The chromosome 17q21.31 microdeletion syndrome is a novel genomic disorder that has originally been identified using high-resolution genome analyses in patients with unexplained mental retardation. Here we report the molecular and/or clinical characterization of 22 individuals with the 17q21.31 microdeletion syndrome. We estimate the prevalence of the syndrome to be 1 in 16,000 and show that it is highly underdiagnosed. Extensive clinical examination, reveals that developmental delay, hypotonia, facial dysmorphisms including a long face, a tubular or pear-shaped nose and a bulbous nasal tip, and a friendly/amiable behavior are the most characteristic features. Other clinically important features include epilepsy, heart defects (ASD, VSD), and kidney/ urologic anomalies. Using high-resolution oligonucleotide arrays, we narrow the 17q21.31 critical region to a 424-kb genomic segment (chr17: 41046729-41470954, hg17), encompassing at least six genes, among which the gene encoding microtubule-associated protein tau (MAPT). Mutation screening of MAPT in 122 individuals with a phenotype suggestive of 17q21.31 deletion carriers, but who do not carry the recurrent deletion, failed to identify any disease-associated variants. In five deletion carriers, we identify a <500-bp rearrangement hotspot at the proximal breakpoint contained within an L2 LINE motif and show that in every case examined, the parent originating the deletion carries a common 900-kb 17q21.31 inversion polymorphism, indicating that this inversion is a necessary factor for deletion to occur (p<10-5). Our data establishes the 17q21.31 microdeletion syndrome as a clinically and molecularly well recognizable genomic disorder.
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