Leri-Weill Dyschondrosteosis (LWD; OMIM 127300) is a dominantly inherited skeletal dysplasia characterized by disproportionate short stature with predominantly mesomelic limb shortening. Expression is variable and consistently more severe in females, who frequently display the Madelung deformity of the forearm (shortening and bowing of the radius with dorsal subluxation of the distal ulna). The rare Langer Mesomelic Dysplasia (LD; OMIM 249700), characterized by severe short stature with hypoplasia/aplasia of the ulna and fibula, has been postulated to be the homozygous form of LWD (refs 4-6). In a six-generation pedigree with LWD, we established linkage to the marker DXYS6814 in the pseudoautosomal region (PAR1) of the X and Y chromosomes (Z max=6.28; theta=0). Linkage analysis of three smaller pedigrees increased the lod score to 8.68 (theta=0). We identified submicroscopic PAR1 deletions encompassing the recently described short stature homeobox-containing gene SHOX (refs 7,8) segregating with the LWD phenotype in 5 families. A point mutation leading to a premature stop in exon 4 of SHOX was identified in one LWD family.

The secreted polypeptide noggin (encoded by the Nog gene) binds and inactivates members of the transforming growth factor beta superfamily of signalling proteins (TGFbeta-FMs), such as BMP4 (ref. 1). By diffusing through extracellular matrices more efficiently than TGFbeta-FMs, noggin may have a principal role in creating morphogenic gradients. During mouse embryogenesis, Nog is expressed at multiple sites, including developing bones. Nog-/- mice die at birth from multiple defects that include bony fusion of the appendicular skeleton. We have identified five dominant human NOG mutations in unrelated families segregating proximal symphalangism (SYM1; OMIM 185800) and a de novo mutation in a patient with unaffected parents. We also found a dominant NOG mutation in a family segregating multiple synostoses syndrome (SYNS1; OMIM 186500); both SYM1 and SYNS1 have multiple joint fusion as their principal feature. All seven NOG mutations alter evolutionarily conserved amino acid residues. The findings reported here confirm that NOG is essential for joint formation and suggest that NOG requirements during skeletogenesis differ between species and between specific skeletal elements within species.

Partial absence of the sacrum is a rare congenital defect which also occurs as an autosomal dominant trait; association with anterior meningocoele, presacral teratoma and anorectal abnormalities constitutes the Currarino triad (MIM 176450). Malformation at the caudal end of the developing notochord at approximately Carnegie stage 7 (16 post-ovulatory days), which results in aberrant secondary neurulation, can explain the observed pattern of anomalies. We previously reported linkage to 7q36 markers in two dominantly inherited sacral agenesis families. We now present data refining the initial subchromosomal localization in several additional hereditary sacral agenesis (HSA) families. We excluded several candidate genes before identifying patient-specific mutations in a homeobox gene, HLXB9, which was previously reported to map to 1q41-q42.1 and to be expressed in lymphoid and pancreatic tissues.

Synpolydactyly (SPD) is a dominantly inherited congenital limb malformation. Typical cases have 3͞4 finger and 4͞5 toe syndactyly, with a duplicated digit in the syndactylous web, but incomplete penetrance and variable expressivity are common. The condition has recently been shown to be caused by expansions of an imperfect trinucleotide repeat sequence encoding a 15-residue polyalanine tract in HOXD13. We have studied 16 new and 4 previously published SPD families, with between 7 and 14 extra residues in the tract, to analyze the molecular basis for the observed variation in phenotype. Although there is no evidence of change in expansion size within families, even over six generations, there is a highly significant increase in the penetrance and severity of phenotype with increasing expansion size, affecting both hands (P ‫؍‬ 0.012) and feet (P < 0.00005). Affected individuals from a family with a 14-alanine expansion, the largest so far reported, all have a strikingly similar and unusually severe limb phenotype, involving the first digits and distal carpals. Affected males from this family also have hypospadias, not previously described in SPD, but consistent with HOXD13 expression in the developing genital tubercle. The remarkable correlation between phenotype and expansion size suggests that expansion of the tract leads to a specific gain of function in the mutant HOXD13 protein, and has interesting implications for the role of polyalanine tracts in the control of transcription.Dominantly inherited disorders frequently display incomplete penetrance (a normal phenotype in some mutation carriers) and variable expressivity (different degrees of phenotypic severity in affected individuals), the molecular basis for which is generally not understood. One such disorder is the rare dominantly inherited congenital limb malformation, synpolydactyly (SPD; OMIM No. 186000). Mutations in the first exon of HOXD13 have recently been found in three American SPD families (1), expanding a 15-residue polyalanine tract encoded by an imperfect trinucleotide repeat sequence by 7, 8, and 10 additional residues, respectively. Similar 9-residue expansions subsequently have been reported in two Turkish SPD families (2). SPD typically consists of 3͞4 finger and 4͞5 toe syndactyly, with a duplicated digit in the syndactylous web (3). Incomplete penetrance and variable expressivity both between and within affected families are common (4-6). From one to four limbs can be involved, and the severity of involvement ranges from partial skin syndactyly to complete reduplication of a digit, extending as far proximally as the metacarpals͞tarsals. Associated distal limb abnormalities include fifth-finger clinodactyly, camptodactyly, or brachydactyly; variable syndactyly of the second to fifth toes; and middle phalanx hypoplasia͞ aplasia.To investigate the molecular basis for this incomplete penetrance and variable expressivity, we analyzed the genotype and phenotype of 16 new SPD pedigrees, including one with an expansion that almost doub...