What About Genetic Counseling in Prenatally Detected Supernumerary Chromosome Marker Cases

Background: Supernumerary chromosome markers (SMC) are rare chromosomal abnormalities that are difficult to identify by conventional cytogenetic techniques alone. They are found in 0.044% of live births and are seven times more frequent in cases of intellectual disability. Their prenatal prevalence is 0.075%. This frequency appears to be reduced because 30 to 50% of pregnancies with a fetus carrying SMC are interrupted. Diagnosis of SMC is always challenging especially when detected prenatally.

Objective: The aim of this study was to characterize prenatally detected SMC in five pregnant women and to offer a genetic counseling to the parents.

Methods: Amniocentesis, R-Banding karyotype and Fluorescent in Situ Hybridization (FISH) were performed in five patients referred to our department at 17 weeks of gestation for advanced maternal age and/or a history of trisomy 21 or abnormalities in ultrasound evaluation.

Results: Amniocentesis showed a small SMC present homogeneously for two patients, mosaic and de NOVO in three. FISH was performed on amniotic cells using specific probes for 15, 22, 20 and 18 chromosomes. The SMC was found to be a derivative 20 in two fetuses, 1 derivative from heterochromatic regions of 13/21 chromosomes and derivative 22 in the latter. Follow-up has been proposed and postnatal Karyotypes were performed in 3 patients. Der 13/21 was inherited from a healthy mother. Interestingly, for the derivative 22, postnatal karyotypes on both lymphocytes and fibroblasts was normal. Likewise, karyotype on lymphocytes was normal for one of the derivatives 20.

Conclusion: Genetic counseling was given to parents who decided to continue the pregnancy, which had normal outcome in all cases. Discordant results from amniocentesis and postnatal karyotypes suggest the existence of confined placental mosaicism. Our study confirms that amniotic fluid cells alone are not sufficient to predict mosaic SMC’s outcome.