Lack of HLA class II antigen expression in microsatellite unstable colorectal carcinomas is caused by mutations in HLA class II regulatory genes

Abstract

Colorectal cancers (CRC) develop on the basis of a deficient DNA mismatch repair (MMR) system in about 15% of cases. MMR-deficient CRC lesions show high level microsatellite instability (MSI-H) and accumulate numerous mutations located at coding microsatellite loci that lead to the generation of immunogenic neopeptides. Consequently, the host's antitumoral immune response is of high importance for the course of the disease in MSI-H CRC patients. Accordingly, immune evasion mediated by impairment of HLA class I antigen presentation is frequently observed in these cancers.In the present study, we aimed at a systematic analysis of alterations affecting HLA class II antigen expression in MSI-H CRC. HLA class II antigens are expressed by only two-thirds of MSI-H CRCs. The mechanisms underlying the lack of HLA class II antigens in a subset of MSI-H CRCs still remained unknown. We here screened HLA class II-regulatory genes for the presence of coding microsatellites and identified mutations of the essential regulator genes RFX5 in 9 (26.9%) out of 34 and CIITA in 1 (2.9%) out of 34 MSI-H CRCs. RFX5 mutations were related to lack of or faint HLA class II antigen expression (p=0.006, Fisher's exact test). Transfection with wild type RFX5 was sufficient to restore interferon gamma (IFN-γ) -inducible HLA class II antigen expression in the RFX5-mutant cell line HDC108. We conclude that somatic mutations of the RFX5 gene represent a

HLA class II antigen-processing pathway in tumors: Molecular defects and clinical relevance

“…In addition, a heterogeneous intra-tumoral HLA class II antigen expression pattern was detected in HLA class II antigen-positive tumor lesions. 33 While there is concordant information in the literature about the frequency of HLA class II antigen expression in some tumor types, in others the information is conflicting. For example, the frequency of HLA class II positivity in colorectal carcinoma (CRC) reported in the literature varied between 21 and 55% depending on the study.…”

“…[34][35][36][37][38] These different results may reflect differences in the methodology and antibodies used, in the characteristics of the patients' population included in the studies and in the molecular pathogenesis of the disease. In this context, it is noteworthy that HLA class II antigen expression in CRC is closely related to the high-level microsatellite instability (MSI-H) phenotype 33,39 underlining that a precise molecular classification is required to obtain reliable information about the frequency of HLA class II antigen expression in defined tumor subtypes. 37 This conclusion is further supported by the observation in head and neck cancers, where HLA class II antigen expression is related to human papillomavirus infection of the lesions.…”

“…They appear to be mainly mediated by structural abnormalities, by epigenetic, transcriptional and post-transcriptional regulation of HLA class II molecules and/or APM components (Table 2) 42 and by genomic instability. 33 Furthermore, the function of genes encoding IFNg as well as components of the IFNg signal pathway, which regulate HLA class II antigen expression, may be impaired by multiple mechanisms. They include either gene-inactivating mutations, silencing through promoter methylation, transcriptional downregulation or post-translational alterations, such as an altered phosphorylation pattern.…”

“…Mutations in the HLA class II-regulatory gene RFX5 were found at frequency of about 30% of HLA class II antigen-negative microsatellite-unstable (MSI-H) CRC (28.9%), while CIITA mutations were only found in about 5% of these lesions. 33 Also in tumor types distinct from CRC, structural alterations of HLA class II antigen-regulatory genes have been detected. In melanoma A to G substitutions in the 5 0 flanking region of the CIITA-PIII were found, which were associated with higher levels of constitutive HLA-DR expression.…”

“…55 The hypermethylation of the CIITA promoter, in particular of CIITA-PIV was found at a high frequency in gastric cancers, HNSCC and ocular melanoma, but not in CRC suggesting tumor type-specific CIITA methylation. 33,55,56 Promoters of the HLA class II antigenencoding genes could also be directly silenced by methylation. In RFX5-negative B-lymphoma cells, the HLA-DO, HLA-DR and HLA-DQ promoters are methylated.…”