PMS2 Mutations in Childhood Cancer

Until recently, the PMS2 DNA mismatch repair gene has only rarely been implicated as a cancer susceptibility locus. New studies have shown, however, that earlier analyses of this gene have had technical limitations and also that the genetic behavior of mutant PMS2 alleles is unusual, in that, unlike MLH1 or MSH2 mutations, PMS2 mutations show low heterozygote penetrance. As a result, a dominantly inherited cancer predisposition has not been a feature reported in families with PMS2 mutations. Such families have instead been ascertained through childhood-onset cancers in homozygotes or through apparently sporadic colorectal cancer in heterozygotes. We present further information on the phenotype associated with homozygous PMS2 deficiency in 13 patients from six families of Pakistani origin living in the United Kingdom. This syndrome is characterized by café-au-lait skin pigmentation and a characteristic tumor spectrum, including leukemias, lymphomas, cerebral malignancies (such as supratentorial primitive neuroectodermal tumors, astrocytomas, and glioblastomas), and colorectal neoplasia with an onset in early adult life. We present evidence for a founder effect in five families, all of which carried the same R802-->X mutation (i.e., arginine-802 to stop) in PMS2. This cancer syndrome can be mistaken for neurofibromatosis type 1, with important management implications including the risk of the disorder occurring in siblings and the likelihood of tumor development in affected individuals.

An I47L substitution in the HOXD13 homeodomain causes a novel human limb malformation by producing a selective loss of function

The 5' members of the Hoxa and Hoxd gene clusters play major roles in vertebrate limb development. One such gene, HOXD13, is mutated in the human limb malformation syndrome synpolydactyly. Both polyalanine tract expansions and frameshifting deletions in HOXD13 cause similar forms of this condition, but it remains unclear whether other kinds of HOXD13 mutations could produce different phenotypes. We describe a six-generation family in which a novel combination of brachydactyly and central polydactyly co-segregates with a missense mutation that substitutes leucine for isoleucine at position 47 of the HOXD13 homeodomain. We compared the HOXD13(I47L) mutant protein both in vitro and in vivo to the wild-type protein and to an artificial HOXD13 mutant, HOXD13(IQN), which is completely unable to bind DNA. We found that the mutation causes neither a dominant-negative effect nor a gain of function, but instead impairs DNA binding at some sites bound by wild-type HOXD13. Using retrovirus-mediated misexpression in developing chick limbs, we showed that wild-type HOXD13 could upregulate chick EphA7 in the autopod, but that HOXD13(I47L) could not. In the zeugopod, however, HOXD13(I47L) produced striking changes in tibial morphology and ectopic cartilages, which were never produced by HOXD13(IQN), consistent with a selective rather than generalised loss of function. Thus, a mutant HOX protein that recognises only a subset of sites recognised by the wild-type protein causes a novel human malformation, pointing to a hitherto undescribed mechanism by which missense mutations in transcription factors can generate unexpected phenotypes. Intriguingly, both HOXD13(I47L) and HOXD13(IQN) produced more severe shortening in proximal limb regions than did wild-type HOXD13, suggesting that functional suppression of anterior Hox genes by more posterior ones does not require DNA binding and is mediated by protein:protein interactions.