Human Molecular Genetics Advance Access published online on March 25, 2008
Human Molecular Genetics, 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, Radboud University Nijmegen Medical Centre, 6500 HB Nijmegen, The Netherlands 2 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 3 Laboratory of Skin Biology and Experimental Dermatology, Radboud University Nijmegen Medical Centre, 6500 HB Nijmegen, The Netherlands 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, WC1 N 3EH, UK 6 Genetic Health Services Victoria, Murdoch Children's Research Institute, Royal Children's Hospital, Victoria 3052, Australia
* Correspondence author: Dr. Hans van Bokhoven Department of Human Genetics 588 Radboud University Nijmegen Medical Centre P.O. Box 9101, 6500 HB Nijmegen The Netherlands Tel. +31 (0)24 361 6696 Fax. +31 (0)24 366 8752 Email: h.vanbokhoven{at}antrg.umcn.nl
Received January 28, 2008; Revised March 19, 2008; Accepted March 19, 2008
Missense mutations in the 3’ end of the p63 gene are associated with either RHS or AEC 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 to 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.