Heterozygous individuals bearing a founder mutation in the XPA DNA repair gene comprise nearly 1% of the Japanese population

Abstract

Individuals who are homozygotes for mutations in DNA repair genes are at high risk for cancer. It is not well documented, however, if the heterozygous carriers of the mutation are also predisposed to cancer. To address the issue, xeroderma pigmentosum (XP) in Japan is an interesting candidate because of three major reasons: XP is an autosomal recessive disorder with an enormously elevated risk of skin cancer, the frequency of XP patients is higher in Japan than in other parts of the world, and more than half of Japanese XP patients are homozygous for the same founder mutation in the XPA gene. We screened archival blood samples from Japanese individuals who resided in Hiroshima or Nagasaki. A simple PCR–RFLP method was developed that is highly specific for detection of XPA heterozygotes carrying the founder mutation. We identified nine XPA heterozygotes among 1020 individuals screened for a prevalence of 0.88%. This rate, if representative, implies that there are about 1 million carriers of the XPA founder mutation in the Japanese population. Thus, investigation of their cancer risk may be warranted.

Introduction

Xeroderma pigmentosum (XP) is a rare autosomal recessive disorder, and cells from patients are hypersensitive to the killing effect of ultraviolet light (UV) [1]. In addition, patients are characterized by a roughly 1000-fold higher risk of developing skin cancers [2]. The underlying mechanism is defective repair of DNA damage inflicted by exposure to the UVB component contained in sunlight [3]. Eight genes, denoted as XPA, XPB, …, XPG, and pol η are known to be responsible for the development of XP. XPA–XPG proteins work in a coordinate manner during the process of nucleotide excision repair (NER) of UV-induced DNA damage. Namely, the XPA protein is recognized as being involved in damage verification (gene location: 9q22.3), XPB as a 3′–5′ helicase (2q21), XPC as a damage binding (3p25), XPD as a 5′–3′ helicase (19q13.2), XPE as a damage binding (E3 ligase, 11p11–12), XPF as a 5′ nuclease (16p13.3), and XPG as a 3′ nuclease (13q32–33). In contrast, patients with the XP variant form are nearly normal in NER but defective in post-replication repair due to mutation in the DNA polymerase η which is a translesional DNA polymerase (6p21) [3].

In Japan, the overall frequency of XP patients is estimated as 1 per 40,000–100,000 newborns [4], which appears to be more than 10 times higher than that in Western countries (about 1 per million, based on observations of 100–200 XP patients among the 250 million people in the USA) [5]. Interestingly, nearly 60% of Japanese XP patients belong to group A (XPA, OMIM 278700), clinically the severest form of XP, and this proportion is nearly twice that seen in other countries (e.g., 30%) [5], [6]. Further, more than 90% of the mutant XPA alleles in Japanese XPA patients bear an identical G to C base-change mutation [5]. This founder mutation at the 3′ splice acceptor site of intron 3 results in no detectable protein production [7] and creates a sensitive site within the restriction enzyme AlwNI recognition sequence (denoted as the AlwNI mutation) [8], [9] (Fig. 1a).

In a study published before identification of the underlying genes, clinically normal appearing relatives of XP patients were reported to have increased cancer risk [10]. As a first step toward a genetic epidemiologic assessment of cancer risks in XP heterozygotes, we developed a simple PCR–RFLP method to identify the XPA founder mutation heterozygotes. We screened about 1000 Japanese individuals in the general population and collected direct information on the frequency of the XPA heterozygotes.