Activation of <i>Notch</i> and <i>Myc</i> Signaling via B-cell-Restricted Depletion of <i>Dnmt3a</i> Generates a Consistent Murine Model of Chronic Lymphocytic Leukemia.

Abstract

Chronic lymphocytic leukemia (CLL) is characterized by disordered DNA methylation, suggesting these epigenetic changes might play a critical role in disease onset and progression. The methyltransferase <i>DNMT3A</i> is a key regulator of DNA methylation. Although <i>DNMT3A</i> somatic mutations in CLL are rare, we found that low <i>DNMT3A</i> expression is associated with more aggressive disease. A conditional knockout mouse model showed that homozygous depletion of <i>Dnmt3a</i> from B cells results in the development of CLL with 100% penetrance at a median age of onset of 5.3 months, and heterozygous <i>Dnmt3a</i> depletion yields a disease penetrance of 89% with a median onset at 18.5 months, confirming its role as a haploinsufficient tumor suppressor. B1a cells were confirmed as the cell of origin of disease in this model, and <i>Dnmt3a</i> depletion resulted in focal hypomethylation and activation of <i>Notch</i> and <i>Myc</i> signaling. Amplification of chromosome 15 containing the <i>Myc</i> gene was detected in all CLL mice tested, and infiltration of high-<i>Myc</i>-expressing CLL cells in the spleen was observed. Notably, hyperactivation of <i>Notch</i> and <i>Myc</i> signaling was exclusively observed in the <i>Dnmt3a</i> CLL mice, but not in three other CLL mouse models tested (<i>Sf3b1-Atm</i>, <i>Ikzf3</i>, and <i>MDR</i>), and <i>Dnmt3a</i>-depleted CLL were sensitive to pharmacologic inhibition of <i>Notch</i> signaling <i>in vitro</i> and <i>in vivo</i>. Consistent with these findings, human CLL samples with lower <i>DNMT3A</i> expression were more sensitive to <i>Notch</i> inhibition than those with higher <i>DNMT3A</i> expression. Altogether, these results suggest that <i>Dnmt3a</i> depletion induces CLL that is highly dependent on activation of <i>Notch</i> and <i>Myc</i> signaling. SIGNIFICANCE: Loss of <i>DNMT3A</i> expression is a driving event in CLL and is associated with aggressive disease, activation of <i>Notch</i> and <i>Myc</i> signaling, and enhanced sensitivity to <i>Notch</i> inhibition.