SF3B1 MUTATIONS IN CLL ARE FUNCTIONALLY EQUIVALENT TO ATM GENE ALTERATIONS AND CAUSE DEFECTIVE PUMA AND P21 UPREGULATION IN RESPONSE TO DNA DAMAGE

Background:

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 also correspond with poor prognosis. Specifically, recurrent mutations in the splicing factor SF3B1 and the Notch1 proto-oncogene have been found. These mutations were (in part) mutually exclusive with TP53 aberrations, which suggested their overlap in biological function.

Aims:

To investigate whether SF3B1 and Notch1 mutations affect the p53/ATM axis.

Results:

Here, we report results of a comparative analysis of p53 target genes and in vitro responses to cytotoxic drugs in CLL samples with TP53 (n=13), ATM (n=18), SF3B1 (n=20) and Notch1 (n=10) mutations. Upon irradiation, mRNA induction of p53 targets genes (p21, Puma, CD95, Bax, PCNA, FXDR) was decreased in SF3B1 (overall P<0.01), but not in Notch1 mutated CLL samples. SF3B1 mutated samples resembled ATM mutated CLL in displaying a defective but not absent p53 response. At protein level, Puma and p21 induction were defective or absent. 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. Since it is emerging that SF3B1 mutation correlates with 11q deletion we performed extensive analysis of the coding sequence of the ATM gene (exons 1-62) in all SF3B1 mutated cases. In this cohort there was an overlap of SF3B1 mutationswith ATMmutations and/or 11qdeletions in 59% of SF3B1 mutated cases (10/17; in 3 cases ATM analysis is ongoing), and of 18% (3/17) with TP53 mutation. Importantly, 4 SF3B1 mutated cases did not have an ATM mutation, 11q deletion or TP53 mutation, but still these samples displayed an impaired response to irradiation and cytotoxic drugs, indicating that the functional defect can occur independently of ATM or TP53 mutation/deletion.

Summary / Conclusion:

In conclusion, the recently described mutations in a splicing factor SF3B1 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

Abstrakta v časopise Haematologica 2013, Suppl1

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