Intrachromosomal partial triplication of chromosome 13 secondary to a paternal duplication with mild phenotypic effect

Intrachromosomal triplications are rare and can be mistaken for duplications. The majority of triplications reported are de novo, mostly involving chromosome 15q, and have a middle inverted repeat. We report on the clinical, cytogenetic, and molecular analyses of a patient with a novel triplication 13q21.1-q21.33 secondary to a familial duplication 13q21.1-q21.33 with mild phenotypic effect in three generations. The propositus was an 8-year-old boy referred because of language delay and mild mental retardation. His weight, height and OFC were above the 97th centile. He had delayed tooth eruption and subtle dysmorphic features. Chromosome analysis (550 band stage) showed extra material in 13q21. Family history was unremarkable except for adult-onset sensorineural hearing loss in the father and paternal grandfather. Their karyotypes and those of both brothers of the propositus also showed an abnormal chromosome 13 but with less extra genetic material. FISH analysis with several BAC clones showed a triplication in the propositus between 204N9 and 184B18 (which mapped to 13q21.1 and 13q21.33, respectively) and a direct duplication for the same fragment (around 12 Mb) in the rest of the family members with the abnormal chromosome 13. The FISH signals did not show a middle inverted repeat. We describe the first intrachromosomal triplication 13q21.1-q21.33 derived from a paternal duplication. Meiotic instability in the transmission of a duplication has not been previously observed. Phenotypic variability may be explained by chromosomal non-penetrance or dosage critical loci located in the triplicate/duplicate segment. © 2008 Wiley-Liss, Inc.