Unstable telocentric chromosome produced after centric misdivision of a 21q/21q translocated element

Sex-specific chromosome instability in early human development

The predominance of females segregating chromosome aberrations to their offspring has been explained mostly by selection disadvantage of unbalanced products of spermatogenesis. However, analysis of data from the literature supports the idea that somatic cells of early female embryos are similar to female germ cells in that they are prone to malsegregation. The goal of this study was to compare the sex ratio (male to female ratio) of carriers of presumably mitotic-occurring chromosome abnormalities to identify any sex biases. In examining the literature, we found a female prevalence in cases of mosaicism associated with uniparental disomy (UPD) (26 male individuals/conceptions and 45 female individuals/conceptions, sex ratio is 0.58, significantly different from 1.06 in newborn population, P = 0.0292). This predominance was highest at gestational age <16 week (8 male and 22 female conceptuses, sex ratio is 0.36, significantly different from expected figure of 1.28, P = 0.0025), which diminished at later stages of fetal development indicating potential correction of trisomies predominantly in females. There is a threefold prevalence of 46,XX/45,X mosaics over 46,XY/45,X mosaics in prenatally diagnosed cases, which also suggests a gender-specific postzygotic chromosome loss. The male prevalence in Prader-Willi syndrome with maternal UPD of chromosome 15 also can be explained by sex-specific trisomy correction, with predominant loss of a maternal chromosome causing biparental inheritance and therefore, complete correction of trisomy in females (without UPD). Finally, there is a female predominance in carriers of chromosome rearrangement with pericentromere break (mosaicism for Robertsonian translocation/isochromosome, centric fission, nonacrocentric isochromosome, and whole arm rearrangement), in both prenatal (21 males and 36 females, sex ratio is 0.58, P < 0.0184) and postnatal ill-defined cases (14 males and 35 females, sex ratio is 0.40, P = 0.001). Thus, the findings presented in this paper suggest that, in addition to reduction in male fertility, and to probable selection against abnormal cell line(s), there are two mechanisms that contribute to female preponderance among carriers of mosaicism: sex-specific chromosome loss and sex-specific centromere instability. The data obtained suggest that females may have gonadal mosaicism for aneuploidies and structural rearrangements more often than males. This may lead to the maternal origin bias in offspring with trisomies or structural rearrangements.

Moving satellites and unstable chromosome translocations: clinical and cytogenetic implications

We describe 2 families in which acrocentric short arm material moved from one chromosome to another. In case 1, a meiotic event resulted in movement of an unusually large paternal 21p to chromosome 13 in the fetus. In case 2, a mitotic event resulted in fetal mosaicism. The short arm material from a paternal chromosome 15 moved to chromosome 14 in some of the fetal cells. Movement of acrocentric short arm material resulted from breakage and exchange in centromeric areas of repetitive DNA. We suggest the mechanism may be similar to that of previously reported "jumping" translocations. Failure to recognize movement of the short arms of acrocentric chromosomes can result in erroneous interpretation of prenatal cytogenetic results and of other cytogenetic assays dependent on acrocentric short arm polymorphisms.

De novo 13q partial duplication identified by cytogenetic, biochemical and molecular approaches

A 3.5-month-old female infant manifesting dysmorphic facies, developmental delay and failure to thrive was referred for cytogenetic evaluation. Peripheral lymphocytes revealed three chromosomally distinct cell lines: 46,XX/46,XX,10p+/47,XX,10p+,+mar. Dermal fibroblasts revealed only the 46,XX,10p+cell line. High resolution G-, R-, and Q-banding suggested that the extra chromosomal material (10p+) represented a duplication of the segment 13q14----13qter. Parental karyotypes were normal. As absolute identification of de novo chromosomal abnormalities, based solely on cytogenetic studies, is sometimes difficult, both biochemical and molecular approaches were undertaken to elucidate this abnormality in more detail. Dosage effects were examined using esterase D (localized to 13q14.1) and the DNA probes p1E8 and p9A7 (localized to 13q22 and 13q31/32, respectively). These studies suggested the presence of only 2 copies of esterase D, but 3 copies of both DNA probes, allowing identification of the breakpoint at 13q14.2.

An unusual case of mosaic Down's syndrome involving two different Robertsonian translocations

A baby girl with some of the stigmata of Down's syndrome was found to be a mosaic with three different cell lines: 45,XX,-13,-21,+t(13q21q)/(46,XX/46,XX, -21,+t(21q21q). The chromosome rearrangements detected in this patient appear to have arisen de novo. In the normal cell line the terminal end of the p arm of one chromosome 21 is thought to have been damaged. It seems probable that this is related to the other chromosomal anomalies found.

Trisomy 22 in a newborn with multiple malformations

A case of complete trisomy 22 in a live-born female child with multiple malformations is reported. The karyotype of the index patient had 46 chromosomes, with one chromosome 22 missing and one supranumerary metacentric chromosome. Different banding methods and in situ hybridization revealed that the extra chromosome consists of the long arms and a part of the short arms of two chromosomes 22. Our report supplies further proof that a fetus with complete trisomy 22 can occasionally survive to term, but the condition is not compatible with life over a long period.

Dissociation of a t(12;21) resulting in a normal cell line in two trisomic 21 sons of a nonmosaic t(12;21) father?

A normal cell line arising from a translocation, t(12;21), possibly by dissociation, was observed in two brothers in early life. Each was conceived as trisomic 21 by their 45,XY,-12,-21,+t(12;21) father, who was phenotypically normal. Each brother showed morphologic manifestations of trisomy 21 syndrome, and each was mildly mentally retarded. Dermatoglyphic indices were not diagnostic of trisomy 21 syndrome. At 4 months the younger brother had a 50:50 proportion of trisomic:normal blood cells which became 25:75 of trisomic 21:normal at 36 months. The older brother had a 25:75 proportion of trisomic 21:normal when first studied at 41/2 years. A similar t(12;21) has not previously been reported. The occurrence of an apparently normal cell line arising spontaneously is unique.

Balanced transmission of centromeric fission products in man

Centrometric fission of chromosome 10 is reported in a 25-year-old psychiatric patient and his 5 1/2-year-old normal daughter. The relevant literature on this subject is briefly reviewed.