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 threemarkers were identified as ring chromosomes
containing different amounts of euchromaticmaterial 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.