SF3B1 Mutations in CLL Are Equivalent to p53/ATM Dysfunction and Cause Defective Puma Upregulation in Response to Chemotherapy

Abstract 711

Mutations or deletions of the tumor suppressor p53 or its upstream kinase ATM are well-known determinants of poor prognosis in Chronic Lymphocytic Leukemia (CLL). In recent years, genome wide sequencing has uncovered novel gene mutations that correspond with poor prognosis. Specifically, recurrent mutations in the splicing factor SF3B1 and the Notch and NRAS/KRAS oncogenes have been found. These mutations were (in part) mutually exclusive with p53 and/or ATM mutations, which suggested overlap in biological function.

Here, we report results of a comparative analysis of p53 target genes and in vitro drug responses in CLL samples with either p53 (n=9), ATM (n=10), SF3B1 (n=11), Notch (n=6), or NRAS/KRAS (n=4) gene deletions/mutations. We found that upon irradiation, mRNA induction of all tested p53 targets genes (p21, Puma, CD95, Bax, PCNA, FXDR) was clearly decreased in all SF3B1 mutated CLL samples (overall p<0.001). SF3B1 samples resembled ATM mutated/11q⁻ CLL in displaying a defective but not absent p53 response. In contrast, Notch and KRAS/NRAS mutations did not affect RNA induction of apoptosis inducers Puma and Bax. At the protein level, Puma and p21 induction were defective or absent in SF3B1 mutated CLL. This corresponded with decreased apoptosis after in vitro treatment with fludarabine. Treatment with nutlin, either alone or in combination with fludarabine, restored cell death induction, again indicating an overlap with ATM dysfunction. To establish possible causality between SF3B1 mutation and ATM dysfunction, more genetic and functional studies are ongoing and will be reported.

In conclusion, the recently described mutations in a splicing factor in CLL can be linked at the functional level to defective ATM and/or p53 target gene responses, providing an explanation for the poor clinical prognosis of CLL patients with SF3B1 mutations.