Chromosomal imbalance reportA case of an atypically large proximal 15q deletion as cause for Prader-Willi syndrome arising from a de novo unbalanced translocation

•Patient with de novo 3;15 unbalanced translocation with large atypical chromosome 15 deletion.•Patient is also deleted for critical region of 15q13.3 microdeletion syndrome.•Phenotype is Prader-Willi syndrome with severe global developmental delay.•FISH can detect more complicated rearrangements in cases where patients have an abnormal chromosomal microarray.

We describe an 11 month old female with Prader-Willi syndrome (PWS) resulting from an atypically large deletion of proximal 15q due to a de novo 3;15 unbalanced translocation. The 10.6 Mb deletion extends from the chromosome 15 short arm and is not situated in a region previously reported as a common distal breakpoint for unbalanced translocations. There was no deletion of the reciprocal chromosome 3q subtelomeric region detected by either chromosomal microarray or FISH. The patient has hypotonia, failure to thrive, and typical dysmorphic facial features for PWS. The patient also has profound global developmental delay consistent with an expanded, more severe, phenotype.