Head and neck squamous cell carcinoma susceptibility genes in the HLA region

Abstract

The Human Leukocyte Antigen (HLA) region on the short arm of chromosome 6 includes the classical HLA genes and in addition HLA-related and non-HLA related genes. The majority of the genes located in this region are directly or indirectly involved in the immune response. The polymorphic HLA molecules, encoded by the HLA genes, bind endogenous and exogenous peptides. Stable bound peptides are recognized by CD4+ and CD8+ T-cells, which elicit a humoral or cytotoxic immune response to pathogens. The immunological function of HLA molecules in transplantation has been widely studied. The role of HLA in autoimmune disease and cancer however is less obvious. In head and neck squamous cell carcinoma (HNSCC) aberrant HLA expression patterns have been observed on primary tumour cells and lymph node metastases, suggesting an aberrant immune response. Whether the aberrant expression resides in the HLA genes themselves or is as a result of other genes within the HLA region is poorly understood. This thesis describes how the HLA region on the short arm of chromosome 6 is associated with the pathogenesis of HNSCC.

Associations with particular HLA-B alleles and HLA-B-DRB1 haplotypes were identified. However, since few patients carried these alleles and haplotype, this could not be the primary risk factor for the pathogenesis of HNSCC. The MICA gene is in linkage disequilibrium with HLA-B and involved in the innate and adaptive immune system. Allelic polymorphism for the MICA gene was evaluated with a refined method for determination of MICA allele polymorphism based upon the combination of nucleotide polymorphisms in exons 2, 3 and 4 and repeat polymorphism in exon 5. No differences in MICA allelic polymorphism were identified between patients and controls. Repeat length polymorphism, present in the MICA transmembrane region and encoded by exon 5, revealed a significant decrease in the frequency for the MICA-A9 repeat in the patients. Analysis of MICA protein expression indicated that the majority of tumours expressed MICA. The expression of MICA was not correlated with the length of the repeat present in the transmembrane region. To evaluate the role of non-HLA related genes, located within the HLA region and which confer susceptibility to HNSCC, disease-associated regions were defined using microsatellite analysis in pooled DNA analysis. Microsatellite analysis revealed three associated regions, one in the class I region and two in the class II region. From the associated regions 18 genes were selected and evaluated for their RNA expression in tumour tissue compared to surrounding healthy tissue. MICA RNA expression was significantly increased in tumour tissue while a significant decrease in RNA expression was observed for hydroxysteroid (17-beta) dehydrogenase 8 (HSD17B8) in tumour tissue. For retinoid X receptor β (RXRß) and NOTCH4 a decreased, but non-significant, RNA expression in tumour tissue was observed. These results show that apart from HLA genes also other genes in the HLA region are associated with the pathogenesis of HNSCC.