Understanding The Molecular Basis Of Skeletal Defects In Brachydactyly Type A1

Brachydactyly type A1 (BDA1; MIM 112500), characterized by shortening or absence of the middle phalanges in digits, is the first recorded disorder of the autosomal dominant Mendelian trait in human. Recently, heterozygous missense mutations in the Indian hedgehog gene (IHH) were reported for BDA1. However, the underlying pathogenesis of BDA1 and the role of Ihh during early digit morphogenesis were remained unclear. As it is a developmental abnormality, we carefully studied a mouse model carrying a human equivalent E95K missense mutation to address the molecular and developmental consequences. Homozygous mice for the E95K allele displayed mild dwarfism and typical characteristics of BDA1 with severe shortening (digit II-IV) or missing (digit V) of the middle phalanges. Moreover, like in human cases, this mutation exerts a dominant effect in mice, causing mild brachydactyly type A1 phenotype in Ihh+/E95K mice. We studied the molecular basis of the E95K mutation in vitro and in vivo. Our results suggest that there is a slight reduction in the signaling capacity of the E95K Ihh protein, but the effective signaling range in the cartilage element of the developing digits and long bones is broader. In long bones, the alteration in the signaling range of Ihh signal disturbs the Ihh-PthrP negative feedback loop and impairs chondrocyte proliferation and differentiation. In digits, this increased Ihh signaling range allows additional signals extend into the interzone and enhance PthrP expression, a potential negative feedback regulator of Ihh expression. We further showed that distal Ihh signal is reduced in BDA1 mice, impairing mesenchymal recruitment and growth of the distal cartilage element, contributing to the abnormal segmentation between the middle and distal phalanges causing BDA1. In addition, inter-phalangeal interzone development is abnormal in BDA1 mice. In support of these observations, we show that the distal outgrowth and interzone development of digital primordia are almost completely abolished in the digital context correspond to Ihh signaling. Thus, our results suggest that IHH signaling is needed in regulating elongation of distal digit elements during early digit patterning.