Patient with three euchromatic supernumerary marker chromosomes derived from chromosomes 1, 12, and 18: characterization and evaluation of the aberrations

The genetic relevance of small supernumerary marker chromosomes (sSMCs) depends on their content of euchromatin. In case of mosaicism, the phenotype of the carrier furthermore is influenced by the distribution of the marker in the body. In the majority of reported cases no correlation of the degree of mosaicism in the tissue(s) analyzed and the phenotype could be detected. In particular, non-acrocentric derived sSMCs show a strong tendency to appear in mosaic state irrespective of the clinical picture. We present a patient with cognitive disability and mild craniofacial dysmorphisms with mosaicism of three different autosomal marker chromosomes. The extra chromosomes were analyzed by a combination of SNP array and a variety of fluorescence in situ hybridization (FISH) probes. All three markers were identified as ring chromosomes containing different amounts of euchromatic material derived from chromosome 1 (1p12 → q21), 12 (12p13.1 → q13.11) and 18 (18p11.21 → q11.2). The size and the frequency of the sSMCs were strikingly different, besides, we observed an unequal combination of the three derivates.