Identification of Autosomal Supernumerary Chromosome Markers (SMCs) by Fluorescent In Situ Hybridization (FISH)

Supernumerary marker chromosomes (SMCs) are rare chromosomal abnormalities resulting in partial trisomy of specific genomic regions with characteristic phenotypic effects. Twenty six cases with autosomal SMCs are reported. Four were identified prenatally and 22 postnatally in children, aged from 8 days to 15 years, who were referred for genetic evaluation because of various congenital anomalies and developmental delay. In 22 of the 26 cases, the SMCs were de novo, in two they were familial and in another two a 11;22 reciprocal translocation was revealed in the mothers. In only one patient was the SMC present in a mosaic form. Sequential fluorescent in situ hybridization studies (FISH) using Whole Chromosome Paint (WCP) probes were performed in order to determine the chromosomal origin of the SMCs. Sixteen of them originated from chromosome 15, five were shown to be an isochromosome 18p and one was derived from chromosome 22, but did not contain the DiGeorge/ VCFS critical region. In two instances, the SMCs were derivatives of chromosome 13 and in two the SMCs resulted from a 11;22 maternal translocation and contained material from both chromosomes 11 and 22. Molecular investigation of two of the patients with an SMC[15] revealed three copies of the SNRPN gene, but the diagnosis of PW/AS due to possible imprinting was excluded in both patients by a methylation-specific PCR. FISH and molecular studies have greatly facilitated the characterization of marker chromosomes. As more SMCs are classified, better genetic counseling and risk evaluation can be achieved.