A case of atypical Down's syndrome with mosaic 46,XX/46,XX-21+t(21q21q)

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.

Discordant prenatal diagnosis of trisomy 21 due to mosaic structural rearrangements of chromosome 21

OBJECTIVES

Trisomy 21 mosaicism associated with a structural rearrangement of chromosome 21 is uncommon. We report on two prenatal diagnoses in which karyotypes showed mosaicism with an aberrant cell line, including a structural rearrangement of chromosome 21.

METHODS

Both these cases were associated with increased nuchal translucency. Conventional and molecular cytogenetic analyses were performed on uncultured and cultured trophoblast and amniotic fluid cells.

RESULTS

In Case 1, analysis of trophoblast cells revealed an abnormal karyotype of 47,XX,+mar.ish der(13/21)(D13Z1/D21Z1+)/46,XX. The amniocentesis showed a free non-mosaic trisomy 21. In Case 2, the trophoblast direct analysis showed a normal male karyotype whereas the long-term culture revealed a mosaicism for a dicentric long-arm isochromosome 21: 46,XY,idic(21)(p11)/45,XY,-21/46,XY. Amniocentesis showed an unbalanced non-mosaic karyotype 46,XY,idic(21)(p11) resulting therefore in trisomy for the long arm of chromosome 21.

CONCLUSION

Our cases underline the importance of combining the direct analysis and long-term culture of trophoblast and emphasise the need for confirmatory studies in other tissues when mosaicism of structural rearrangement is encountered in chorionic villi. The meiotic and mitotic mechanisms of formation of these structural rearrangements of chromosome 21 are discussed.

Prenatal diagnosis of mosaicism 46,XX/46,XX,-21,+t(21q21q)

Mosaicism for a structural chromosome abnormality in amniotic cell cultures indicative of true fetal mosaicism is a rare event. In addition to the laboratory findings the clinical interpretation for counseling in such cases is based on observation of the same abnormality in liveborns as well as previous experience with prenatal diagnosis of the same or similar abnormalities. We report here the prenatal diagnosis of 46,XX/46,XX,-21,+t( 21q21q ) which was confirmed in fetal skin cell and amnion cell cultures.