Familial medullary thyroid carcinoma secondary to an <i>SLC30A9</i> intragenic deletion and translation reinitiation

While most individuals with familial medullary thyroid carcinoma (fMTC) carry <i>RET</i> mutations, in some instances the causative mutations remain unknown. We studied two related families with <i>RET</i> -negative fMTC in 21 affected individuals through linkage analysis, exome/genome sequencing, and high-density array comparative genomic hybridization. We identified a novel heterozygous 40kb intragenic <i>SLC30A9</i> deletion which segregated with the disease in all affected individuals. The mutant transcript escaped nonsense-mediated decay and resulted in the production of N-terminally truncated proteins via translation reinitiation from in-frame AUG codons located downstream of the deletion. These proteins showed increased stability and their expression in an MTC cell line increased cell proliferation and clonogenic capacity, supporting an oncogenic role. These findings expand the genetic background of fMTC beyond <i>RET</i> mutations and implicate translation reinitiation in the etiology of cancer susceptibility syndromes secondary to structural genomic variants.