A Functional Promoter Polymorphism (−8C>G) of the Proteasome Subunit α6 Gene Is Associated with Multiple Myeloma and Poor Outcome Independent of Circulating Proteasome Serum Levels.

Introduction: The proteasome system plays a crucial role in several malignant diseases, especially in multiple myeloma (MM). Recently, the serum 20S circulating proteasome level (CPL) was shown to be an independent prognostic factor in MM.

A single nucleotide polymorphism (SNP) −8C>G in PSMA6, one of seven α-subunit genes of the 20S proteasome, was currently demonstrated to be associated with myocardial infarction. Additionally, it has been shown that PSMA6 expression is genotype-dependently altered e.g. in human B cells, whereas the G allele is associated with a 1.8 fold higher expression. Demonstrating the extensive role of the proteasome system in MM we investigated the role of the novel SNP in our cohort of 116 patients with MM.

Methods: DNA-samples of 116 patients with MM, all treated at the University Hospital Essen, and 125 healthy controls were genotyped for PSMA6 −8C>G. CPL of 70 patients were studied by an anti-20S proteasome enzyme-linked immunoabsorbant assay (ELISA). PSMA6 −8C>G genotypes were correlated with patients’ survival and CPL.

Results: Patients’ genotype distribution (69 CC, 44 CG, 3 GG) and genotype distribution of healthy controls (90 CC, 31 CG, 4 GG) were consistent with Hardy-Weinberg equilibrium. Genotypes were significantly associated with MM in a dominant genetic model (CC vs. CG+GG), with an odds ratio of 1.75 (95% confidence interval (CI): 1.02–3.00, p=0.043). Kaplan-Meier curves revealed a significant association of PSMA6 −8C>G with 5-year survival (p=0.014). Median survival time was 43 months for the GG genotype and 50 months for the CG genotype. It was not reached within follow-up by the CC genotype (CC 5-year survival rate 61.2%). Following hazard ratios (HR) were calculated: CC vs. CG: 2.007, 95%CI 1.11–3.63, p=0.022; CC vs. GG: 2.515, 95%CI 0.58–10.86, p=0.217 and in the dominant genetic model CC vs. CG+GG: 2.038, 95%CI 1.14–3.65, p=0.017. In multivariate analysis the GG/GC genotypes were independent prognostic factors (HR 2.1, p=0.014).

To proove if the detected effect of individual PSMA6 genotypes was dependent or independent from CPL, ELISA experiments were performed. There was no detectable difference in CPL between the genotypes. Mean CPL was 255 ng/mL for CC homozygous and 205 ng/mL for G allele carriers (p=0.718).

Conclusions: These results suggest the PSMA6 −8C>A polymorphism as a survival prognosticator as well as indicator of a high risk group within patients with MM. PSMA6 genotypes were not associated with CPL. Therefore, the SNP is independent of this known prognostic factor and could lead to additional prognostic information for MM patients.