A case of de novo partial tetrasomy of distal 6p and review of the literature.

We report on a 3-year-old girl with bilateral eyelid colobomas, bulbous nose, blepharophimosis, blepharoptosis, sensorineural hearing loss, atrial septal defect, psychomotor retardation, and growth delay. Cytogenetic analysis showed additional material of unknown origin on the short arm of chromosome 8. Whole chromosome paint FISH identified the additional material to originate from chromosome 6. Subtelomeric metaphase FISH analysis detected a bright signal pattern for the 6p subtelomere probe on the derivative 8 as well as two short arm signals for the normal chromosomes 6. Interphase FISH with the 6p subtelomere probe demonstrated four 6p signals. Interestingly, metaphase FISH with a probe for the 8p subtelomere region demonstrated a signal for 8p just proximal to the translocated material. Comparative genomic hybridization studies confirmed tetrasomy of the 6p subtelomere region from 6p25.1 --> 6p25.3. Thus our patient represents the first reported case of "pure" partial tetrasomy 6p, meaning the tetrasomy was not associated with a significant deletion of chromosome arm 8p. We compare here this case with previously reported cases of partial trisomy 6p and the resulting phenotypes.