© 2001 Oxford University Press
Human Molecular Genetics, 2001, Vol. 10, No. 3
243–250
Maternal sex chromosome non-disjunction:
evidence for X chromosome-specific risk factors
N. Simon Thomas1,+, Sarah Ennis2, Andrew J. Sharp1, Miranda Durkie1, Terry J. Hassold3,
Andrew R. Collins2 and Patricia A. Jacobs1,2
1Wessex
Regional Genetics Laboratory, Salisbury District Hospital, Salisbury, Wiltshire SP2 8BJ, UK,
Genetics, Duthie Building, Southampton General Hospital, Southampton SO16 6YD, UK and
3Department of Genetics, Case Western Reserve University, BRB 724 10900 Euclid Avenue,
Cleveland, OH, USA
2Human
Received 17 October 2000; Revised and Accepted 23 November 2000
Human trisomy is attributable to many different
mechanisms and the relative importance of each
mechanism is highly chromosome specific. The
association between altered recombination and
maternal non-disjunction is well documented:
reductions in recombination have been reported for
maternal meiosis I (MI) errors involving chromosomes 15, 16, 18 and 21 and increased recombination
has been reported for meiosis II (MII) errors
involving chromosome 21. We therefore investigated
maternal X chromosome non-disjunction, to determine
whether the effects of recombination are unique to
the X chromosome or similar to any of the autosomes
thus far studied. We genotyped 45 47,XXX females
and 95 47,XXY males of maternal origin. Our results
demonstrate that 49% arose during MI, 29% during
MII and 16% were postzygotic events; a further 7%
were meiotic but could not be assigned as either MI
or MII because of recombination at the centromere.
Among the MI cases, a majority (56%) had no
detectable transitions and so absent recombination
is an important factor for X chromosome nondisjunction. However, similar to trisomy 15 and
unlike trisomy 21, we observed a significant increase
in the mean maternal age of transitional MI errors
compared with nullitransitional cases. In our studies
of MII errors, recombination appeared normal and
there was no obvious effect of maternal age,
distinguishing our results from MII non-disjunction
of chromosomes 18 or 21. Thus, surprisingly, the risk
factors associated with both MI and MII non-disjunction
appear to be different for virtually every chromosome
that has been adequately studied.
INTRODUCTION
Trisomy is the most common chromosomal abnormality in
humans and affects 4% of all clinically recognized human
+To
pregnancies (1). The rate of non-disjunction varies between
different chromosomes and the factors underlying nondisjunction appear to be highly chromosome specific. Most
trisomies arise from an error during maternal meiosis (MI).
However, there are exceptions and among 47,XXY males the
origin is divided almost equally between maternal and paternal
non-disjunction (2,3). Among autosomes, paternal nondisjunction accounts for a significant number of cases of
trisomies 2 and 8 (4) and, for trisomy 18, maternal meiosis II
(MII) errors predominate (5).
The mechanisms underlying non-disjunction are poorly
understood, although both increased maternal age and altered
recombination are strong aetiological factors. Reduction in
recombination has been reported for all autosomal maternal
trisomies. For MI errors involving chromosomes 18 (5) and 21
(6,7) this is caused both by absent recombination (achiasmate
tetrads) and diminished recombination (a reduced number of
exchanges per tetrad). However, for chromosome 16 only
diminished recombination is important (8). The effects of
maternal age on recombination in MI and MII errors are less
clear and may be chromosome specific. For example, in
maternal MI errors, there is a direct correlation between
exchange frequency and maternal age for trisomy 15 (9), but
no such effect has been reported for other autosomal trisomies;
in maternal MII errors, increases in maternal age are associated
with both trisomies 18 and 21, but recombination is normal for
trisomy 18 and increased for trisomy 21.
For maternal sex chromosome trisomies, Macdonald et al.
(10) reported that both absent and altered recombination were
important in non-disjunction. However, the X centromere
marker used in this study, DXZ1, is extremely difficult to interpret
reliably (3) and retyping of 94 cases revealed that many actual
MII and postzygotic mitotic event (PZM) errors had mistakenly
been interpreted as MI. In total, the centromere status had been
misassigned in >30% of cases and thus some of the conclusions of
McDonald et al. (10) are wrong. We have now determined the
stage of origin and the extent and location of recombination in
these 94 cases and an additional 46 trisomies. From these data
we have determined that the effects of recombination and
maternal age on the X chromosome show both similarities and
differences to the autosomes thus far studied.
whom correspondence should be addressed. Tel: +44 1722 429080; Fax: +44 1722 338095; Email: [email protected]
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Human Molecular Genetics, 2001, Vol. 10, No. 3
RESULTS
Table 1. Loci/megaloci used and their position on the standard map
Nomenclature
Locus
Standard map location Locus
Standard map location
The terminology used in this paper is similar to that of
Robinson et al. (9). Briefly, a chiasma, also called an
exchange, occurs between pairs of chromatids during the
4 strand stage. A transition refers to an observed change in
marker state from non-reduced (N) to reduced (R), or vice
versa, in two non-disjoined strands. Therefore, non-disjoined
strands in which no transitions are observed are referred to as
nullitransitional, instead of achiasmate, because this term
should be applied only to the absence of chiasma in a tetrad. A
crossover refers to an observable change in the marker state in
a single chromosome.
It should be noted that not all of the exchanges which have
occurred in a meiotic tetrad will subsequently be observed as
crossovers or transitions. 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 which is not represented in the two
non-disjoined strands recovered.
SHOXa
0
Origin of additional X chromosome
Utilizing 54 loci, which were grouped into 40 megaloci (Table 1)
covering the entire X chromosome, we were able to subdivide
all 140 cases into groups according to the origin of trisomy. A
total of 68 cases (49%) were consistent with an error at MI, of
which 38 showed no transitions and 30 showed at least one
transition. There were 40 cases (29%) with MII errors, by
definition having at least one transition. A total of 22 cases did
not exhibit any transitions and were reduced at all informative
loci: these were either the result of a PZM or an MII error
following a nullitransitional but disjoined MI. In the remaining
10 cases we observed a change in marker state (see Materials
and Methods), from N to R or vice versa, between the proximal
loci on one chromosome arm and the 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
MII-derived. A summary of the distribution of the origins of
aneuploidy throughout the sample is shown in Table 2.
Non-disjunction mapping
Using the map+ program (11) the standard map length of the X
chromosome was calculated to be 190.13 cM. Non-disjunction
analysis was also performed using the map+ program (Table 3).
When all MI cases were considered together, the calculated
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
significantly different from the standard map length. We also
compared the non-disjunction linkage maps with the standard
map to assess the possible role of aberrant recombination (Fig. 1).
Although there is a small increase in recombination in the p arm,
the MI transitional class non-disjunction map did not deviate
significantly from the standard curve. This suggests that aberrant
recombination is not involved in transitional MI non-disjunction.
DXS8380
88.69
DXYS232X b 2.45
DXS8092
93.83
DXS2497
3.52
DXS6793
94.07
DXS1060
8.54
DXS6803
94.87
DXS996
9.24
DXS990
102.12
DXS8378
12.85
DXS6809
103.41
DXS207
23.55
DXS6789
104.91
DXS999
25.83
DXS8063
111.79
DXS1226
33.08
DXS6797
114.68
DXS451
35.7
DXS6804
118.39
DXS989
38.74
DXS1220
123.5
DMD
41.48
DXS1001
132.95
DXS538
51.93
DXS8057
137.21
DXS1068
53.21
DXS1047
141.77
DXS993
62.7
DXS984
154.02
DXS8379c
63.37
DXS8043
167.75
DXS8083
72.4
DXS6806
169.92
DXS1199d
84.82
DXS998
174.41
DXS8032e
84.82
DXS1073g
189.13
DXS7132f
85.25
SKK h
190.13
aSHOX
= DXYS233, SHOX, DXS201, DXS6814, DXS234.
= DXYS228, DXYS232X.
cDXS8379 = DXS8379, MAOB.
dDXS1199 = DXS1199, DXS1204.
eDXS8032 = DXS8032, AI7, DXS991.
fDXS7132 = DXS7132, HUMAR, DXS1213.
gDXS1073 = DXS1073, DXS1108, DXS1107.
hSKK = SKK–1, DXYS225.
bDXYS232X
For the MII class there was also a small increase in p arm
recombination, but overall no significant deviation in recombination pattern and no significant change in total map length.
Non-disjunction maps were also prepared by incorporating
the unassigned meiotic group into each of the three
groups: MI-all, MI transitional and MII. The effect of the 10
unassigned cases was to increase the level of recombination
around the centromere but with little effect on recombination
in the chromosome arms (data not shown).
Transition and chiasma frequency estimates
The distribution of transitions observed for each non-disjunction
class is shown in Table 4. We applied the EXCHANGE
program (5) to these data to estimate the chiasma frequency in
tetrads and the corresponding map length. There are small
differences in the map lengths generated by the EXCHANGE
and map+ programs because the EXCHANGE program does
not incorporate error filtration or chiasma interference. The
length of the standard map, constructed using the map+
program, was 190 cM, corresponding to a mean of 3.8 chiasmata
per tetrad. For the MI group, the EXCHANGE program
Human Molecular Genetics, 2001, Vol. 10, No. 3
245
estimated that 47% of tetrads have no chiasmata, χ21 = 25.95,
when tested against the null hypothesis that the frequency of
achiasmate tetrads (q0) is zero (Table 5). Therefore, of the 38
observed nullitransitionals, ∼32 are derived from achiasmate
tetrads. The chiasma distribution estimated for all the MI cases
gives a mean of 1.5 chiasmata per tetrad (∼75 cM map length).
The same analysis carried out on the MI transitional and MII
classes gave mean chiasma frequencies of 3.2 per tetrad (∼160 cM
map length) and 3.4 per tetrad (∼170 cM map length), respectively.
P = 0.0001). A direct comparison between the two MI classes
showed the MI nullitransitionals to have a significantly
younger mean maternal age (P = 0.0163).
Correlation analysis was then performed to investigate
whether this advanced maternal age effect was related to the
observed number of transitions (Table 4). This was not significant
for the MI transitional class (P = 0.7525) and therefore the
number of transitions is independent of any maternal age
effect. In a study on the origin of trisomy 15, Robinson et al.
(9) found the mean maternal age of the three or four transition
class was significantly higher compared with the one and two
transition class considered together. When tested in our cohort
of X chromosome aneuploidies, this result was also non-significant.
To test whether significant differences in mean maternal age
between MI transitionals and MI nullitransitionals is a feature
of other non-disjoined chromosomes, we re-analysed our
sample of trisomy 18 data (5). Much of this sample was ascertained through advanced maternal age and so could not be
compared with a normal mean maternal age. However, withinsample comparisons are valid. The mean maternal age for the
MI nullitransitionals is 32.94 (n = 16, SE = 1.73) and for the
MI transitionals is 36.48 (n = 25, SE = 1.31). Although the
difference between these groups was not statistically significant
(two-tailed P = 0.1059), a larger sample of these data may
corroborate the results found in this study.
Neither the MII nor the unassigned classes had significantly
different mean maternal ages compared with the controls.
However, a statistically significant difference was found
between the PZM mean maternal age of 31.65 years and the
control mean maternal age (two-tailed P = 0.04). A maternal
age effect would not be expected for a postzygotic event and so
we tested the ascertainment of the PZM class compared with
the others. All data were divided on the basis of ascertainment
into postnatal and prenatal groups. Comparison of the PZMs
with the remainder of the cohort showed that the PZMs were
not enriched for either group (χ21 = 0.09).
Parental age
Assessment of PZM cases
Parental ages were available for the majority of the 140 cases
and we have calculated a mean maternal age for each class of
non-disjunction (Table 6). The families in this study were
recruited via multiple centres worldwide, over a 12 year period
and with a variety of prenatal and postnatal referral reasons.
The dates of birth and prenatal sampling ranged from 1948 to
1999. During this time there has been a marked increase in the
overall mean maternal age. Therefore, it is not possible to use
an appropriate normal control for maternal age. Instead we
have calculated a control mean maternal age (28.47 years)
from a cohort of mothers of paternally derived sex chromosome
trisomies, which are unlikely to be associated with an age
effect (12,13).
The t-test statistic was used to ascertain whether any of the
groups had a significantly different mean maternal age
compared with the control maternal age. The mean maternal
age for the group containing all MIs was found to differ
significantly from that of the control group (two-tailed P = 0.0003).
When the MI group was split into transitionals and nullitransitionals,
the nullitransitionals’ mean maternal age was non-significant
compared with controls. However, the difference between MI
transitionals and normals was extremely significant (two-tailed
For genuine PZM cases there should be an equal probability of
the duplicated chromosome being maternal or paternal. Therefore, we compared the incidence of maternal and paternal PZM
cases among our cohort of 47,XXX females. Seven of the
45 maternal 47,XXX cases were apparently mitotic. From the
same project we had ascertained seven 47,XXX cases of
paternal origin (10) (N.S. Thomas, unpublished data). Five
were informative for the most distal PAR1 or PAR2 markers
with three cases resulting from MII errors and only two from a
PZM error. Thus there is a slight excess of maternal cases.
Although this is not statistically significant, it suggests that
a proportion of maternal PZM errors may arise from a
nullitransitional MI that does not disjoin at MII. However, as
the nullitransitional MI class is not associated with an
increased maternal age, contamination with a proportion of
nullitransitional cases would not explain the increased
maternal age of the PZM class.
From the observed transmission data for the MII class, the
EXCHANGE program estimated that the frequency of achiasmate
tetrads among MII errors was zero (Table 5). However, by
definition all MII errors had at least one observed transition.
Therefore, we used the SEGRAN program (14) to determine
Table 2. Distribution of all aneuploidies
Origin
n
Percentage of total
MI, nullitransitional
38
27.1
MI, transitional
30
21.4
MII
40
28.6
PZM
22
15.7
Unassigned meiotic errors
Total
10
140
7.1
100
Table 3. Map length estimates from map+ program
Map
Length (cM)
Standard
190.13
6.09
85.57
11.43
MI, all
SE
MI, transitionals
186.25
24.06
MII
204.07
16.80
Unassigned meiotic errors
213.94
50.16
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Human Molecular Genetics, 2001, Vol. 10, No. 3
Figure 1. Comparison of the standard and non-disjunction genetic maps. The non-disjunction maps are plotted against the standard map on the x-axis with the p telomere
at the origin. All genetic distances are given in centiMorgans. The standard map itself appears as a straight line through the origin.
whether any of the apparent PZM cases were due to an MII
error following a nullitransitional MI. We used a hypergeometric
distribution (14) to calculate the probable number of zero
transitions that occur given the actual distribution of the
number of transitions within the MII class. These raw data are
3, 15, 17, 3 and 2, respectively, for the number of transitions 1–5.
The hypergeometric distribution was used because it had two
advantages over the Poisson distribution, producing a better fit
for these data and incorporating interference modelling. The
program estimates a value for Π which is the probability of
there being >0 transitions given the complete distribution.
Given an estimate of Π = 0.9921, the number of cases with zero
transitions (x) was calculated, by the formula x = [(1 – Π)N]/Π, to
be 0.3185 of the 22 apparent PZM cases. As this value was
close to zero and, in agreement with the estimate of the
EXCHANGE program, we were content that these cases had
been accurately assigned and that the actual PZM distribution
did not need correcting with this estimate.
DISCUSSION
Analysis of 140 families has shown that maternal sex chromosome trisomy can arise by several different mechanisms. Some
of these mechanisms have parallels with autosomal trisomies
whereas others are unique to the X chromosome.
MI nullitransitional
Consistent with most maternal trisomies, MI errors were the
single biggest class for the X chromosome, accounting for
nearly half of all cases. A majority, 56%, had no observable
transitions and 47% were estimated to have derived from
achiasmate tetrads. Therefore, complete absence of recombination accounts for a significant proportion of maternal sex
chromosome trisomy. This effect appears to be independent of
maternal age, in which case the absence of chiasmata may be
incompatible with normal chromosome segregation, resulting
in non-disjunction irrespective of maternal age. Absent recombination is also important for maternal trisomy of chromosomes 15 (9), 18 (5) and 21 (6,15) and for paternal sex
chromosome non-disjunction (12). However, for the maternal
trisomies 18 and 21 achiasmate tetrads are associated with
advanced maternal age.
MI transitional
The remaining MI cases have apparently normal recombination
with no change in map length and no significant deviation from
the normal pattern of recombination. Therefore, the overall
decrease in recombination associated with all maternal MI
trisomies can now be explained in three different ways: (i) a
significant proportion of achiasmate tetrads (X chromosome);
Human Molecular Genetics, 2001, Vol. 10, No. 3
247
Table 4. Observed distribution of transitions within groups
Group
No. of transitions
MI, all
0
1
2
3
4
5
0.5588
0.0882
0.2353
0.0882
0.0294
0.0000
Total
Mean
SE
64
0.94
0.1955
MI, transitional
0.0000
0.2000
0.5333
0.2000
0.0666
0.0000
64
2.13
0.1496
MII
0.0000
0.0750
0.3750
0.4250
0.0750
0.0500
106
2.65
0.1457
Unassigned meiotic errors
0.0000
0.1000
0.1000
0.4000
0.2000
0.2000
33
3.30
0.3958
Table 5. Estimated chiasma distribution from EXCHANGE program
Group
MI, all
Frequency of chiasmata
q0
q1
q2
q3
q4
q5
Mean
0.4698
0.0278
0.1577
0.2239
0.1208
0.0000
1.4981
MI, transitional
0.0000
0.0000
0.0959
0.6312
0.2729
0.0000
3.1770
MII
0.0000
0.0000
0.1085
0.5796
0.1183
0.1937
3.3975
Table 6. Effect of maternal age
Maternal age
Versus mean control age
n
Mean
SE
Difference
P value
Control
70
28.47
0.73
–
–
MI, all
63
32.71
0.88
+4.24
0.0003
MI, nullitransitional
33
30.73
1.04
+2.26
Not significant
MI, transitional
30
34.90
1.36
+6.43
0.0001
MII
38
28.29
1.18
–0.18
Not significant
PZM
20
31.65
1.39
+3.18
0.0400
9
28.89
2.37
+0.42
Not significant
Unassigned meiotic errors
The mean paternal age for the entire sample was 32.99 years (n = 114, SE = 0.75).
(ii) a decrease in the mean number of exchanges per tetrad
among the MI transitional class (chromosome 16); and
(iii) both absent and altered recombination (chromosomes 15,
18 and 21).
However, this class was associated with a significant increase
in maternal age compared with both the controls (P = 0.0001) and
the MI nullitransitional class (P = 0.016). This is unlike
maternal trisomies for chromosomes 16 (8), 18 (5) and 21 (6),
which display age-related alterations in recombination.
Maternal MI transitional errors of chromosome 15 are associated
with a 50% decrease in pericentromeric recombination (9),
otherwise the significant difference in mean maternal age
between the transitional and nullitransitional classes closely
parallels the X chromosome. However, although maternal age
was independent of the number of transitions for the X chromosome, for chromosome 15 the maternal age was associated
with the level of recombination.
Although non-significant, our re-analysis of trisomy 18 data
(5) suggested that for MI errors the mean maternal age of the
transition class was also greater than the mean maternal age of
the nullitransitional class. For trisomy 21, Lamb et al. (16)
found no significant difference in the map lengths of young
and old mothers and no difference in the distribution of
exchanges. Data from chromosome 16 suggest that the
maternal age effect on MI errors is linked to the frequency and
location of recombination events (8). Thus, there is no simple
relationship between maternal age and recombination.
However, the diverse effects of maternal age could all be
encompassed by the ‘two-hit’ model proposed by Lamb et al.
(16). It is possible that with increasing maternal age the ability
to process tetrads with certain ‘at risk’ exchange configurations
will decrease and these configurations will be more likely to
non-disjoin in older mothers. What constitutes an ‘at risk’
configuration, i.e. the number and/or position of chiasmata on
which the maternal age-dependent second hit will act, is
chromosome specific. 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-specific
factors which ensure normal segregation (9).
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Human Molecular Genetics, 2001, Vol. 10, No. 3
MII
The third mechanism of X chromosome non-disjunction
occurs during MII. Amongst 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. Increases in maternal age are
associated with both trisomies 18 (5) and 21 (6), but recombination is normal for trisomy 18 and increased for trisomy 21.
Therefore, the effects of maternal age and recombination on
MII errors appear to be highly chromosome specific.
Unassigned meiotic errors
The fourth mechanism of X chromosome non-disjunction
occurs meiotically but, because of recombination at or very
close to the centromere, cannot be classified as MI or MII. This
class of error was rare but, given the exceptionally low rate of
recombination around the human X centromere (17), the
identification of 10 of 140 cases is highly significant. Like MII
errors, they do not appear to be associated with advanced
maternal age, although they have a greater number of mean
transitions than either the MII class (P = 0.0699) or the MI
transition class (P = 0.0016). 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 seems to be a novel type of nondisjunction.
A similar mechanism for autosomal trisomies has not been
reported, but would be difficult to detect in short or acrocentric
chromosomes. Nevertheless, altered recombination around the
centromere is an important factor in non-disjunction. The
unassigned class closely parallels MII errors of trisomy 21
which are associated with increased proximal recombination
and also show an overall excess of recombination (6,16). Proximal
chiasmata may predispose to ‘chromosome entanglement’,
preventing the bivalent from separating (16), or the resolution
of proximal chiasmata may result in premature chromosome
separation (18). 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 α and γ satellite DNA (19).
PZM
The fifth mechanism of X chromosome non-disjunction results
from a mitotic duplication early in the zygote. This class of
maternal error appears more common for the X chromosome
than for autosomal trisomies. Since the zygote is presumably
formed from two normal monosomic gametes and the error
occurs after conception there should be no bias regarding the
parental origin of the additional chromosome and no effect of
advanced maternal age. Surprisingly, however, this class was
associated with a significantly elevated maternal age and there
was a slight excess of maternal cases. It is possible that some
of these cases are actually MII errors following a nullitransitional
MI, as by molecular analysis such cases would also appear to
be reduced along the whole length of the chromosome. This
cannot explain the effect of maternal age because, for the
X chromosome, both maternal MII errors and nullitransitional
MI errors are independent of maternal age. However, for chromosome 21, Lamb et al. (7,16) have demonstrated that certain
chiasmata distributions laid down during MI predispose to agedependent non-disjunction at MII, i.e. events at MI and MII are
not always independent. We can speculate that for the X chromosome, nullitransitional cases that survive MI may be peculiarly
susceptible to some form of maternal age-dependent nondisjunction at MII. If the observed PZM cases can arise by two
different mechanisms, one maternal age-dependent and the
other maternal age-independent, in a larger sample we would
expect the maternal ages of the PZM case to be distributed
bimodally.
For MII/nullitransitional MI errors to occur, achiasmate
tetrads must be able to segregate at MI. Although this may be
able to happen at random, two pieces of evidence from our
X chromosome data suggest that there is no ‘back up’ system
in humans to allow proper segregation of non-recombined
homologous pairs. Firstly, achiasmate MI non-disjunction
occurs frequently and, secondly, the SEGRAN program calculated
that nearly all MII errors were chiasmate. Similarly, for the
autosomes there is no evidence for normal disjunction of achiasmate tetrads (5). The frequency of achiasmate tetrads in
normal meiosis is estimated to be zero or near zero for chromosomes 18 (5) and 21 (16), although there may be a low background rate of achiasmate tetrads for chromosome 15 (9). For
most autosomal trisomies the numbers of maternal and
paternal PZM cases are reported to be roughly equal (7,20–22).
Therefore, there is no compelling evidence to re-classify any of
the apparent PZM cases.
Conclusions
At least five 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 five mechanisms are associated with increased
maternal age, including, surprisingly, the PZM cases. The
unassigned class appears to be a novel type of non-disjunction
and both the MI and MII mechanisms are distinct from autosomal
trisomies. The relationship between maternal age, recombination
and non-disjunction appears to be highly chromosome specific.
Thus the risk factors associated with non-disjunction appear to
be different for virtually every chromosome that has been
adequately studied.
MATERIALS AND METHODS
Study population
We have studied 140 cases of maternal sex chromosome
trisomy, 95 with a 47,XXY constitution and 45 with a 47,XXX
constitution. All karyotypes were apparently non-mosaic. A
total of 51 of the 47,XXY cases and 43 of the 47,XXX cases
have been described previously by McDonald et al. (10). Of
the 44 new 47,XXY cases, 29 were recruited as part of a study
into the effect of the parental origin of an additional sex chromosome on behavioural and cognitive phenotypes, 10 were
diagnostic referrals and the remaining five were identified
during a school survey screening for carriers of the fragile X.
Both new cases of 47,XXX were diagnostic referrals. Overall
Human Molecular Genetics, 2001, Vol. 10, No. 3
the 47,XXY population comprised 80 postnatal referrals and
15 prenatal referrals (11 for advanced maternal age, 2 because
of an elevated serum screen risk, 1 because of maternal anxiety
and 1 intrauterine death). The 47,XXX population comprised
35 postnatal referrals and 10 prenatal referrals (8 for advanced
maternal age, 1 because of maternal anxiety and 1 because of
in vitro fertilization). DNA was extracted from all probands
and their parents from either peripheral blood or cheek cells.
Molecular studies
Each case was tested using a panel of markers spanning the
X chromosome and, where informative, their results were
recorded as either R (where heterozygosity in the mother is
reduced to homozygosity in the proband) or N (where
heteozygosity in the mother is retained in the proband). The
X chromosome contains two pseudoautosomal regions, PAR1
and PAR2, and these are located at the distal tips of the p and q
arms, respectively. Diallelic intercrosses with the Y chromosome in these regions were scored as D (if aaa or bbb) or X (if
aab or abb) (23).
The panel contains 54 microsatellites. Ten markers within
PAR1 and PAR2 have been described previously (12),
otherwise all primer sequences and conditions are available
from the Genome Data Base (http://www.gdb.org ). There are
21 markers on the p arm and 23 on the q arm. Adjacent markers
were grouped into megaloci when recombination was absent
between a set of markers. There were 40 megaloci in the
standard map (Table 1). The average spacing was 4.75 cM
with a maximum gap between loci of 12 cM, with the
exception of one interval of 14 cM and one of 15 cM. The
meiotic stage of non-disjunction was assigned using a number
of pericentromeric markers on the p arm (megalocus e) and on
the q arm (megalocus f). The genetic distance between these
markers is <1 cM (17) and the intervals between the centromere
and megaloci 5 and 6 are 5–6 Mb and <2 Mb, respectively (24)
(Location
Database:
http://cedar.genetics.soton.ac.uk/
public_html/ ).
Non-disjunctional mapping
The standard map for the X chromosome was created using
the map+ program (http://cedar.genetics.soton.ac.uk/public_html/
programs.html ) (11) (for pairwise lods generated from CEPH
version 8.2) which constructs maps with an estimable typing error
frequency (ε) and a mapping parameter (p). The location of SKK1 (25) is not known in relation to DXS1073 and so the distance
between these loci was arbitrarily assumed to be 1 cM. Nondisjunction analysis was also performed using the map+ program
to generate linkage maps for the disjoined chromosomes
according to the origin of aneuploidy.
ACKNOWLEDGEMENTS
The authors are grateful to all the families and the referral
centres who took part in this study and to the staff at the
Wessex Regional Genetics Laboratory. This work was
supported by the Wellcome Trust.
249
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