Investigating the role Ikaros variants in multiple myeloma pathophysiology and drug sensitivity

Investigating the role of Ikaros variant in multiple myeloma pathophysiology and drug sensitivity Multiple myeloma (MM) is a complex and heterogeneous disease in which malignant plasma cells grow and accumulate within bone marrow, causing end-organ dysfunctions and morbidity. Although survival in MM has

improved significantly, acquisition of resistance in MM patients results in relapse thereby leading to challenges in disease management. Thus, better understanding of molecular mechanisms underlying the disease is required for identification of novel resistance/sensitivity prognostic markers. The initial impact in MM treatment came from

the introduction of lenalidomide. Lenalidomide causes selective ubiquitination and degradation of two lymphoid transcription factors, Ikaros (IKZF1) and Aiolos (IKZF3), by the Cereblon ubiquitin ligase. This established the dependency of MM cells on IKZF1 for their sustained proliferation and survival. Also, IKZF1 is a critical regulator

of lymphocyte development and differentiation. It is known to act both as activator and repressor of gene transcription. While examining the differences in 3' end RNA processing between MM patients and normal plasma cells, it was surprising to find that normal plasma cells express high levels of an IKZF1 variant which is

lost in a group of MM patients. Patients exhibiting this loss also had shorter progression-free survival. As this variant is generated by combination of alternative splicing and early cleavage and polyadenylation in the intron of the gene, it is referred as Ikaros intronic cleavage and polyadenylation isoform (IKZF1-IPA). IKZF1-IPA lacks

five 3' exons but contains two alternate 3' exons, and it thus differs in its 3' coding region and 3' untranslated region (3' UTR), compared to IKZF1-full-length (IKZF1-FL). The encoded IKZF1-IPA has a distinct C-terminus, which is significantly shorter than IKZF1-FL and lacks all the known DNA binding as well as homo- and hetero-

dimerization domains. Based on these findings and well-established oncogenic role of IKZF1-FL, it is hypothesized that loss of IKZF1-IPA could contribute to MM pathogenesis by increasing oncogenic IKZF1-FL levels and loss of tumour suppressive activity of IKZF1-IPA. Another hypothesis is that MM cells exhibit different

sensitives to lenalidomide treatment in presence or absence of IKZF1-IPA. Thus, in AIM1, how loss of IKZF1- IPA promotes MM pathophysiology will be investigated. As IKZF1-IPA exhibits limited similarity to known IKZF1 isoforms, there is lack of understanding about the binding partners of IKZF1-IPA and its localization preferences.

Identifying its binding partners will provide insight about its molecular modus operandi. It has been demonstrated that the interaction between IKZF1-FL and other IKZF1 variants impacts its chromatin remodeling activity. Thus, in AIM 2, the role of IKZF1-IPA in shaping epigenetic landscape, its binding partners and its localization

preferences will be investigated. Understanding the functional role of IKZF1-IPA in MM will advance knowledge about the resistance and sensitivity pathways in MM. It will also allow to determine if IKZF1-IPA can serve as a prognostic marker for MM treatment and response. Given the central role of IKZF1 in development, disease, and

treatment, the knowledge gained from this study can be extended to other diseases and malignancies.