True fetal mosaicism of an isochromosome of the long arm of a chromosome 20: the dilemma persists

Mosaic isochromosome 20q at amniocentesis: Prenatal diagnosis, genetic counseling and literature review

OBJECTIVE

We present prenatal diagnosis of mosaic isochromosome 20q [i(20q)] at amniocentesis, and we review the literature.

CASE REPORT

A 36-year-old woman underwent amniocentesis at 17 weeks of gestation because of advanced maternal age. Amniocentesis revealed a karyotype of 46,XY,i(20)(q10)[27]/46,XY[29]. Prenatal ultrasound findings were unremarkable. The parental karyotypes were normal. Repeat amniocentesis was performed at 20 weeks of gestation. During repeat amniocentesis, array comparative genomic hybridization (aCGH), interphase fluorescence in situ hybridization (FISH) and quantitative fluorescent polymerase chain reaction (QF-PCR) assay were performed on uncultured amniocytes, and conventional cytogenetic analysis, interphase FISH and aCGH were performed on cultured amniocytes. In the repeat amniocentesis, the cultured amniocytes revealed a karyotype of 46,XY. Interphase FISH analysis showed the i(20q) signal in 5.2% (5/96) of the uncultured amniocytes compared with 2% in the control, and in 0.98% (1/102) of the cultured amniocytes compared with 2% in the control. aCGH detected no genomic imbalance in both uncultured and cultured amniocytes. QF-PCR analysis excluded uniparental disomy 20. At 38 weeks of gestation, a healthy 2870-g male baby was delivered with no phenotypic abnormality. The postnatal blood karyotype was 46,XY. FISH analysis on urinary cells showed 2.1% (2/95 cells) mosaicism compared with 1.9% (2/105 cells) in the control.

CONCLUSION

Mosaic i(20q) at amniocentesis is a benign condition associated with a favorable outcome in most cases and can be a cell culture artifact confined to cultured amniocytes. Molecular cytogenetic analysis using uncultured amniocytes is useful for rapid confirmation. Prenatal diagnosis of very high percentage of mosaicism for i(20q) at amniocentesis should alert the presence of fetal structural abnormalities. Prenatal diagnosis of mosaic i(20q) at amniocentesis should include a detail examination of fetal brain and spine.

Pregnancy and postnatal outcome of mosaic isochromosome 20q

Prenatally diagnosed mosaicism for isochromosome 20q is generally reported in association with a normal outcome at birth and is rarely confirmed postnatally. However, the origin of these abnormal cells is unclear and there are few reports of long-term outcomes. We present an additional case of prenatally detected isochromosome 20q, with normal outcome up to age 3.6 years. The abnormal cells, while present at high levels in the amniotic fluid, could not be confirmed in placenta or fetal blood. Nonetheless, based on a review of the literature, the level of isochromosome 20q cells found is associated with risk of abnormal outcome, suggesting a possible effect in some cases.

Prenatal detection of mosaic isochromosome 20q: a fourth report with abnormal phenotype

We described a new case of mosaic isochromosome 20q revealed by amniocentesis. The propositus presented with craniofacial dysmorphism, clubfeet, and vertebral abnormalities. A 46,XX,i(20)(q10)[14]/46,XX[1] karyotype was confirmed by FISH on cultured cells. The pregnancy was terminated. From review of literature, fetus with mosaic isochromosome 20q identified on amniocentesis are most likely to be phenotypically and cytogenetically normal after birth. So we performed CGH and array-CGH to exclude another possible imbalance. We discuss here the possible relation between this chromosomal abnormality and the abnormal phenotype.

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.