A HoxD11 Homeobox Gene Mutation Associated With Congenital Absent Radius But Without The Thrombocytopenia Of The TAR Syndrome Or a HoxA11 Mutation

HoxA11 and HoxD11 are homeobox genes critical for normal development of the forearm and thus are potential candidate genes for involvement in the pathogenesis of the thrombocytopenia/absent radius (TAR) syndrome. However, we previously reported an absence of coding sequence mutations in either HoxA11 or HoxD11 in a series of 10 unrelated TAR syndrome patients (Fleischman RA et al., Br J Haematol., 116:367-75, 2002). Despite this negative finding, interest in the potential role of homeobox genes in the TAR syndrome has been supported by a report of a HoxA11 mutation occurring in two kindreds with amegakaryocytic thrombocytopenia and radio-ulnar synostosis, a less pronounced more proximal pattern of radial malformation (Thompson AA and Nguyen LT. Nat Genet., 26:397-8, 2000). Unlike HoxA11, however, no mutations in the human HoxD11 gene have been described thus far that would help elucidate the potential role of this paralogous gene in megakaryopoiesis or the TAR syndrome. We now describe a novel mutation in human HoxD11 that results in a polyalanine sequence expansion, (GCG)6→ (GCG)8, and report that this mutation is associated with a unilateral absent radius in the affected propositus. A familial syndrome is suggested in this kindred, moreover, by the prior observation of a bilateral absent radius in a deceased maternal aunt. This mutation was not present in more than 100 unrelated normal subjects or 8 other unrelated individuals with sporadic absence of the radius. Two other living maternal relatives also carried the mutation but did not exhibit any radial defects, a finding consistent with autosomal dominance with incomplete penetrance, an inheritance pattern reported for short polyalanine expansion mutations in the related homeobox gene HoxD13 which cause synpolydactyly. In contrast to the reported HoxA11 mutation, however, neither the propositus nor the mutation carriers of this HoxD11 mutation exhibited thrombocytopenia or any other cytopenias or congenital defect. The results suggest that at least one class of mutation in human HoxD11 may be sufficient to cause an absent radius syndrome but unlike the reported HoxA11 mutation, does not adversely affect megakaryopoiesis. The findings further suggest that additional studies of the TAR syndrome may be necessary to exclude as yet undetected non-coding mutations in promoter or enhancer sequences that alter the expression of HoxA11, HoxD11 or other homeobox genes critical for radial development and/or megakaryopoiesis. This work was supported by a VA Merit Award. Disclosures: No relevant conflicts of interest to declare.