Prenatal diagnosis of trisomy 21 by i(21q): a rare case of fetoplacental chromosomal discrepancy

Isochromosome 21q is overrepresented among false-negative cell-free DNA prenatal screening results involving Down syndrome

False-negative cell-free DNA (cfDNA) screening results involving Down syndrome are rare, but have high clinical impact on patients and their healthcare providers. Understanding the biology behind these results may allow for improved diagnostic follow-up and counseling. In 5 different centers offering cfDNA prenatal screening, 9 false-negative results were documented in 646 confirmed cases of trisomy 21; a false-negative rate of 1.4% (95% CI, 0.7-2.6). False-negative results included 4 cases of classical trisomy 21 and 5 cases with a de novo 21q;21q rearrangement. Two out of five rearrangements had molecular studies and were confirmed as isochromosomes. When combined with reports from the cfDNA screening literature, 8 out of 29 (28%) Down syndrome cases with a false-negative "non-invasive prenatal test" (NIPT) were associated with a 21q;21q rearrangement, compared with 2% reported in live born children with Down syndrome. In our laboratory series, evidence for placental or fetal mosaicism was present in 3 out of 3 true-positive cases involving a 21q;21q rearrangement and was confirmed in one false-negative case where placental material was available for study. Isochromosome 21q rearrangements are thus overrepresented among false-negative cfDNA screening results involving Down syndrome. Postzygotic isochromosome formation leading to placental mosaicism provides a biological cause for the increased prevalence of these rearrangements among false-negative cases. For clinical practice, a low trisomic fraction (z-score or equivalent measure) relative to the fetal fraction suggests placental mosaicism. Care should be taken as these cases may not reflect confined placental mosaicism, but rather full trisomy in the presence of a placenta containing normal cells.

Complete discrepancy between QF-PCR analysis of uncultured villi and karyotyping of cultured cells in the prenatal diagnosis of trisomy 21 in three CVS

OBJECTIVE

To investigate complete discrepancies in the prenatal diagnosis of trisomy 21 between QF-PCR analysis of uncultured villi and karyotyping of cultured cells in three chorion villus samples.

METHODS

Clinical details were obtained from all three patients. Follow-up studies were undertaken where possible by evaluation of chromosome 21 copy number with QF-PCR, interphase FISH, MLPA and karyotyping, and by post-mortem examination.

RESULTS

Case 1: severe oligohydramnios and microcephaly on scan. QF-PCR: trisomy 21; MLPA: trisomy 21; cultured karyotype: 46,XY[48]. Placental and fetal tissue results and post-mortem examination indicated a euploid fetus with trisomy 21 mosaicism confined to the placenta. Case 2: Down screen risk 1:16; NT = 4.4 mm; absent nasal bone (Caucasian mother). QF-PCR: disomy 21; cultured karyotype: 47,XY,+ 21[23]. Neck thickening noted at delivery-post-mortem refused, no fetal tissue available. Placental tissue indicated mosaicism for trisomy 21. Case 3: Down screen risk 1:91; NT = 6.7 mm. QF-PCR: disomy 21; cultured karyotype: 46,XX,der(21;21)(q10;q10)[60]. No follow-up possible. PCR genotyping of cultured cells confirmed sample identity in all three cases. Chromosome 21 markers observed by PCR were biallelic in all three cases, indicating that a mitotic error could account for the presence of the abnormal cell lines in each case.

CONCLUSION

QF-PCR analysis of uncultured villi and cultured karyotyping may rarely show complete discrepancy in the prediction of fetal trisomy 21 in CVS. Within-biopsy sample mosaicism, together with the testing of different cell populations, provide an explanation for these results. Practical ways to minimise the risk of such discrepancy are proposed.

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.