Genomic inversion at 6p22.3 supports ID4 dysregulation as the pathogenic mechanism of Mesomelic dysplasia Savarirayan-type.

Mesomelic dysplasia Savarirayan-type or ID4-related (MDST) is an ultra-rare skeletal dysplasia caused by chromosome 6p22.3 microdeletions. To date, only four cases have been reported. Here, we report a fifth case, a 9 year-old female with severe mesomelic lower limb shortening and characteristic radiographic findings, highly resembling those identified in previous MDST patients. No deletion was identified by array. However, whole genome sequencing (WGS) revealed a de novo inversion at 6p22.3. As hypothesized for deletions detected in this disorder we predict that the structural variant disrupts several topologically associated domains (TADs) in the region and is likely to place ID4 in closer proximity to more telomerically located limb enhancers, which could result in enhancer adoption and potentially lead to ID4 limb misexpression. Thus, this case broadens the genetic spectrum in MDST and provides further support to the role of ID4 dysregulation as the main underlying molecular mechanism of this ultra-rare skeletal disorder.