The Atypical 16p11.2 Deletion: A Not So Atypical Microdeletion Syndrome?

One of the recently recognized microdeletion syndromes is the 16p11.2 deletion syndrome (593 kb; similar to 29.5Mb to similar to 30.1 Mb), associated with developmental delay, autism spectrum disorder, epilepsy, and obesity. Less frequently reported is a smaller 220 kb deletion, adjacent and distal to this 16p11.2 deletion, which has been referred to as the atypical 16p11.2 deletion (220 kb; similar to 28.74Mb to similar to 28.95 Mb). We describe three patients with this deletion and update the manifestations in two sibs who have been described as possibly new entity in this Journal in 1997 [Bakker and Hennekam (1997); Am J Med Genet 70: 312-314] and were recently found to have the "atypical 16p11.2 deletion" as well. Patients show a developmental delay, behavioral problems, and unusual facial morphology (prominent forehead, downslanted, and narrow palpebral fissures), and some are obese. We suggest that this "atypical" deletion may turn out to become a microdeletion syndrome that will be recognizable in the future, or at least to show a phenotype that is recognizable in retrospect. As it may no longer be so "atypical," we suggest renaming the entity "distal 16p11.2 deletion," to distinguish it from the common proximal 16p11.2 deletion. (C) 2011 Wiley-Liss, Inc.