title: Presence of harmless small supernumerary marker chromosomes hampers molecular genetic diagnosis: a case report.
abstract: Mental retardation is correlated in approximately 0.4% of cases with the presence of a small supernumerary marker chromosome (sSMC). However, here we report a case of a carrier of a heterochromatic harmless sSMC with fragile X syndrome (Fra X). In approximately 2% of sSMC cases, similar heterochromatic sSMC were observed in a clinically abnormal carriers. In a subset of such cases, uniparental disomy (UPD) of the corresponding sister chromosomes was shown to be the cause of mental retardation. For the remainder of the cases, including the present one, the sSMC was just a random finding not related to the clinical phenotype. Thus, it is proposed to test patients with heterochromatic sSMC and mental retardation of unclear cause as follows: i) exclude UPD, ii) test for Fra X as it is a major cause of inherited mental retardation, and iii) perform chip-based assays or tests for special genetic diseases according to the phenotype. In any case, the diagnosis of a cytogenetic aberration such as an sSMC should not automatically be considered the resolution of a clinical case.
authors.affiliation: Institute of Human Genetics and Anthropology, Jena University Hospital, 07740 Jena, Germany.
authors.forename: Heike
authors.initials: H
authors.lastname: Nelle
authors.forename: Isolde
authors.initials: I
authors.lastname: Schreyer
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Ewers
authors.forename: Kristin
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authors.lastname: Mrasek
authors.forename: Nadezda
authors.initials: N
authors.lastname: Kosyakova
authors.forename: Martina
authors.initials: M
authors.lastname: Merkas
authors.forename: Ahmed Basheer
authors.initials: AB
authors.lastname: Hamid
authors.forename: Raimund
authors.initials: R
authors.lastname: Fahsold
authors.forename: Anikó
authors.initials: A
authors.lastname: Ujfalusi
authors.forename: Jasen
authors.initials: J
authors.lastname: Anderson
authors.forename: Nikolai
authors.initials: N
authors.lastname: Rubtsov
authors.forename: Alma
authors.initials: A
authors.lastname: Küchler
authors.forename: Ferdinand
authors.initials: F
authors.lastname: von Eggeling
authors.forename: Julia
authors.initials: J
authors.lastname: Hentschel
authors.forename: Anja
authors.initials: A
authors.lastname: Weise
authors.forename: Thomas
authors.initials: T
authors.lastname: Liehr