. (1998) Non-disjunction of chromosome 18. Hum. Mol.
Genet., 7, 661–669.
82. MacDonald, M., Hassold, T., Harvey, J., Wang, L.H., Morton, N.E. and
Jacobs, P. (1994) The origin of 47,XXY and 47,XXX aneuploidy: hetero-
83.
84.
85.
86.
geneous mechanisms and role of aberrant recombination. Hum. Mol.
Genet., 3, 1365–1371.
Koehler, K.E. and Hassold, T.J. (1998) Human aneuploidy: lessons from
achiasmate segregation in Drosophila melanogaster. Ann. Hum. Genet.,
62, 467–479.
Lamb, N.E. et al. (1996) Susceptible chiasmate configurations of chromosome 21 predispose to non-disjunction in both maternal meiosis I and
meiosis II. Nature Genet., 14, 400–405.
Brown, A.S., Feingold, E., Broman, K.W. and Sherman, S.L. (2000)
Genome-wide variation in recombination in female meiosis: a risk factor
for non-disjunction of chromosome 21. Hum. Mol. Genet., 9, 515–523.
Hunter, N. and Borts, R.H. (1997) Mlh1 is unique among mismatch repair
proteins in its ability to promote crossing-over during meiosis. Genes
Dev., 11, 1573–1582.
2420 Human Molecular Genetics, 2000, Vol. 9, No. 16 Review