Human Reproduction Update, Vol.9, No.4 pp. 309±317, 2003
DOI: 10.1093/humupd/dmg028
Aberrant recombination and the origin of Klinefelter
syndrome
N.S.Thomas1,2 and T.J.Hassold3,4
1
Wessex Regional Genetics Laboratory, Salisbury District Hospital, Salisbury, UK, 2Human Genetics, Duthie Building,
Southampton General Hospital, Southampton, UK and 3Department of Genetics, Case Western Reserve University,
Cleveland, OH, USA
4
To whom correspondence should be addressed at: Department of Genetics, Case Western Reserve University10900 Euclid Avenue,
Cleveland OH 44106, USA. E-mail: [email protected]
Trisomy is the most commonly identi®ed chromosome abnormality in humans, occurring in at least 4% of all clinically recognized pregnancies; it is the leading known cause of pregnancy loss and of mental retardation. Over the
past decade, molecular studies have demonstrated that most human trisomies originate from errors at maternal
meiosis I. However, Klinefelter syndrome is a notable exception, as nearly one-half of all cases derive from paternal
non-disjunction. In this review, the data on the origin of sex chromosome trisomies are summarized, focusing on the
47,XXY condition. Additionally, the results of recent genetic mapping studies are reviewed that have led to the identi®cation of the ®rst molecular correlate of autosomal and sex chromosome non-disjunction; i.e. altered levels and
positioning of meiotic recombinational events.
Key words: aneuploidy/chromosomal abnormalities/Klinefelter syndrome/meiotic non-disjunction/recombination
Introduction
Chromosome abnormalities occur with astonishing frequency in
humans, being an order of magnitude more frequent than in any
other species. Of the different classes of abnormality, aneuploidyÐthe loss or gain of an entire chromosome; that is,
monosomy or trisomyÐis by far the most common and clinically
the most important. Aneuploidy occurs in some 5% of all
pregnancies that survive long enough to be clinically recognized
(Hassold and Jacobs 1984; Hunt and Hassold, 2002), and an even
higher rate is indicated by the direct cytogenetic analysis of human
gametes. Indeed, it seems likely that as many as 10±25% of all
fertilized human oocytes are either monosomic or trisomic
(Jacobs, 1992; Pellestor et al., 2002).
The consequences of aneuploidy vary, depending on the speci®c
abnormality. In general, monosomies are eliminated early in
gestation, with sex chromosome monosomy (45,X) being the only
monosomic condition compatible with survival to term. Trisomies
are also incompatible with normal development, and the observed
level of trisomy varies enormously depending upon the developmental time point being examined. For example, only 0.3% of
newborns are trisomic (Hassold, 1998), but the incidence increases
10-fold among stillbirths and 10-fold again among spontaneous
abortions (Table I). The consequences of trisomy are less severe
for the sex chromosomes than the autosomes, so that sex
chromosome trisomies are `enriched' among liveborns by com-
parison with autosomal trisomies. Thus, the 47,XXY condition
(the chromosome constitution associated with Klinefelter syndrome) is identi®ed in approximately 1 of every 1000 male births
(Hook and Hamerton, 1975), making it one of the most commonly
identi®ed chromosome abnormalities among liveborn individuals.
This relatively high incidence has made it possible to investigate
intensively the mechanism of origin of the additional chromosome
in XXY individuals.
The parent and meiotic stage of origin of Klinefelter
syndrome
Over the past decade, DNA polymorphisms have been used to
analyse over 1000 cases of trisomy, and the results show
remarkable variation among trisomies with regard to the parental
and meiotic stage of origin of the additional chromosome. In the
great majority of trisomies the additional chromosome is of
maternal origin and results from an error during the ®rst meiotic
division. Paternal non-disjunction accounts for only about 10% of
all autosomal trisomies (Zaragoza et al., 1994; Savage et al.,
1998), although certain autosomal trisomies (e.g. trisomies 2 and
8) appear to have an increased incidence of paternally derived
cases of non-disjunction (Hassold, 1998). In contrast to autosomal
abnormalities, paternal errors contribute more frequently to sex
chromosome aneuploidy, since they account for about 10% of
47,XXX females (May et al., 1990) and all 47,XYY males
Human Reproduction Update 9(4) ã European Society of Human Reproduction and Embryology 2003; all rights reserved
309
N.S.Thomas and T.J.Hassold
Table I. Incidence of XXY and all trisomies among different populationsa
Table II. Relative contribution of maternal and paternal errors to XXY
Population
XXY
(%)
All trisomies
(%)
Proportion
of XXY
Reference(s)
Spontaneous abortions
Stillbirths
Livebirths
All clinically recognized pregnancies
0.20
0.20
0.05
0.08
26.1
4.0
0.3
4.1
1:131
1:20
1:6
1:51
Jacobs et al. (1988); Hassold et al. (1991)
112
Lorda-Sanchez et al. (1992)
47
N.Thomas, D.Skuse and P.A.Jacobs (unpublished) 73
Total
232
aMales
56
24
38
118
56
23
35
114
and females combined
(Robinson and Jacobs, 1999), and in 80% of 45,X females the
missing sex chromosome is paternal (Jacobs et al., 1997).
The 47,XXY condition has also been extensively studied, with
around one half of all cases being paternally derived (Table II). In
theory, the nature of the errors by which maternal and paternal
XXYs can arise are very different. Maternal XXY can arise by an
error at either meiosis I (MI) or meiosis II (MII), or through an
error at an early mitotic division in the developing zygote [postzygotic mitotic (PZM) errors]. From the data in Table III it is clear
that all such errors occur, with errors at MI being the most
common source of maternal non-disjunction.In contrast, XXY of
paternal origin can only arise by an MI errorÐthat is, errors at MII
or during an early cleavage division result only in 47,XXX or
47,XYY conceptuses, not in 47,XXYs.
Meiotic recombination and the aetiology of meiotic
non-disjunction
Ever since the detection of the ®rst human trisomy over 40 years
ago, considerable effort has been directed at identifying predisposing factors to trisomy. Until recently, these efforts went
unrewarded, as the well-known association between maternal age
and trisomy remained the only factor incontrovertibly linked to
human non-disjunction. However, over the past decade it has
become increasingly clear that aberrant meiotic recombination is
an important contributor to human trisomies. In the following
sections, the importance of recombination to meiotic chromosome
segregation is ®rst discussed, after which the available evidence
linking altered recombination to maternally and paternally derived
cases of 47,XXY is reviewed.
Meiosis serves to generate haploid gametes through a
specialized cell division process that consists of one round of
DNA replication followed by two stages of cell division, MI and
MII. During prophase of MI the pairs of homologous chromosomes (`bivalents') synapse and undergo recombination, with
chiasmata being formed at the sites of exchange. These chiasmata
serve to link together the homologues and thus play a crucial role
in the proper disjunction of chromosomes during the ®rst meiotic
division. The number and position of recombination events is
tightly controlled, and for any chromosomal pair there is a
requirement for at least one crossover (Hawley et al., 1994).
In yeast and Drosophila, mutants that reduce meiotic recombination invariably have increased frequencies of non-disjunction
(for a review, see Roeder, 1997). While it seemed likely that this
relationship would also apply to humans, there was no easy way to
test for this. However, with the advent of DNA polymorphism
analysis during the 1980s, it ®nally became possible to ask whether
310
Total Maternal Paternal
(n)
(n)
(n)
Table III. Distribution of sex chromosome trisomiesa
Origin
Meiosis I ± achiasmate
Meiosis I ± chiasmate
Meiosis II
PZM
Unassigned meiotic errors
Paternal
(%)
67.1
32.9
0
0
0
Maternal
(%)
22.8
25.7
28.6
15.7
7.1
aEstimated using the computer program EXCHANGE.
PZM = post-zygotic mitotic.
such an association might exist for humans. The methodological
approach is relatively straightforward, and is an extension of
`tetrad analysis' techniques long applied to organisms such as
yeast. In essence, it involves three steps:
1. DNA polymorphisms are compared between the trisomic
fetus/child and the parents to determine the parent of origin of the
extra chromosome. For example, if at a given chromosome 21
locus a trisomy 21 individual has three different alleles, A,B,C,
while his mother is A,B and his father, C,D, it would be concluded
that the trisomy was maternally derived.
2. The meiotic stage of non-disjunction is assigned by using
markers at or near the centromere and asking whether heterozygosity in the parent of origin is maintained (non-reduced, or `N')
or whether heterozygosity is reduced to homozygosity (reduced or
`R'); if the former, the error occurred at MI, but if the latter, it
occurred at MII or post-zygotically. Using the above example of
trisomy 21, if the mother were A,B, the father C,D and the child
A,B,C for a chromosome 21 centromeric marker, a maternal MI
error would be concluded. However, if the child were A,A,C, a
maternal MII or a mitotic error would be concluded.
3. Finally, recombination is assessed by examining adjacent
pairs of markers all along the relevant chromosome in the trisomic
individual and the parent of origin, asking whether a `transition'
from R to N or N to R is observed between any of them. If such a
transition is identi®ed, an exchange must have occurred between
the markers during the trisomy-generating meiosis; if no transitions are identi®ed, there would be no evidence for the occurrence
of exchanges. Using the above example, if the mother were
heterozygous (e.g., A,B) at two different chromosome 21 loci and
her trisomic offspring A,B,C at the ®rst locus and A,A,C at the
second, it would be concluded that an exchange had occurred; if
the trisomic individual were A,B,C or A,A,C at both loci, there
would be no evidence that an exchange had occurred.
Aberrant recombination and Klinefelter syndrome
Formally, a transition is de®ned as an observed change in
marker state from N to R, or vice versa, in two non-disjoined
strands; non-disjoined strands in which no transitions are observed
are sometimes referred to as `nullitransitional'. Chiasmata occur
during the four- strand stage, and tetrads in which no crossovers
have occurred are termed `achiasmate'. In this type of genetic
study it is worth bearing in mind that not all of the exchanges
which have occurred in a meiotic tetrad will subsequently be
observed as transitions; for example, if genetic markers are widely
spaced, then two exchanges may occur between the same two
chromatids, leaving ¯anking regions unchanged. Similarly, not all
nullitransitional cases observed will have derived from an
achiasmate tetrad; rather, a proportion will have derived from a
tetrad in which an exchange took place but neither recombinant
chromatid was transmitted to the gamete.
Using this general approach, the ®rst evidence of an association
between aberrant recombination and human trisomy was obtained
15 years ago, when reduced levels of chromosome 21 recombination in meioses leading to trisomy 21 were reported (Warren et al.,
1987). Subsequently, several laboratories have extended these
observations, and it is now clear that mostÐif not allÐhuman
autosomal trisomies are associated with alterations in recombination. Speci®cally, signi®cant reductions in recombination have
now been reported for maternally derived cases of trisomies 15,
16, 18 and 21, and for paternally derived cases of trisomy 21 (for a
review, see Hassold and Hunt, 2001). As described below, these
observations also extend to the XXY condition.
The effect of recombination on maternally derived XXY
During the early 1990s, two groups investigated the association
between meiotic recombination and the origin of maternally
derived cases of 47,XXY (Lorda-Sanchez et al., 1992; Macdonald
et al., 1994). The latter group (Macdonald et al., 1994) reported
that both absent and altered recombination were important in nondisjunction. However, the X centromere marker that they used,
DXZ1, is extremely dif®cult to interpret reliably (Lorda-Sanchez
et al., 1992), and retyping of 94 cases revealed that many cases of
MII and PZM origin had mistakenly been interpreted as MI errors
(Thomas et al., 2001). In total, the centromere status had been
misassigned in over 30% of cases, and thus some of the original
conclusions (Macdonald et al., 1994) clearly were in error.
In a later, more comprehensive study, the stage of origin and
extent and location of recombination in these 94 cases, as well as
an additional 46 maternally derived XXY or XXX cases, were
determined (Thomas et al., 2001). These 140 cases had been
collected over a 12-year period from multiple centres worldwide,
and included both post-natal and prenatal referrals. The results
from both XXXs and XXYs were almost indistinguishable, as
would be expected since they differ only according to the type of
sperm by which the disomic 24,XX oocyte was fertilized. Thus,
the combined results are summarized below.
To determine precisely the effects of recombination on maternal
X chromosome non-disjunction, DNA from each trisomic sample
and both parents was ampli®ed at a total of 54 polymorphic loci
spanning the entire X chromosome, and the results analysed as
described above. All 140 maternally derived cases could be subdivided into ®ve groups according to the origin of trisomy
(Table III). Sixty-eight cases (49%) were consistent with an error
at MI, of which 38 showed no transitions and 30 showed at least
one transition. Using the EXCHANGE program (Bugge et al.,
1998) to reconstruct crossover distributions from the observed
number of transitions, it was calculated that 47% of MI errors
arose from homologues with no crossovers (chiasmata). That is, of
the 38 cases observed to have no transitions, it was estimated that
six arose from a chiasmate meiosis (i.e. crossovers had occurred
but could not be detected) and approximately 32 were genuine
achiasmate errors. There were 40 cases (29%) with MII errors, by
de®nition having at least one transition. Twenty-two cases did not
exhibit any transitions and were reduced at all informative loci:
these were classed as PZM events but could in theory have arisen
as MII errors following a nullitransitional MI. In the remaining 10
cases, a change in marker state was observed, from N to R or vice
versa, between the most proximal (i.e. closest to the centromere)
loci on one chromosome arm and the most proximal loci on the
other chromosome arm. Thus, these cases were clearly meiotic but,
with recombination apparently occurring at or near the centromere, it was not possible to classify the cases as MI- or MIIderived.
Genetic maps were generated from the trisomy data and
compared with a `standard' genetic map of the X chromosome
constructed from normal meioses (Figure 1). When all MI
trisomies were considered together, the trisomic map was considerably shorter than the standard map, suggesting that reduced
recombination is an important factor in MI non-disjunction.
However, when the MI transitional cases were considered
separately the map length was not signi®cantly different from
the standard map length. The non-disjunction linkage maps were
also compared to the standard map to assess the possible role of
altered location of exchanges. Although there was a small increase
in recombination in the short arm, the MI transitional nondisjunction map did not deviate signi®cantly from the standard
map. Similarly, for the MII class there was no signi®cant deviation
in the distribution or number of exchanges.
Thus, from these observations it appears that maternal sex
chromosome trisomy can arise by several different mechanisms,
some of which have parallels with autosomal trisomies while
others are unique to the X chromosome.
MI `nullitransitional'
Consistent with most autosomal trisomies, maternal MI errors
were the single most common class for the extra X chromosome, accounting for nearly one-half of all cases. A majority
(56%) had no observable transitions, and 47% were estimated
to have derived from non-crossover homologues. Therefore,
complete absence of recombination accounts for a signi®cant
proportion of maternal sex chromosome trisomy. In a small
proportion of oocytes, X chromosomes lacking synaptonemal
complexes have been observed immunocytologically (Tease
et al., 2002), and these may serve as a source of achiasmate
homologues. This class of non-disjunction was independent of
maternal age.
MI `transitional'
The remaining MI cases had apparently normal recombination,
with no change in map length and no signi®cant deviation from
the normal pattern of recombination. However, this class was
311
N.S.Thomas and T.J.Hassold
Figure 1. Comparison of X chromosome recombinational maps based on normal and abnormal meioses. The `standard' map estimates the amount of
recombination occurring in a normal female meiotic event; it was constructed using conventional linkage analysis to examine recombination in a wellestablished registry of human pedigrees (i.e. the CEPH families). This map can then be compared with maps generated from female meioses in which the X
chromosome non-disjoined, yielding XXX or XXY offspring; these `non-disjunctional' maps were generated using tetrad analysis to examine genetic markers
in trisomic individuals and their parents, as described in the text. In this ®gure, three different non-disjunctional maps have been displayed, corresponding to
three different categories of maternal non-disjunctional errors. For the pooled meiosis I (MI) cases, there was a signi®cant reduction in recombination by
comparison with normals, while for `transitional' MI cases and meiosis II (MII) cases there was no evidence for a difference from the standard map. Map
distances are expressed in centiMorgans. Reproduced from Thomas et al. (2001) with permission from Oxford University Press.
associated with a signi®cant increase in maternal age compared
with both the controls and the MI nullitransitional class.
Meiosis II
The third mechanism of X chromosome non-disjunction occurs
during MII. For this class, recombination was normal and thus
appeared independent of events at MI. There was also no
association with increased maternal age, distinguishing maternal MII non-disjunction of the X chromosome from autosomal
trisomies.
As recombination occurs at MI, it would not be expected that
altered recombination would be a factor in MII non-disjunction. However, for maternal trisomy 21, one group (Lamb et al.,
1997) observed an increase in recombination, especially near
the centromere, in maternal MII-derived cases compared with
controls. These authors hypothesized that MI and MII are
linked and that mostÐif not allÐcases of human female nondisjunction originate at MI. However, the normal recombination patterns associated with maternal MII errors in the present
authors' study of the sex chromosomes (Thomas et al., 2001)
and of chromosome 18 (Bugge et al., 1998) suggested that the
effect might be speci®c to chromosome 21.
312
Unassigned meiotic errors
The fourth mechanism of maternal X chromosome nondisjunction occurs meiotically, but because of recombination
at or very close to the centromere, it cannot be classi®ed as MI
or MII. This class of error was rare but, given the exceptionally
low rate of recombination around the human X centromere
(Mahtani and Willard, 1998), the 10 cases identi®ed from 140
was surprising. The distribution of transitions in the unassigned
cases was also unusual with increased proximal recombination,
in addition to recombination at the centromere, and the
presence of double recombinations within relatively short
distances. Thus, this appears to be a novel type of human nondisjunction. There was no association with increasing maternal
age.
A similar mechanism has not been reported for autosomal
trisomies, but would be dif®cult to detect in small or
acrocentric chromosomes. Nevertheless, altered recombination
around the centromere is an important factor in some cases of
human non-disjunction. Indeed, the unassigned class of the
present authors' report closely parallels MII errors of trisomy
21 which are associated with increased proximal recombination, and also show an overall excess of recombination
Aberrant recombination and Klinefelter syndrome
(Sherman et al., 1994; Lamb et al., 1997). Proximal chiasmata
may predispose to `chromosome entanglement', preventing the
bivalent from separating (Lamb et al., 1997), or the resolution
of proximal chiasmata may result in premature chromosome
separation (Koehler et al., 1996). Alternatively, the effect of
recombination on the X chromosome may be unique because
the X centromere is different from nearly all autosomal
centromeres, being composed of both alpha and gamma
satellite DNA (Lee et al., 1997).
Post-zygotic mitotic (PZM)
The ®fth mechanism of maternal X chromosome nondisjunction results from a mitotic error in an early cell division
of the conceptus. This class of maternal error appears to be
more common for the X chromosome than for autosomal
trisomies (16% versus to 0±4%). Perhaps surprisingly, the
PZM class was associated with a signi®cantly elevated
maternal age. Possibly, this indicates that a proportion of
these cases are actually MII errors following a nullitransitional
MI.
Thus, altogether at least ®ve different mechanisms are
associated with maternal sex chromosome non-disjunction.
Absent recombination is an important factor but, except for the
10 `unassigned' cases, there was no other association with
aberrant recombination. Only two of the ®ve mechanisms were
associated with increased maternal age including, surprisingly,
the PZM cases. The `unassigned' class appears to be a novel
type of non-disjunction and in general, the mechanisms by
which errors of MI and MII occur are distinct from those
causing autosomal trisomies.
The effect of recombination on paternally derived XXY
In the human female, both X chromosomes are equivalent in size
and genetic content; thus, pairing and recombination can occur
along the entire length of the chromosome and, on average, three
or four crossovers link the homologues (Tease et al., 2002). In
human males, however, only small regions of homology exist
between the sex chromosomes, and thus crossing-over is restricted
to a relatively small interval. Speci®cally, the identity between the
X and Y chromosomes is limited to two regions of 2.5 Mb and 0.33
Mb located at the distal (i.e. furthest from the centromere) tips of
the short (p) and long (q) arms of the X and Y chromosomes
respectively. As these regions show autosomal patterns of
inheritance and undergo recombination with sexual phenotype,
they have been described as `pseudoautosomal' (Simmler et al.,
1985). The Xp/Yp region is referred to as the pseudoautosomal
region (PAR) 1, and the smaller Xq/Yq region is referred to as
PAR 2.
Although crossovers may occur in either PAR 1 or PAR 2, the
nature and extent of recombination is very different between the
two regions. During male meiosis a single crossover normally
occurs within PAR 1 to ensure proper disjunction (Burgoyne,
1982); indeed, this is frequently referred to as an `obligatory
chiasma'. In contrast, electron microscopy has demonstrated that
exchanges within PAR 2 are rare (Chandley et al., 1984) and,
consistent with this, molecular studies have identi®ed PAR 2
recombination in only about 2% of meioses (Freije et al., 1992; Li
et al., 1995). Further, PAR 2 recombination is thought neither
necessary nor suf®cient for normal disjunction to occur (Kvaloy
et al., 1994; Li et al., 1995).
As in earlier studies of maternally derived sex chromosome
trisomies, two studies have been undertaken to determine whether
there is an association between aberrant recombination and XY
chromosome non-disjunction. In these studies, attention was
focused on PAR1, comparing recombination rates in male meioses
resulting in 47,XXY with those resulting in 46,XX or 46,XY
offspring.
Initially, 41 paternally derived 47,XXYs and their parents were
tested, using six polymorphic loci spanning PAR 1 (Hassold et al.,
1991). For each locus where the father was heterozygous, the
question was asked whether the proband had retained heterozygosity (N) or had been reduced to homozygosity (R); as for
maternal sex chromosome trisomies, a transition from R to N
between markers indicates an exchange. All 41 individuals were
scored as N at the centromere as they had inherited both their
father's X and Y chromosomes. The PAR 1 telomeric markers
were then examined, with the assumption being made thatÐif
recombination were normalÐapproximately one-half of these
individuals would be R in this region; that is, individual crossovers
involve only two of the four chromatids and thus an exchange
would be recovered in only one-half of all XXY individuals. In
fact, it was found that only six cases were R for the telomeric
markers DXYS14 or DXYS20 loci, indicating a highly signi®cant
reduction in crossing-over. Consistent with this, another group
(Lorda-Sanchez et al., 1992) observed no PAR 1 recombinants in
their analysis of 10 paternal XXYs.
More recently, the ®nding of reduced recombination was
con®rmed in an additional 35 paternally derived cases, as
recombination was observed in only ®ve subjects instead of the
expected 17.5 (Thomas et al., 2000). By combining these data with
those of others (Hassold et al., 1991), it was estimated that some
67.1% of the paternally derived XXYs were achiasmate. Thus, the
vast majority of all paternal cases of Klinefelter syndrome derive
from meioses in which the X and Y chromosomes did not
recombine with one another. Importantly, no obvious effect of
paternal age on this process could be discerned.
The importance of reduced recombination in XY non-disjunction has also been documented in a direct analysis of human sperm.
By applying genetic linkage methodology to analyse a single
spermatozoon, Shi et al. (2001) identi®ed a highly signi®cant
decrease in PAR 1 recombination for 24,XY disomic sperm
compared with that observed in normal sperm. Thus, data from the
genetic studies of products of abnormal meioses and from the
direct analysis of human sperm suggest there are at least two
distinct mechanisms underlying paternal non-disjunction: MI
`nullitransitional' and MI `transitional'.
MI `nullitransitional'
This class of non-disjunctional error is associated with a
complete absence of recombination between the X and Y
chromosomes in PAR 1, and appears to be the most common
source of XY non-disjunction. The methodology does not
allow any differentiation to be made between two possible
sources of this error, namely achiasmate non-disjunction
313
N.S.Thomas and T.J.Hassold
resulting from failure of homologous pairing (i.e. `nonconjunction') or non-disjunction resulting from absent recombination despite normal pairing. It was hypothesised (Hale,
1994) that, for mice, synapsis could occur without recombination but that both X-Y pairing and recombination were required
for normal progression of meiosis and completion of
spermatogenesis. A complete absence of pairing in human
males causes infertility associated with arrested germ cell
development (Gabriel-Robez et al., 1990; Speed and Chandley,
1990; Mohandas et al., 1992). However, the effect of pairing
failure in a single chromosome in a small proportion of cells is
unknown, and could be compatible with germ cell development. Indeed, one group (Ashley et al., 1994) suggested that
while most unrecombined sex chromosomes are eliminated at
MI, a small proportion of aneuploid gametes with unrecombined sex chromosomes might proceed to MII and beyond.
Whilst it may be assumed that most cases of XY nondisjunction are sporadic, there are also genetic conditions that
might lead to failure of XY pairing/recombination, and thus
predispose to XY non-disjunction. For example, complete loss
of PAR 1 in males, either through a deletion (Mohandas et al.,
1992) or a translocation (Gabriel-Robez et al., 1990) results in
failure of X-Y pairing. Further, submicroscopic deletions
encompassing the SHOX gene within PAR 1 have recently
been identi®ed as the causative mutation in the majority of
families with Leri-Weill dyschondrosteosis (LWD), a dominantly inherited skeletal dysplasia (Belin et al., 1998; Shears
et al., 1998). Such deletions would reduce the extent of X/Y
homology and could have an adverse effect on pairing or
recombination. Thus, to test whether PAR 1 deletions could
explain some cases of paternal sex chromosome non-disjunction, a series of paternal 47, XXYs was screened for PAR 1
microdeletions using a total of seven microsatellites across the
region. There was no evidence for the presence of any
microdeletions within PAR, though the presence of very small
deletions involving one locus or between loci could not be
excluded.
MI `transitional'
In total, 11 cases (10 single and one double crossover) were
identi®ed in which crossing-over had occurred in the meioses,
leading to XXY progeny. The approximate locations of all 12
crossovers were indicated on a standard genetic map of PAR 1
constructed using the Centre d'Etude du Polymorphisme
Humain (CEPH) version 8.2 map+ program (Collins et al.,
1996) and appeared to be relatively evenly distributed across
PAR 1 (Figure 2).
Role of recombination in human non-disjunction:
comparisons between autosomal and sex chromosome
trisomies
Changes to the normal number and/or distribution of crossovers
have emerged as important factors in the aetiology of human nondisjunction. Since the role of chiasmata is to maintain the physical
connections between homologues, and they occur at the sites of
recombination, it is not surprising that disturbances in recombina-
314
tion are associated with abnormalities in chromosome segregation.
Reduced recombination is associated with MI errors for all
maternal autosomal trisomies studied and for paternal MI errors
causing trisomy 21. Similarly, reduced recombination is important
in both maternal and paternal sex chromosome trisomies.
However, while reduced recombination appears to be a universal
feature of all trisomies, the exact nature of the alterations in
recombination and how these interact with the other strong risk
factor, increased maternal age, appear to be chromosome-speci®c.
For example, depending on the chromosome, the overall reduction
in recombination associated with the MI errors may involve one of
two mechanisms:
1. In a proportion of cases the chromosomes failed to
recombine; that is, the non-disjoining bivalent derived from an
achiasmate meiosis.
2. Among those cases in which an exchange occurred (i.e. MI
transitional errors), there was a lower than expected number of
exchanges.
Both mechanisms have been observed, although their relative
importance varies considerably for different chromosomes. For
example, for maternal trisomy 16, achiasmate errors appear to be
rare or non-existent, with a decrease in the number of exchanges
being the sole recombination-associated mechanism (Hassold
et al., 1995). In contrast, both absent and decreased recombination
have been reported for maternal trisomies 15 (Robinson et al.,
1998), 18 (Fisher et al., 1995; Bugge et al., 1998) and 21 (Sherman
et al., 1994; Lamb et al., 1996). That is, achiasmate errors are an
important source of maternal MI errors for these chromosomes,
but fewer than expected crossovers have been noted among the MI
transitional errors. Finally, sex chromosome trisomies appear to be
unique, since failure to recombine is important in both maternally
and paternally derived cases, yet among the MI transitional cases
there was no evidence for reduced recombination.
Alterations in recombination can also exert a more subtle effect
mediated through changes in the placement of crossovers. For
maternal trisomies 15, 16 and 21, MI transitional errors tend to
have an excess of distal crossovers, while maternal trisomy 21 MII
errors have an excess of more proximal crossovers (Lamb et al.,
1997). However, alterations in the number and placement of
crossovers appear to be related and exert a combined effect on
recombination. Thus, for trisomy 21 a relative increase in the
proportion of distal crossovers is associated with an overall
decrease in the mean level of recombination, and an excess of
proximal crossovers is associated with an overall increase in
recombination. However, in sex chromosome non-disjunction
there is no evidence for these effects. Among non-disjoined
chromosomes, when X/X or X/Y recombination does occur, both
the number and distribution of exchanges are indistinguishable
from normal meiosis. Only among the rare unassigned meiotic
errors were any changes observed in the position and frequency of
crossovers.
Interaction between recombination, non-disjunction and parental
age
The identi®cation of aberrant recombination as an important
aetiological factor in human non-disjunction begs an obvious
question; namely, how does aberrant recombination interact
with the only other known predisposing factor to trisomy,
Aberrant recombination and Klinefelter syndrome
Figure 2. Approximate locations of pseudoautosomal region crossovers in the 12 paternally derived XXY cases in which exchanges were identi®ed.
namely, increasing maternal age? Perhaps not surprisingly, it
transpires that the relationship is complex, and appears to be
chromosome-speci®c. For trisomies 16 and 21, exchange
distributions do not appear to be affected by maternal age, and
the genetic maps are very similar in younger and older women
(Hassold and Hunt, 2001). However, for trisomy 15 (Robinson
et al., 1998), sex chromosome trisomies (Thomas et al., 2001)
and possibly trisomy 18 (Bugge et al., 1998; Thomas et al.,
2001), there is a link between maternal age and the number of
exchanges observed in MI errors. For these trisomies, direct
comparisons between the two classes of MI error show that MI
`nullitransitionals' are more common in younger than older
women. For example, for maternal sex chromosome trisomies,
68% of MI errors involving women aged under 30 years are
nullitransitional, but the ®gure is reduced to 21% for mothers
aged over 40 years (Thomas et al., 2001).
Thus, there appear to be two types of susceptible meiotic
con®guration: achiasmate tetrads without any exchanges
(`nullitransitionals'); and chiasmate tetrads (`transitionals')
with an abnormal distribution of exchanges. An absence of
chiasmata may be incompatible with normal chromosome
segregation, particularly for larger chromosomes like X that
should have several exchanges per tetrad, resulting in nondisjunction irrespective of maternal age. In contrast, the
chiasmate class appears to be age-dependent, though the effect
varies considerably among chromosomes.
The diverse effects of maternal age can be encompassed by
the `two-hit' model of non-disjunction proposed earlier (Lamb
et al., 1997). In this model, the ®rst hit is assumed to be the
establishment of a susceptible chiasmate con®guration in the
fetal oocyte, with a second hit acting on the periovulatory
oocyte to decrease successful segregation of susceptible
chromosomes. With increasing maternal age, the ability to
process tetrads with `at-risk' exchange con®gurations may
decrease, and so these con®gurations will be more likely to
non-disjoin in older mothers. What constitutes an `at-risk'
con®guration (i.e. the number and/or position of chiasmata) on
which the maternal age-dependent second hit will act, is
chromosome-speci®c. Thus, the effect of maternal age will be
related to the normal distribution of chiasmata for a particular
chromosome, or to the presence of chromosome-speci®c
factors which ensure normal segregation (Robinson et al.,
1998). Another hypothesis (Wolstenholme and Angell, 2000)
proposes that non-disjunction is attributable to premature
separation of sister chromatids at MI, and that increasing
maternal age does not directly cause malsegregation but acts to
reduce the cohesion within bivalents of older mothers. The
stability of the bivalent and its ability to undergo a reductional
division is then determined by the number and location of
chiasmata.
While the association of non-disjunction and increased
maternal age is well established, the relationshipÐif anyÐ
between increasing paternal age and trisomy is unclear, with
con¯icting results produced by epidemiological and molecular
studies (Hassold et al., 1996). Testing for a paternal age effect
is complicated by the vast excess of maternally derived
trisomies, and although molecular studies can speci®cally
identify paternal cases, with the exception of sex chromosome
trisomies, the numbers are very small. Previously, it was
reported (Lorda-Sanchez et al., 1992) that there was a
signi®cant increase in the paternal age of paternally derived
47,XXY cases compared with maternally derived cases. In
contrast, in the present authors' studies there was no evidence
for any increase in paternal age among paternal 47,XXYs
(Macdonald et al., 1994; Thomas et al., 2000).
Conclusions
Despite the extraordinary frequency and clinical importance of
human trisomy, there remains a distinct lack of understanding as to
how and why human meiosis goes wrong. Furthermore, analyses
of the origin of sex chromosome trisomies indicate that when the
answers are eventually obtained, they will not necessarily be
simple. A variety of non-disjunctional mechanisms involving the
sex chromosomes has already been identi®edÐsome of these are
associated with aberrant recombination and others not, while some
are maternal age-dependent and others are apparently independent
of increasing maternal age. While some of these features are
shared with non-disjunction of autosomes, others are unique to the
X and Y chromosomes. Indeed, the speci®c combination of risk
factors associated with non-disjunction appear to be different for
315
N.S.Thomas and T.J.Hassold
virtually every chromosome, and it is becoming increasingly
obvious that no single chromosome will serve as a paradigm for
human meiotic non-disjunction. As observations on non-disjunction are currently available for only a small number of autosomal
chromosomes, there is clearly much work to be done before an
understanding of the basis of human aneuploidy is obtained. This
level of ignorance notwithstanding, the present observations may
have certain practical implications for genetic counselling involving sex chromosome aneuploidies. That is, the variety of nondisjunctional mechanisms observed have provided little evidence
for simple, repetitive causes of sex chromosome trisomy. Thus,
consistent with other previous analyses, it appears unlikely that the
occurrence of a sex chromosome trisomy imparts a greatly
increased risk for a second such event.
Acknowledgements
Studies conducted in the Hassold and Thomas laboratories and
discussed in this review were supported by NIH grant HD21341 (to
T.J.H.) and funding from the Wellcome Trust (for N.S.T.). The
authors thank Prof. Patricia A.Jacobs for her support of this research
programme, and for her critical appraisal of this manuscript.
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