Detection of a cryptic translocation t(13;20)(q34;p13) in an unexplained case of MCA/MR: Value of FISH over high resolution banding

Cryptic unbalanced chromosome rearrangements in the telomeric bands of the chromosomes may constitute a significant cause of unexplained mental retard ation with or without congenital anomalies. We report on a boy with a termi nal deletion of the long arm of chromosome 13, combined with a partial dupl ication of the short arm of chromosome 20, owing to a cryptic balanced tran slocation in his father. The karyotype of the father was 46XY,t(13;20) (q34 ;p13), The propositus presented with severe mental and growth retardation, microcephaly, facial anomalies including ptosis of the right upper eyelid, a high nasal bridge, small palpebral fissures, and bilateral epicanthus, hy pospadias, and scoliosis, A younger brother died at birth and had a low bir th weight, hypospadias, and a horseshoe kidney, Repeated chromosome analyse s with high resolution banding in the propositus and his parents were appar ently normal. Chromosome painting eventually disclosed the cryptic transloc ation in the father with unbalanced karyotype in the propositus. The import ance of additional FISH analysis in patients with unexplained mental retard ation, physical anomalies, and apparently normal chromosomes is emphasized. Am. J. Med, Genet, 86:385-388, 1999, (C) 1999 Wiley-Liss, Inc.