Human Molecular Genetics Advance Access originally published online on March 25, 2008
Human Molecular Genetics 2008 17(13):1968-1977; doi:10.1093/hmg/ddn094
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A novel translation re-initiation mechanism for the p63 gene revealed by amino-terminal truncating mutations in Rapp-Hodgkin/Hay-Wells-like syndromes
1 Department of Human Genetics 2 Laboratory of Skin Biology and Experimental Dermatology, Radboud University Nijmegen Medical Centre, 6500 HB Nijmegen, The Netherlands 3 St John's Institute of Dermatology, Division of Genetics and Molecular Medicine, The Guy's King's College and St Thomas' Hospitals' School of Medicine, London SE1 9RT, UK 4 Department of Cancer Biology and Genetics, Memorial Sloan-Kettering Cancer Center, NY 10065, USA 5 Department of Clinical Genetics, Great Ormond Street Hospital for Children, London WC1N 3EH, UK 6 Genetic Health Services Victoria, Murdoch Children's Research Institute, Royal Children's Hospital, Victoria 3052, Australia
* To whom correspondence should be addressed at: Department of Human Genetics 588, Radboud University Nijmegen Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands. Tel: +31 243616696; Fax: +31 243668752; Email: h.vanbokhoven{at}antrg.umcn.nl
Received January 28, 2008; Accepted March 19, 2008
Missense mutations in the 3' end of the p63 gene are associated with either RHS (Rapp-Hodgkin syndrome) or AEC (Ankyloblepharon Ectodermal defects Cleft lip/palate) syndrome. These mutations give rise to mutant p63
protein isoforms with dominant effects towards their wild-type counterparts. Here we report four RHS/AEC-like patients with mutations (p.Gln9fsX23, p.Gln11X, p.Gln16X), that introduce premature termination codons in the N-terminal part of the p63 protein. These mutations appear to be incompatible with the current paradigms of dominant-negative/gain-of-function outcomes for other p63 mutations. Moreover it is difficult to envisage how the remaining small N-terminal polypeptide contributes to a dominant disease mechanism. Primary keratinocytes from a patient containing the p.Gln11X mutation revealed a normal and aberrant p63-related protein that was just slightly smaller than the wild-type p63. We show that the smaller p63 protein is produced by translation re-initiation at the next downstream methionine, causing truncation of a non-canonical transactivation domain in the 
N-specific isoforms. Interestingly, this new Np63 isoform is also present in the wild-type keratinocytes albeit in small amounts compared with the p.Gln11X patient. These data establish that the p.Gln11X-mutation does not represent a null-allele leading to haploinsufficiency, but instead gives rise to a truncated Np63 protein with dominant effects. Given the nature of other RHS/AEC-like syndrome mutations, we conclude that these mutations affect only the Np63 isoform and that this disruption is fundamental to explaining the clinical characteristics of these particular ectodermal dysplasia syndromes.