STUDIES ON MICROSATELLITE MARKERS WITHIN THE HUMAN MAJOR HISTOCOMPATIBILITY COMPLEX AND NASOPHARYNGEAL CARCINOMA

Abstract

Nasopharyngeal carcinoma (NPC) is an endemic disease in certain well-defined populations, although in most parts of the world, this disease remains rare. High frequencies have been reported in various ethnic groups, in particular, the southern Chinese, both in the southern provinces of China as well as in the other parts of the world where they have migrated to. These observations have led to the hypothesis that, apart from dietary factors and Epstein Barr virus which have been shown to be associated with the development of NPC, genetic factors also play important roles in the pathogenesis of this disease. Indeed, a linkage study based on affected sib-pairs in the Chinese population from, Singapore, Hong Kong and the southern provinces of China lend strength to this hypothesis. The study found that there exists a gene within or close to the major histocompatibility complex (MHC) region which confers a greatly increased risk of NPC. The maximum likelihood estimate of relative risk (RR) was approximately 21 and its 95% confidence interval (CI) was from 5.1 to infinity. The location and identity of this NPC disease susceptibility (DS) gene, however, has since remained unknown. The MHC region has more than four million nucleotides and contains numerous genes. To test each and every one of these genes as to their role in the development of NPC, if any at all, is unfeasible. There is hence a need to narrow down the region where the NPC DS gene may reside in. This is the main thrust of this thesis. With the use of highly polymorphic microsatellite markers that are distributed throughout the MHC region, the locus or loci which confers high relative risk of NPC can then be identified, thereby yielding information on the presence and location of the NPC DS gene. A total of 90 histologically confirmed NPC patients and 101 normal healthy controls were studied in various sections of this thesis. All subjects were Singapore Chinese. In a study involving 54 NPC patients and 58 controls, the results showed that allele 4 of D6S306 microsatellite conferred a nine-fold increase in RR of NPC (p = 0.0017, pc = 0.02, 95% Cl = 1. 9 - 41.1). The telomerically adjacent microsatellite, D6S105 also showed similar findings where allele 5 of that microsatellite was associated with an eight-fold increase in RR of NPC (p<0.001, pc<0.012, 95% Cl = 2.7 - 22.6). The results also showed that the simultaneous presence of both these microsatellite alleles conferred an infinite increase in RR of NPC (p = 0.01). The above results were shown to be reproducible in another study involving 36 NPC patients and 43 controls. D6S306 allele 4 was associated with a 12-fold increase in RR of NPC (p = 0.007, 95% Cl = I .4 - 101) whilst D6S 105 allele 5 was associated with a 4.3-fold increase in RR of NPC (p<0.01, 95% Cl = 1 .4 13 .6 ). The simultaneous presence of both these microsatellite alleles also conferred an infinite increase in RR of NPC in the second study (p = 0.02). Thus, in a total of 90 NPC patients and 101 controls, D6S306 allele 4 was shown to confer a RR of 9.9 (p = l.4x 10-5, pc = 0.00017, 95% Cl = 2.8 - 34.5) whilst D6S105 allele 5 conferred a RR of 5.5 (p<0.0005, pc<0.006, 95% Cl = 2.6 - 11.7). The combination of these two microsatellite alleles resulted in an infinite increase in RR of NPC (p = 0.0008). The increase in the frequencies of these two microsatellite alleles was also shown to be specific to NPC. A study involving a total of 67 patients with Graves' disease, rheumatoid arthritis or myasthenia gravis, which were also HLA B46 associated diseases, did not show significantly increased frequencies compared to the 101 controls. The findings of this thesis indicate that there exists a gene or genes at the telomeric end of the MHC class I region, as marked by the microsatellites D6S306 and D6S105. This thesis further suggests that more than one gene may exist within the MHC region and that these genes may act synergistically in enhancing the development of NPC.