<scp>LncRNA H19</scp> ‐Encoded Micropeptide <scp>altH19</scp> Promotes <scp>DNA</scp> Replication and Mitosis in Myeloma Cells by Enhancing the Phosphorylation of <scp>CDK2</scp> at Threonine 160

Micropeptides are endogenous peptides translated from alternative open reading frames (alt-ORFs) within coding or non-coding genes. Emerging evidence suggests that some micropeptides play critical roles in both physiological and pathological processes. Multiple myeloma (MM), a haematological malignancy, remains incurable due to frequent relapses and a limited understanding of its underlying mechanisms. In this study, we sought to investigate the function and molecular mechanism of a novel micropeptide in MM pathogenesis. We identified a novel micropeptide, altH19, encoded by the lncRNA H19, which is highly expressed in patients of MM. Functional assays revealed that altH19 promotes myeloma cell proliferation and colony formation significantly. Furthermore, altH19 induces multipolar mitosis by upregulating the expression of Aurora B, Centrin 2 and phosphorylated histone H3. Flow cytometry analyses confirmed that overexpression of altH19 enhances DNA replication and accelerates the transition from early to mid-late stages of the DNA replication process. Conversely, knockout of altH19 reverses these effects. Mechanistically, altH19 directly interacts with phosphorylated CDK2 at threonine 160, thereby enhancing CDK2 T160 phosphorylation and activating the downstream E2F1 target RB phosphorylation. Notably, altH19 was able to restore phosphorylation levels of CDK2 and RB that were otherwise suppressed by the CDK2-selective inhibitor Seliciclib. In summary, we identify altH19 as a novel lncRNA-derived micropeptide with a pivotal role in myeloma progression, highlighting the therapeutic potential of targeting the altH19-CDK2-RB axis in MM treatment.