Gastric cancers with and without high-frequency
microsatellite instability (MSI-H) represent distinctive pathways of
carcinogenesis. The aim of this study was to clarify if
human leukocyte antigen (HLA)
class I antigen subunits and antigen processing machinery (APM) components are differentially downregulated in these two groups of tumours. Using reverse transcription PCR (RT-PCR), loss of heterozygosity (LOH) analysis, methylation-specific PCR (MSP),
DNA sequencing, immunohistochemistry, and flow cytometry, we analysed expression and/or alteration of HLA
class I antigen subunits and APM components, including low molecular weight
polypeptide proteasome subunit (LMP)2, LMP7, LMP10,
transporter associated with antigen processing (TAP)1, TAP2,
tapasin,
proteasome activator (PA) 28alpha, and PA28beta in two stage-matched panels of 30 MSI-H and 30 microsatellite stable (MSS)
gastric cancers. Mutations at coding microsatellites (cMS) located within beta2-microglobulin (beta2m) and genes encoding APM components, including endoplasmic reticulum (ER) chaperone
protein genes, such as
calnexin, SEC63, SEC31, and P4HB (p55), were also analysed. HLA class Ia transcripts were totally downregulated in 18.3% of
cancer cases. Locus-specific downexpression of
HLA-A, -B, and -C was detected in 41.7%, 45.0%, and 31.7% of cases. Loss of
HLA-A was significantly more frequent in MSI-H
cancers. The LOH ratios of the
HLA-A, -B, and -C loci microsatellite markers were relatively low: 5/32 (15.6%) for D6S306, 4/32 (12.5%) for D6S258, 4/33 (12.1%) for D6S273, and 4/30 (13.3%) for D6S1666. Methylation of
HLA-A, -B, and -C was detected in 38.3%, 40.0%, and 28.3% of cases. A significant association between methylation and reduction in expression was observed in
gastric cancer tissues. Mutations at cMS of beta2m and APM components were detected in 3.3-46.7% of MSI-H
cancers but in none of MSS
cancers. These data show that
gastric cancers have various defects in HLA
class I antigen subunits and APM components and that the MSI phenotype is associated with frequent
HLA-A inactivation and frameshift mutations of the beta2m and APM genes.