A Small Molecule That Selectively Targets BLM Helicase Has a Therapeutic Interest in Multiple Myeloma

Hematological malignancies, including MM, are characterized by genomic instability that is possibly related to underlying defects in DNA repair or genomic stability maintenance. RECQ helicases are at the crossroad between replication, recombination, DNA repair and transcription and could represent potent therapeutic targets for cancer therapy.

The Bloom's syndrome helicase, BLM, has DNA annealing and unwinding activities. Through its interaction with TOPOIIIα, BLM unwinds the short stretches of naked duplex DNA and processes homologous recombination (HR) intermediates containing a double holiday junction. It may suppress hyper-sister chromatid exchange (SCE) by disruption of D-loop recombination intermediates and also might be involved in the suppression of crossing over during homology mediated recombination. This helicase facilitates telomere replication by resolving G4 structures. BLM deregulation is also associated with cancer.

We recently identified a significant overexpression of BLM in malignant plasma cells compared to normal bone marrow plasma cells. Since BLM, plays important roles in the maintenance of chromosomal stability, we investigated if BLM could play a role in MM pathophysiology and drug resistance.

A high BLM expression in MMCs could predict for shorter overall survival (OS) in four independent cohorts of patients (P = 0.003 in the HM cohort (N=206), P = 0.0002 in the UAMS-TT2 cohort (N=345), P = 0.0008 in TT3 cohort (N=186)) and P = 0.04 in Hovon cohort (N=282). A high BLM expression in MMCs could also predict for shorter event free (EFS) survival in the HM and UAMS-TT2 cohorts. BLM expression was significantly higher in the poor prognosis proliferation MM subgroup (P < 0.05). Moreover, we found that BLM was overexpressed in HMCLs (median 1848, range 246 - 10901) compared to primary MMCs or BMPCs (P< .001).

The small molecule, ML216, is a membrane permeable selective inhibitor of BLM helicase. ML216 appears to act at the BLM-nucleic acid substrate binding site, inhibiting DNA binding and blocking BLM's helicase activity.

We investigated the interest of the BLM inhibitor, ML216, to eradicate MM cells. The effect of ML216 was tested using 10 different human myeloma cell lines (HMCL). ML216 induced a dose dependent inhibition of cell growth in all investigated HMCL with a median IC50 of 4.1 μM (range: 1.2 - 30 μM). Moreover, our tests on HMCL showed that the BLM inhibitor synergizes with Melphalan and Bortezomib, two drugs currently used in myeloma therapy. Furthermore, ML216 induced a significant apoptosis of primary myeloma cells of patients co-cultured with their bone marrow environment and recombinant IL-6 (n=7). BLM inhibitor significantly reduced the median number of viable myeloma cells by 54%, 60% and 74% at respectively 3, 6 and 10 μM (P = 0.001, P < 0.001 and P<0.001 respectively; n=7). Of interest, the normal non myeloma cells present in the culture were significantly less affected by ML216 when used at 3 and 6 μM. These data demonstrated a significant higher toxicity of ML216 on myeloma cells compared to normal bone marrow cells.

Taken together, our results underline the therapeutic interest of BLM inhibitor in MM and especially in patients characterized by high BLM expression and a poor prognosis.