ABSTRACT
Humans typically have 22 pairs of autosomal chromosomes in cells,and a pair of sex chromosomes.Some individuals have an extra,autosomal chromosome called a small supernumerary marker chromosome (sSMC).sSMC is a structurally abnormal chromosome fragment.The fragments are too small and no-specific banding pattern to be identified by conventional banding cytogenetic analysis.Array-based comparative genomic hybridization (aCGH),fluorescence in situ hybridization (FISH) or other molecular biological methods are necessary for the diagnosis.This article summarized the karyotype,pathogenesis,and the clinical manifestations of the sSMC-related chromosome 18p abnormalities.The patients with sSMC usually presented with abnormal chromosome syndrome.Some syndromes are relative common,such as Pallister-Killian syndrome,isochromosome 18p syndrome,Cat eye syndromes or Emanuel syndrome.sSMC is considered to be the frequent cause of mental retardation.The patients have no specific symptoms.With the progress of molecular cytogenetics,more sSMC has been identified.Genetic counseling and prenatal diagnosis are important to prevent sSMC.Molecular cytogenetic techniques are necessary to the diagnosis.
ABSTRACT
A de novo supernumerary marker chromosome (SMC) was identified in a 13- month-old girl who presented with microcephaly and mild mental retardation. On further characterization by oligo-nucleotide array-comparative genomic hybridization [array-CGH], the SMC was confirmed to be 18p.
ABSTRACT
Partial trisomy 2p is a rare but relatively well-defined syndrome with distinctive clinical features, including marked psychomotor delay, dysmorphic face, and congenital heart disease. The phenotype of trisomy 18p is variable, from normal appearance to moderate mental retardation. Most cases of trisomy 2p and trisomy 18p result from the inheritance of an unbalanced segregant from a balanced parental translocation or due to de novo duplication. Here, we present the first report of a combined partial trisomy 2p and trisomy 18p due to a supernumerary marker chromosome (SMC). The final karyotype of the patient was 47,XX,+der(18)t(2;18)(p23.1;q11.1)[22]/46,XX[8]. The patient had typical dysmorphic features of partial trisomy 2p23-pter syndrome and congenital heart disease. SMCs are remarkably variable in euchromatic DNA content and mosaicism level. The precise identification of the origin and composition of SMCs is essential for genotype-phenotype correlation and genetic counseling.