Suspected gonadal mosaicism for isochromosomes 18p and 18q unsubstantiated by fluorescence in situ hybridization analysis of sperm

Prenatal diagnosis of recurrent mosaic ring chromosome 13 of maternal origin

OBJECTIVE

We present prenatal diagnosis of recurrent mosaic ring chromosome 13 [r(13)] of maternal origin.

CASE REPORT

A 27-year-old woman underwent amniocentesis at 17 weeks of gestation because of a past history of fetal abnormality caused by mosaic r(13) in the previous fetus associated with fetal intrauterine growth restriction (IUGR), a karyotype of 46,XY,r(13)[23]/45,XY,-13[10]/46,XY,idic r(13)[2] and a maternal origin of abnormal r(13). The parental karyotypes were normal. During this pregnancy, amniocentesis revealed a karyotype of 46,XX,r(13)[12]/45,XX,-13[8] and a 22.80-Mb deletion of chromosome 13q31.3-q34. The pregnancy was subsequently terminated, and a malformed fetus was delivered with craniofacial dysmorphism. Repeat amniocentesis revealed a karyotype of 46,XX,r(13)(p11.1q31)[18]/45,XX,-13[12]. The placenta had a karyotype of 46,XX,r(13)(p11.1q31)[27]/45,XY,-13[13]. Polymorphic DNA marker analysis using the DNA derived from the parental bloods and umbilical cord confirmed a maternal origin of the abnormal r(13).

CONCLUSION

Prenatal diagnosis of mosaic r(13) in consecutive pregnancies should raise a suspicion of parental gonadal mosaicism, and polymorphic DNA marker analysis is useful for determination of the parental origin of recurrent aneuploidy under such a circumstance.

Recurrence risk in de novo structural chromosomal rearrangements

According to the textbook of Gardner and Sutherland [2004], the standard on genetic counseling for chromosome abnormalities, the recurrence risk of de novo structural or combined structural and numeric chromosome rearrangements is less than 0.5-2% and takes into account recurrence by chance, gonadal mosaicism, and somatic-gonadal mosaicism. However, these figures are roughly estimated and neither any systematic study nor exact or evidence-based risk calculations are available. To address this question, an extensive literature search was performed and surprisingly only 29 case reports of recurrence of de novo structural or combined structural and numeric chromosomal rearrangements were found. Thirteen of them were with a trisomy 21 due to an i(21q) replacing one normal chromosome 21. In eight of them low-level mosaicism in one of the parents was found either in fibroblasts or in blood or in both. As a consequence of the low number of cases and theoretical considerations (clinical consequences, mechanisms of formation, etc.), the recurrence risk should be reduced to less than 1% for a de novo i(21q) and to even less than 0.3% for all other de novo structural or combined structural and numeric chromosomal rearrangements. As the latter is lower than the commonly accepted risk of approximately 0.3% for indicating an invasive prenatal diagnosis and as the risk of abortion of a healthy fetus after chorionic villous sampling or amniocentesis is higher than approximately 0.5%, invasive prenatal investigation in most cases is not indicated and should only be performed if explicitly asked by the parents subsequent to appropriate genetic counseling.