Orientation of the Hoxa complex and placement of the Hd locus distal to Hoxa2 on mouse chromosome 6

Polyalanine expansion in HOXA13: three new affected families and the molecular consequences in a mouse model

Polyalanine expansions in two of three large imperfect trinucleotide repeats encoded by the first exon of HOXA13 have been reported in hand-foot-genital syndrome (HFGS). Here we report additional families with expansions in the third repeat of 11 and 12 alanine residues, the latter being the largest expansion reported. We also report a patient with a novel, de novo 8-alanine expansion in the first large repeat. Thus, expansions in all three large HOXA13 polyalanine repeats can cause HFGS. To determine the molecular basis for impaired HOXA13 function, we performed homologous recombination in ES cells in mice to expand the size of the third largest polyalanine tract by 10 residues (HOXA13(ALA28)). Mutant mice were indistinguishable from Hoxa13 null mice. Mutant limb buds had normal steady-state Hoxa13 RNA expression, normal mRNA splicing and reduced levels of steady-state protein. In vitro translation efficiency of the HOXA13(ALA28) protein was normal. Thus, loss of function is secondary to a reduction in the in vivo abundance of the expanded protein likely due to degradation.

Human HOX gene mutations

HOX genes play a fundamental role in the development of the vertebrate central nervous system, axial skeleton, limbs, gut, urogenital tract and external genitalia, but it is only in the last 4 years that mutations in two of the 39 human HOX genes have been shown to cause congenital malformations; HOXD13, which is mutated in synpolydactyly, and HOXA13, which is mutated in Hand-Foot-Genital syndrome. Here we review the mutations already identified in these two genes, consider how these mutations may act, and discuss the possibility that further mutations remain to be discovered both in developmental disorders and in cancer.

The molecular basis of hypodactyly (Hd): a deletion in Hoxa 13 leads to arrest of digital arch formation

Hypodactyly (Hd) is a semidominant mutation in mice that maps in a genetic interval overlapping the Hoxa cluster. The profound deficiency of digital arch structures in Hd/Hd mice is consistent with a defect in a gene activated late in limb morphogenesis. We have determined the structure of the Hoxa13 gene and describe a 50-base pair deletion in the first exon of the Hd allele that probably arose from unequal recombination or misalignment between triplet repeats. It is predicted that no Hoxa13 protein is made from Hd mRNA. The hypodactyly limb phenotype is similar to that of Hoxd13-deficient mice in sharing defects along multiple axes and alterations in cartilage maturation; however, the overall effects on digital arch formation are more severe in Hd/Hd mice. Our results confirm the critical role of AbdB-like Hox genes in the development of the autopod, and add to the spectrum of mutations involving triplet repeats.

An efficient method for isolating putative promoters and 5'-transcribed sequences from large genomic clones

Efficient strategies to isolate promoters and flanking exons from large genomic clones would facilitate the assembly of transcription units, complement existing techniques to isolate expressed sequences, and provide 5' regulatory elements. We have developed a rapid and simple method to isolate promoters from large mammalian genomic DNA clones by exploiting the abundance of binding sites for the ubiquitous transcription factor Sp1 in gene promoters. Using this method, putative promoter sequences with Sp1-binding sites are enriched approximately 100-fold from fragmented P1 clone DNA. Based on the abundance of Sp1-binding motifs in promoters, we predict that a significant subset of vertebrate promoters could be isolated by this method.

Cellular analysis of limb development in the mouse mutant hypodactyly

The limb defect in the mouse Hypodactyly (Hd) affects only the distal structures. Heterozygotes (Hd/+) lack all or part of the distal phalanx and the terminal claw of digit on the hindlimbs; mice homozygous (Hd/Hd) for the mutation have just one digit on each of the four limbs. Early limb development in the mutant appears normal and a change in morphology can only be detected later. Limb buds of Hd/+ and Hd/Hd embryos become reduced in width, with Hd/Hd buds becoming very pointed instead of rounded. This change in bud shape is correlated with an increase in cell death anteriorly in Hd/+ hindlimbs and both anteriorly and posteriorly in Hd/Hd fore- and hindlimb buds. The apical ectodermal ridge is very pronounced in pointed Hd/Hd limb buds. Mesenchyme cells from the Hd/Hd mutant in culture show a cell-autonomous change in behaviour and less cartilage differentiates.