Molecular Resistance Mechanisms in Multiple Myeloma
Authors: Barrio Garcia Santiago , Munawar Umair , Stuehmer Thorsten , Einsele Hermann , Kortüm K. Martin 
Published: October 2019

Abstract
Mechanisms of drug resistance in Multiple Myeloma (MM) are poorly understood. Mutations and/or changes in the protein expression of the CRBN pathway and proteasome subunits have been identified to induce resistance to IMiDs and PIs. However, only few patients are affected by these alterations. To determine the specific genomic fingerprint of MM relapse we selected 57 MM patients from the CoMMpass trial (version IA11) that have genomic data of paired samples available (diagnosis/relapse). 35 of them have also sequential FISH-seq data. We focused on acquired mutations in first relapse and filtered all mutations and genetic alterations already present at diagnosis. Doing so, we found 1.274 mutations, representing an average of 23 new mutations/patient (range; 2-76). Of interest, 66% of the acquired mutations were present in a sub-clonal level (Variant read frequency (VRF) < 25%).
Most common mutations include known hotspots of the RAS pathway (NRAS 12%, KRAS 7% and BRAF 4%). Notably, all 7 NRAS mutations in relapse were located at Q61K, suggesting a functional role of disease progression for this specific and known hotspot location. In total 5 of 35 cases (14%) with FISH-seq data developed a 17p13 deletion in relapse. Of these, three patients acquired a bi-allelic alteration in addition to a preexisting TP53 mutation and one developed a biallelic inactivation of TP53 (VRF = 100%), through parallel acquisition of del17p and TP53 mutation. Gain of 1q21 was observed in relapse in 5 of 35 (14%) cases, and one 1q gain was lost from diagnosis to relapse. Two cases (4%) presented mutations in IMiD treatment related genes, with two mutations in the CRBN pathway. One harbored a missense mutation in the Lenalidomide (LEN) degron sequence of IKZF3 (G159A) (VRF = 36%), known to be essential for the IMiD action in vitro, 45 months after continuous exposition to LEN . The other case presented two subclonal frameshift mutations in CUL4B (VRF = 5% and 32%), detected after more than three years of LEN containing therapy. We functionally validated in vitro LEN resistance through CRISPR/Cas9 knockout of CUL4B, suggesting a resistance inducing effect of the acquired CUL4B mutations. Six cases (11%) harbored acquired mutations in proteasome subunit genes (PSMC2, PSMC6, PSMD8, PSME4, PSMB9 (two mutations)), all of them had undergone prior proteasome inhibitor (PI) containing therapy. We validated earlier the 19S protein subunits PSMC6 and PSMC2 (KO and/or point mutations) as inducers of PI resistance in vitro, thus we hypothesize contribution to resistance induction / disease progression through these 19s mutations.
Remarkably ubiquitin (E3, E2 and SUBs) and histone related genes (histones and histone methylases and deacetylases) were found mutated in 51% and 19% of the relapsed patients. Genes for drug transporters (ATP-binding cassette (ABC) and Solute Carrier (SLC) transporters) were hit in 32% of cases and genes for mucins (previously related with genotoxic agents and immunotherapy resistance) in 19%. Notably, RRBP1 presented 10 mutations in 6 patients (11%) with the mutations clustering within 30 amino-acids (aa) of exons 9 and 10 and 3 hotspots (2 patients each) in aa Q426P, K430R and Q436P. RRBP1 is involved in the binding of the ribosome to the endoplasmic reticulum (ER) and is related with the unfolded protein response and ER stress via GRP78. All the patients with RRBP1 mutations were pretreated with PI inhibitors and exhibited worst survival outcome affecting PFS (Pval<0.001) and OS (Pval=0.0016) in this limited dataset. The mutations were detected on average 433 days (range: 258-568) after diagnosis. Five of the 6 patients died on average 180 days after RRBP1 mutation detection (range: 18-446) further suggesting high risk features of such acquired mutations.
In summary, we observe clonal selection of known high-risk related alterations like TP53 mutations, 17p deletions or 1q13 in early relapse data of the CoMMpass trial. Furthermore we identify RRBP1 mutations as a new acquired high-risk biomarker of MM. Alterations are specifically related to subclonal selection by therapy, thus we suggest that the definition of high-risk disease in MM needs to be revisited and should also include clonal selection processes under anti-tumor therapy.
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Disclosures
No relevant conflicts of interest to declare.