Mutations in bone morphogenetic protein receptor 1B cause brachydactyly type A2

Abstract

Brachydactyly (BD) type A2 is an autosomal dominant hand malformation characterized by shortening and lateral deviation of the index fingers and, to a variable degree, shortening and deviation of the first and second toes. We performed linkage analysis in two unrelated German families and mapped a locus for BD type A2 to 4q21-q25. This interval includes the gene bone morphogenetic protein receptor 1B (BMPR1B), a type I transmembrane serinethreonine kinase. In one family, we identified a T599 3 A mutation changing an isoleucine into a lysine residue (I200K) within the glycine͞serine (GS) domain of BMPR1B, a region involved in phosphorylation of the receptor. In the other family we identified a C1456 3 T mutation leading to an arginine-to-tryptophan amino acid change (R486W) in a highly conserved region C-terminal of the BMPR1B kinase domain. An in vitro kinase assay showed that the I200K mutation is kinase-deficient, whereas the R486W mutation has normal kinase activity, indicating a different pathogenic mechanism. Functional analyses with a micromass culture system revealed a strong inhibition of chondrogenesis by both mutant receptors. Overexpression of mutant chBmpR1b in vivo in chick embryos by using a retroviral system resulted either in a BD phenotype with shortening and͞or missing phalanges similar to the human phenotype or in severe hypoplasia of the entire limb. These findings imply that both mutations identified in human BMPR1B affect cartilage formation in a dominant-negative manner.

Homozygous missense and nonsense mutations in BMPR1B cause acromesomelic chondrodysplasia-type Grebe

“…15 was used as a template for in vitro mutagenesis to introduce the homologous mutation C53R and dLBD. Primers for mutagenesis are available in Supplementary Table 1.…”

“…Micromass cultures were isolated from limb buds of Hamburger Hamilton stage 23-24 33 as described, with minor modifications. 15 Micromass cultures were plated at a density of 2 Â 10 5 cells per 12 ml drop in the middle of a 24-well tissue-culture plate. Infection was performed with a titer of 1 Â 10 7 plaque-forming units of the concentrated RCAS viral supernatants, as described.…”

“…35 Previously reported point mutations in BMPR1B associated with BDA2 showed a dominantnegative effect on chondrogenesis, when overexpressed in this system. 15,21 RCASBP(A) virus expressing the chicken orthologs of BMPR1B wt , BMPR1B C53R , BMPR1B dLBD , and the BDA2-associated dominant-negative mutant BMPR1B I200K . 15 or empty virus as a control were used to infect micromass cultures.…”

“…Heterozygous carriers of dominant-negative mutations in BMPR1B are usually affected by brachydactyly-type A2 (BDA2; MIM number 112600), which is characterized by a shortened middle phalanx of the index finger and variable clinodactyly of the fifth finger. 15 In contrast, carriers of heterozygous mutations in the ligand, GDF5, are usually affected by brachydactyly-type C (MIM number 113100), 16,17 a condition characterized by brachymesophalangy of index, middle, and little fingers as well as shortening of the first metacarpal. Ring fingers are only mildly affected and hyperphalangy of the index and middle fingers might occur.…”

Mechanisms of digit formation: Human malformation syndromes tell the story

“…1). BDA2 is caused by heterozygous mutations in BMPR1B (Lehmann et al, 2003) that are thought to confer dominant-negative properties to the resulting BMPR1B protein. Of interest, BDA2 has also been shown to be associated with dominant mutations in GDF5 (Seemann et al, 2005;Kjaer et al, 2006;Lehmann et al, 2006;Ploger et al, 2008), the ligand for BMPR1B (Nishitoh et al, 1996).…”