Autosomal dominant craniometaphyseal dysplasia is caused by mutations in the transmembrane protein ANK.

Craniometaphyseal dysplasia (CMD) is a rare skeletal disorder characterized by progressive thickening and increased mineral density of craniofacial bones and abnormally developed metaphyses in long bones. Linkage studies mapped the locus for the autosomal dominant form of CMD to an approximately 5-cM interval on chromosome 5p, which is defined by recombinations between loci D5S810 and D5S1954. Mutational analysis of positional candidate genes was performed, and we describe herein three different mutations, in five different families and in isolated cases, in ANK, a multipass transmembrane protein involved in the transport of intracellular pyrophosphate into extracellular matrix. The mutations are two in-frame deletions and one in-frame insertion caused by a splicing defect. All mutations cluster within seven amino acids in one of the six possible cytosolic domains of ANK. These results suggest that the mutated protein has a dominant negative effect on the function of ANK, since reduced levels of pyrophosphate in bone matrix are known to increase mineralization.

Combinatorial signaling through BMP receptor IB and GDF5: shaping of the distal mouse limb and the genetics of distal limb diversity.

In this study, we use a mouse insertional mutant to delineate gene activities that shape the distal limb skeleton. A recessive mutation that results in brachydactyly was found in a lineage of transgenic mice. Sequences flanking the transgene insertion site were cloned, mapped to chromosome 3, and used to identify the brachydactyly gene as the type IB bone morphogenetic protein receptor, BmprIB (ALK6). Expression analyses in wild-type mice revealed two major classes of BmprIB transcripts. Rather than representing unique coding RNAs generated by alternative splicing of a single pro-mRNA transcribed from one promoter, the distinct isoforms reflect evolution of two BmprIB promoters: one located distally, driving expression in the developing limb skeleton, and one situated proximally, initiating transcription in neural epithelium. The distal promoter is deleted in the insertional mutant, resulting in a regulatory allele (BmprIB(Tg)) lacking cis-sequences necessary for limb BmprIB expression. Mutants fail to generate digit cartilage, indicating that BMPRIB is the physiologic transducer for the formation of digit cartilage from the skeletal blastema. Expansion of BmprIB expression into the limb through acquisition of these distal cis-regulatory sequences appears, therefore, to be an important genetic component driving morphological diversity in distal extremities. GDF5 is a BMP-related signal, which is also required for proper digit formation. Analyses incorporating both Gdf5 and BmprIB(Tg) alleles revealed that BMPRIB regulates chondrogenesis and segmentation through both GDF5-dependent and -independent processes, and that, reciprocally, GDF5 acts through both IB and other type I receptors. Together, these findings provide in vivo support for the concept of combinatorial BMP signaling, in which distinct outcomes result both from a single receptor being triggered by different ligands and from a single ligand binding to different receptors.

Structural variation of type XII collagen at its carboxyl-terminal NC1 domain generated by tissue-specific alternative splicing.

This paper reports the identification of two structural variations in the NC1 domain of rat and mouse type XII collagen. The long NC1 domain encoding 74 amino acids showed homology to chicken type XII and XIV collagens. The short NC1 domain was composed of 19 amino acids. Through genomic DNA analyses, two alternative exons were identified, each of which contained the variable NC1 sequence. With the amino-terminal NC3 splicing alternatives, we propose here a new descriptive nomenclature: types XIIA-1 and XIIB-1 which include a long NC1 sequence encoded by exon 1 (from the 3'-end), and types XIIA-2 and XIIB-2 which include a short NC1 sequence encoded by exon 2. Types XIIA-1 and XIIB-1, the predominant transcripts in 15-day old mouse embryos, showed decreased expression in 17-day old embryos when type XIIB-2 expression was sustained at constant levels. In adult mice, type XIIB-1 associates with ligament and tendon, whereas type XIIB-2 is expressed in various other tissues. The long NC1 domain contains an extended acidic region (pI = 3.4) followed by a terminal basic region (pI = 13.8). Because the short NC1 domain lacks these features, structural variations in the type XII collagen NC1 domain suggests different functional roles in a tissue-specific fashion.