Analysis of De Novo HOXA13 Polyalanine Expansions Supports Replication Slippage Without Repair in Their Generation

Department of Pediatrics and Communicable Diseases, University of Michigan, Ann Arbor, Michigan.
American Journal of Medical Genetics Part A (Impact Factor: 2.16). 05/2013; 161(5). DOI: 10.1002/ajmg.a.35843
Source: PubMed


Polyalanine repeat expansion diseases are hypothesized to result from unequal chromosomal recombination, yet mechanistic studies are lacking. We identified two de novo cases of hand-foot-genital syndrome (HFGS) associated with polyalanine expansions in HOXA13 that afforded rare opportunities to investigate the mechanism. The first patient with HFGS was heterozygous for a de novo nine codon polyalanine expansion. Haplotype investigation showed that the expansion arose on the maternally inherited chromosome but not through unequal crossing over between homologs, leaving unequal sister chromatid exchange during mitosis or meiosis or slipped mispairing as possible explanations. The asymptomatic father of the second patient with HFGS was mosaic for a six codon polyalanine expansion. Multiple tissue PCR and clonal analysis of paternal fibroblasts showed only expansion/WT and WT/WT clones, and haplotype data showed that two unaffected offspring inherited the same paternal allele without the expansion, supporting a postzygotic origin. Absence of the contracted allele in the mosaic father does not support sister chromatid exchange in the origin of the expansion. Mosaicism for HOXA13 polyalanine expansions may be associated with a normal phenotype, making examination of parental DNA essential in apparently de novo HFGS cases to predict accurate recurrence risks. We could not find an example in the literature where unequal sister chromatid exchange has been proven for any polyalanine expansion, suggesting that the principal mechanism for polyalanine expansions (and contractions) is slipped mispairing without repair or that the true frequency of unequal sister chromatid exchange involving these repeats is low. © 2013 Wiley Periodicals, Inc.

Download full-text


Available from: Herman Scott Stadler, Dec 04, 2015
  • Source
    • "We have found that the absolute percent occurrence of polyQ and polyA proteins in different species is quite variable, as previously observed in the literature (18,46), with a weak correlation with phylogenetic distances. Amino acid repeats in proteins originate by DNA triplet repeat expansions caused by replication slippage and/or unequal crossing-over (47,48). Species-specific differences in the frequency of these phenomena may contribute to the differences in the overall occurrence of repeats in different proteomes that we and others have observed (49,50). "
    [Show abstract] [Hide abstract]
    ABSTRACT: The expansion of homopolymeric glutamine (polyQ) or alanine (polyA) repeats in certain proteins due to genetic mutations induces protein aggregation and toxicity, causing at least eighteen human diseases. PolyQ and polyA repeats can also associate in the same proteins, but the general extent of their association in proteomes is unknown. Furthermore, the structural mechanisms by which their expansion causes disease are not well understood, and these repeats are generally thought to misfold upon expansion into aggregation-prone β-sheet structures like amyloids. However, recent evidence indicates a critical role for coiled-coil (CC) structures in triggering aggregation and toxicity of polyQ-expanded proteins, raising the possibility that polyA repeats may as well form these structures, by themselves or in association with polyQ. We found through bioinformatics screenings that polyA, polyQ, and polyQA repeats have a phylogenetically graded association in human and non-human proteomes, and associate/overlap with CC domains. Circular dichroism and cross-linking experiments revealed that polyA repeats can form - alone or with polyQ and polyQA - CC structures that increase in stability with polyA length, forming higher-order multimers and polymers in vitro. Using structure-guided mutagenesis, we studied the relevance of polyA CCs to the in vivo aggregation and toxicity of RUNX2 - a polyQ/polyA protein associated with cleidocranial dysplasia upon polyA expansion - and found that the stability of its polyQ/polyA CC controls its aggregation, localization, and toxicity. These findings indicate that, like polyQ, polyA repeats form CC structures that can trigger protein aggregation and toxicity upon expansion in human genetic diseases.
    Full-text · Article · Feb 2014 · Human Molecular Genetics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Hox genes are an evolutionarily conserved family of genes, which encode a class of important transcription factors that function in numerous developmental processes. Following their initial discovery, a substantial amount of information has been gained regarding the roles Hox genes play in various physiologic and pathologic processes. These processes range from a central role in anterior-posterior patterning of the developing embryo to roles in oncogenesis that are yet to be fully elucidated. In vertebrates there are a total of 39 Hox genes divided into 4 separate clusters. Of these, mutations in 10 Hox genes have been found to cause human disorders with significant variation in their inheritance patterns, penetrance, expressivity and mechanism of pathogenesis. This review aims to describe the various phenotypes caused by germline mutation in these 10 Hox genes that cause a human phenotype, with specific emphasis paid to the genotypic and phenotypic differences between allelic disorders. As clinical whole exome and genome sequencing is increasingly utilized in the future, we predict that additional Hox gene mutations will likely be identified to cause distinct human phenotypes. As the known human phenotypes closely resemble gene-specific murine models, we also review the homozygous loss-of-function mouse phenotypes for the 29 Hox genes without a known human disease. This review will aid clinicians in identifying and caring for patients affected with a known Hox gene disorder and help recognize the potential for novel mutations in patients with phenotypes informed by mouse knockout studies.
    Full-text · Article · Oct 2013 · Molecular Genetics and Metabolism