Characterization of a small supernumerary ring marker derived from chromosome 2 by forward and reverse chromosome painting.

A small ring-shaped supernumerary marker chromosome (SMC) was detected in 50% of metaphase cells in an 18-month-old boy with mental retardation and multiple congenital anomalies. Conventional cytogenetic methods had failed to identify the origin of the marker. When the patient was age 11.5 years, we defined the origin of the SMC by fluorescence in situ hybridization using a battery of centromere-specific DNA probes. The marker was positive with the probe for locus D2Z. More detailed characterization was achieved by using chromosome 2 arm-specific and marker-specific DNA libraries, which were constructed by microdissection of the two arms chromosome 2 and SMC with subsequent amplification of the chromosomal material by a degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR). The marker was identified as r(2)(p11.2-->q14.1). The propositus had dolichocephaly, coarse hair, low-set ears, exophthalmos, epicanthal folds, strabismus, depressed nasal bridge, high-arched palate, excess of skin on the neck, tapered fingers with mild clinodactyly, talipes varus on the right, inguinal hernia, hypogenitalism, muscular hypotonia, and mental retardation. This is the first case of SMC derived from chromosome 2 that was characterized by forward and reverse chromosome painting. Copyright 1999 Wiley-Liss, Inc.