RPL5 Is a Candidate Tumor Suppressor on 1p22.1 in Multiple Myeloma of Which the Expression Is Linked to Bortezomib Response

Chromosomal region 1p22 is deleted in ~20% of multiple myeloma patients, suggesting the presence of an unidentified tumor suppressor gene in this region. Using high resolution copy number arrays, we delimit a 58 kb minimal deleted region on 1p22.1 encompassing two genes: ectopic viral integration site 5 (EVI5) and ribosomal protein L5 (RPL5). Although mutations in 1p22 genes are rare in multiple myeloma, the tumor suppressor role of EVI5 and RPL5 may be supported by the fact that these genes show the highest frequency of mutations predicted to impair protein function on 1p22. Interestingly, inactivation of RPL5 was also recently described in T-cell acute lymphoblastic leukemia (T-ALL) and glioblastoma.

We found that 1p22 deleted patients have significantly lower levels of EVI5 and RPL5 mRNA expression (59% and 71% residual expression respectively; p<0.0001 for each gene). Whereas 1p22 deletion status correlates well with EVI5 expression in all patients, it is a bad predictor of RPL5 expression level in some cases, suggesting that other mechanisms besides 1p22 deletion can downregulate RPL5 expression in multiple myeloma. Deletion of 1p22 has been associated with decreased progression free (PFS) and overall survival (OS) in newly diagnosed multiple myeloma (Hebraud et al., Leukemia, 2013). We saw that low mRNA expression of EVI5 and RPL5 also correlates with worse survival in diagnosis but not in relapse cases.

Interestingly, RPL5 but not EVI5 mRNA expression levels were significantly lower in multiple myeloma patients responding to bortezomib compared to patients that are not responding (68% residual RPL5 expression in responders versus non-responders). The clinical relevance of these findings is illustrated by the observation that PFS of newly diagnosed patients with low RPL5 expression was significantly longer when they were treated on a bortezomib containing protocol (median PFS bortezomib protocol 29.8 months versus 18.7 months for non-bortezomib, p=0.03, data phase III HOVON-65/ GMMG-HD4 trial). In contrast, PFS of RPL5 high patients was not influenced by bortezomib. These associations between RPL5 expression level and bortezomib response were confirmed when performing the same analyses on the PFS data from relapse patients in the APEX trial. For low EVI5 expressing patients, there was a non-significant trend towards benefit from bortezomib which was not present in the EVI5 high cases. Taken together, these data suggest that RPL5 expression levels are a better biomarker for bortezomib response than 1p22 deletion.

In conclusion, we provide genetic data that narrow down the list of candidate tumor suppressors on 1p22 to EVI5 and RPL5. Although the exact role of these genes in suppressing multiple myeloma disease progression remains to be determined, we identify low RPL5 expression levels in multiple myeloma as a novel, clinically relevant biomarker associated with initial response and survival benefit upon treatment with bortezomib.