Investigation of the origins of human autosomal inversions.

A significant proportion of both pericentric and paracentric inversions have recurrent breakpoints and so could either have arisen through multiple independent events or be identical by descent (IBD) with a single common ancestor. Of two common variant inversions previously studied, the inv(2)(p11q13) was genuinely recurrent while the inv(10)(p11.2q21.2) was IBD in all cases tested. Excluding these two variants we have ascertained 257 autosomal inversion probands at the Wessex Regional Genetics Laboratory. There were 104 apparently recurrent inversions, representing 35 different breakpoint combinations and we speculated that at least some of these had arisen on more than one occasion. However, haplotype analysis identified no recurrent cases among eight inversions tested, including the variant inv(5)(p13q13). The cases not IBD were shown to have different breakpoints at the molecular cytogenetic level. No crossing over was detected within any of the inversions and the founder haplotypes extended for variable distances beyond the inversion breakpoints. Defining breakpoint intervals by FISH mapping identified no obvious predisposing elements in the DNA sequence. In summary the vast majority of human inversions arise as unique events. Even apparently recurrent inversions, with the exception of the inv(2)(p12q13), are likely to be either derived from a common ancestor or to have subtly different breakpoints. Presumably the lack of selection against most inversions allows them to accumulate and disperse amongst different populations over time.

The origin of trisomy 13.

Trisomy 13 is one of the most common trisomies in clinically recognized pregnancies and one of the few trisomies identified in liveborns, yet relatively little is known about the errors that lead to trisomy 13. Accordingly, we initiated studies to investigate the origin of the extra chromosome in 78 cases of trisomy 13. Our results indicate that the majority of cases (>91%) are maternal in origin and, similar to other autosomal trisomies, the extra chromosome is typically due to errors in meiosis I. Surprisingly, however, a large number of errors also occur during maternal meiosis II ( approximately 37%), distinguishing trisomy 13 from other acrocentric and most nonacrocentric chromosomes. As with other trisomies, failure to recombine is an important contributor to nondisjunction of chromosome 13.

Distribution of the D15Z1 copy number polymorphism.

Using fluorescent in situ hybridization (FISH) with the probe p15 (D15Z1), we investigated the distribution of the polymorphic 15p signal which has been reported to occur on acrocentric chromosomes in addition to chromosome 15. The short arm of chromosome 15 has a characteristic signal pattern when hybridized with the FISH probe D15Z1. However, the D15Z1 signal can occasionally be seen on the short arm of other acrocentric chromosomes. We studied the distribution of the D15Z1 probe in 1657 patients consisting both of individuals with a normal karyotype and those with a variety of chromosome abnormalities involving the acrocentric chromosomes. Our results show that one in six individuals, regardless of their patient ascertainment category or karyotypic status, had one or more additional D15Z1 signals, and that there were no significant differences in the distribution of extra signals among the patient groups.

Mosaic trisomy 6 and maternal uniparental disomy 6 in a 23-week gestation fetus with atrioventricular septal defect.

Trisomy 6 is seen in early miscarriages in association with an intact, empty amniotic sac or as a pseudomosaic in amniotic fluid cultures. We report the finding of mosaic trisomy 6 in a 23-week-gestation pregnancy terminated because of intrauterine death. The post-mortem showed a well formed macerated male fetus with an atrioventricular septal defect and an exomphalos. By conventional cytogenetics, trisomy 6 was found in 12 out of 25 (48%) fibroblast colonies from fetal skin and 21 out of 32 (66%) colonies derived from amnion, while the remaining metaphases showed an apparently normal male karyotype. Molecular genetic studies on DNA from uncultured fetal skin and cord samples using polymorphic microsatellite repeat sequences showed no evidence of trisomy 6, but demonstrated that both chromosome 6 homologs were of maternal origin consistent with maternal uniparental disomy (UPD).

Pallister-Killian syndrome: an unusual cause of epileptic spasms.

Pallister-Killian syndrome (PKS) is a rare, sporadic, genetic disorder characterized by dysmorphic features, learning disability, and epilepsy. It is caused by a mosaic supernumerary isochromosome 12p (i[12p]). The i(12p) is rarely found in peripheral blood but it is present in skin fibroblasts. Recognition is essential for cytogenetic diagnosis. We describe a male aged 2 years 6 months and a female aged 11 years with PKS and epileptic spasms (ES). This type of seizure is not unusual in patients with brain malformations and with severe developmental delay, but it is sometimes difficult to recognize without video-electroencephalogram studies and could be mistaken for other types of seizure or behavioural manifestations. In these two patients with PKS, spasms had late onset, persisted beyond infancy, and were drug resistant. Clinicians should be aware of this possibility in PKS, which appears to be a rare cause of ES.

A study of cryptic terminal chromosome rearrangements in recurrent miscarriage couples detects unsuspected acrocentric pericentromeric abnormalities.

Fifty chromosomally normal couples with three or more miscarriages were examined using fluorescent in situ hybridisation (FISH) and a library of subtelomere-specific probes together with alphoid repeats mapping to the acrocentric centromeres. Six abnormalities were found. Firstly, a cryptic reciprocal subtelomere translocation between the long arm of a chromosome 3 and the short arm of a chromosome 10. The other five cryptic abnormalities involved the acrocentric chromosome pericentromeric regions and in one case also Yp. Two patients had a rearranged chromosome 13, where the centromeric region was found to be derived from the short arm, centromere and proximal long arm of chromosome 15. Another two patients had a derived chromosome 22, where the centromere was replaced by two other centromeres, one derived from chromosome 14 and the other from either chromosome 13 or 21, while one patient had the subtelomere region of Yp translocated onto the short arm of a chromosome 21. These abnormalities may be the underlying cause of the recurrent miscarriages, because they may result in abnormal pairing configurations at meiosis leading to non-disjunction of whole chromosomes at metaphase I. The frequency of rearrangements seen in the recurrent miscarriage patient population was significantly different from that in the control group ( P=0.0096, Fisher's exact test) due to the acrocentric pericentromeric abnormalities.