HUMAN MUTATION 27(11), 1092^1103, 2006 RESEARCH ARTICLE Phenotypic Heterogeneity in the XPB DNA Helicase Gene (ERCC3): Xeroderma Pigmentosum Without and With Cockayne Syndrome Kyu-Seon Oh, 1 Sikandar G. Khan, 1 N.G.J. Jaspers, 2 Anja Raams, 2 Takahiro Ueda, 1 Alan Lehmann, 3 Peter S. Friedmann, 4 Steffen Emmert, 5 Alexi Gratchev, 6 Katherine Lachlan, 7 Anneke Lucassan, 7 Carl C. Baker, 8 and Kenneth H. Kraemer 1Ã 1 DNA Repair Section, Basic Research Laboratory (BRL), Center for Cancer Research (CCR), National Cancer Institute (NCI), Bethesda, Maryland; 2 Department of Genetics, Medical Genetic Cluster, Erasmus University, Rotterdam, The Netherlands; 3 Genome Damage and Stability Centre, University of Sussex, Falmer, Brighton, United Kingdom; 4 Department of Dermatology, Southampton University, Southampton, United Kingdom; 5 Department of Dermatology, Goettingen University, Goettingen, Germany; 6 Department of Dermatology, Heidelberg University, Mannheim, Germany; 7 Wessex Regional Genetics Service, Southampton, United Kingdom; 8 Laboratory of Cellular Oncology, CCR, NCI, Bethesda, Maryland Communicated by Michael Dean Defects in the xeroderma pigmentosum type B (XPB) gene (ERCC3), a DNA helicase involved in nucleotide excision repair (NER) and an essential subunit of the basal transcription factor, TFIIH, have been described in only three families. We report three new XPB families: one has two sisters with relatively mild xeroderma pigmentosum (XP) symptoms not previously associated with XPB mutations and two have severe XP/Cockayne syndrome (CS) complex symptoms. All XP-B cells had reduced NER and post-ultraviolet (UV) cell viability. Surprisingly, cells from the milder XP sisters had the same missense mutation (c.296T4C, p.F99S) that was previously reported in two mild XP/CS complex brothers. These cells had higher levels of XPB protein than the severely affected XP/CS complex patients. An XPB expression vector with the p.F99S mutation partially complemented the NER defect in XP-B cells. The three severely affected XP/CS complex families all have the same splice acceptor site mutation (c.2218–6C4A, p.Q739insX42) in one allele. This resulted in alteration of 41 amino acids at the C terminus, producing partial NER complementation. This limited number of mutations probably reflects the very restricted range of alterations of this vital protein that are compatible with life. We found new mutations in the second allele yielding markedly truncated proteins in all five XP or XP/CS complex families: c.1273C4T, p.R425X; c.47111G4A, p.K157insTSDSX; c.807–808delTT, p.F270X; c.1421–1422insA, p.D474EfsX475; and c.1633C4T, p.Q545X. The remarkable phenotypic heterogeneity of XPB is associated with partially active missense mutations in milder patients while severe XP/CS complex patients have nonsense mutations in both alleles with low levels of altered XPB proteins. Hum Mutat 27(11), 1092–1103, 2006. Published 2006 Wiley-Liss, Inc. y KEY WORDS: DNA repair; TFIIH; transcription; neurodegeneration; skin cancer; XPB; ERCC3 INTRODUCTION The genome is continually exposed to agents that cause damage to its structure, resulting in the loss of vital genetic information. Nucleotide excision repair (NER) is the main cellular repair pathway by which a wide range of DNA lesions, including ultraviolet (UV)-induced photoproducts as well as bulky chemical adducts, are eliminated from the genome [Bootsma et al., 2002]. The human xeroderma pigmentosum type B (XPB) XPB (ERCC3; MIM] 133510) and xeroderma pigmentosum type D (XPD) (ERCC2: MIM] 126340) genes, DNA helicases, are components of the basal transcription factor TFIIH (see GTF2H1; MIM] 189972) and are also involved in the early steps of NER. TFIIH is composed of a 10-subunit protein complex that opens the DNA in the context of transcription initiation and NER [Egly, 2001; Kraemer, 2004; Giglia-Mari et al., 2004]. The 3 0 to 5 0 helicase activity of XPB is essential for both transcription and repair, Published online 31 August 2006 in Wiley InterScience (www.inters- cience.wiley.com). DOI 10.1002/humu.20392 The Supplementary Material referred to in this article can be accessed at http://www.interscience.wiley.com/jpages/1059-7794/ suppmat. Received 20 March 2006; accepted revised manuscript 26 May 2006. Grant sponsor: DFG (Deutsche Forschungsgemeinschaft); Grant number: EM 63/3 -1. y This article is a US Government work, and, as such, is in the public domain in the United States of America. Ã Correspondence to: Kenneth H. Kraemer, M.D., Chief, DNA Repair Section, Basic Research Laboratory, National Cancer Institute, Building 37 Room 4002, Bethesda, MD 20892-4258. E-mail: kraemerk@nih.gov PUBLISHED 2006 WILEY-LISS, INC.