GENOMIC STRUCTURE OF HOXD13 GENE - A 9-POLYALANINE DUPLICATION CAUSESSYNPOLYDACTYLY IN 2 UNRELATED FAMILIES

Synpolydactyly (SPD) is a limb malformation that shows a characteristi c manifestation in both hands and feet. This condition is inherited as an autosomal dominant trait with reduced penetrance. We have recently mapped this locus centromeric to the HOXD8 intragenic marker and sugg ested the HOXD13 gene as a potential candidate for this condition. The genomic structure of HOXD13 established in this study consists of two exons that encodes a polypeptide of 335 amino acids. The downstream e xon at the 3' end of this gene contains the homeodomain sequences that are highly conserved. Sixty-three bp upstream of this exon lies a str etch of intronic CA-repeats that proved to be polymorphic in two diffe rent populations. The upstream exon encodes 75% of the entire protein and contains a stretch of 15 normal alanines at its 5' end, Sequence c omparison at this position in the homozygous affected individuals iden tified a total of 24 alanine residues that resulted from a duplication of nine polyalanines. In two unrelated SPD families, this duplication was directly transmitted from the affected parents to their affected, but not unaffected, offspring; in one family its size has remained co nstant for at least 150 years spanning over seven generations. The pre sence of this duplication confirmed the status of four normal gene car riers, one incomplete penetrance and two affected individuals who were recombinants for HOXD8 or HOXD13-CA repeat markers, This duplication was not present in 150 chromosomes of unrelated healthy subjects of tw o different populations.