Frequency and Clinical Presentation of Polycomb Repressive Complex 2 (PRC2) Component Mutations in Childhood Acute Lymphoblastic Leukemia

Abstract 1474

Acute lymphoblastic leukemia (ALL) is the most common childhood malignancy with approximately 85% of cases being of B-cell lineage and 15% of T-cell lineage. There is growing evidence that epigenetic deregulation plays an important role in the pathogenesis of leukemia. Somatic mutations in polycomb repressive complex 2 (PRC2) components such as EZH2, SUZ12, EED, and JARID2 have recently been described in myeloid and lymphoid malignancies and seem to be associated with a poor prognosis. PRC2 is a highly conserved histone H3 lysine 27 methyltransferase that regulates the expression of developmental genes. We sought to determine the frequency and prognostic impact of mutations within the three PRC2 core component genes EZH2, SUZ12, and EED in a clinical well-defined cohort of 154 randomly selected childhood ALL patients (male, n=82; female, n=72; median age 6.7 years, range 0.2–25.9 years): B-cell lineage, n= 125; T-cell lineage, n=29. A total of 168 samples were investigated at diagnosis (n=109), relapse (n=31), or both (n=14) by analysis of genomic DNA isolated from bone marrow cells. PCRs were performed using standard conditions with primers covering all coding exons including intron-exon boundaries to detect potential splice mutations. Forty-nine amplicons were analyzed for each sample (8,232 amplicons in total). Mutation analysis was performed by Sanger sequencing. So far, the entire cohort has been analyzed for EZH2 and 100/168 samples for mutations in all three genes. The complete mutation status will be presented at the meeting. We identified four EZH2 mutations (missense mutations, n=3, frameshift deletion, n=1) in four patients classified as c-ALL (n=2), pro-B-ALL (n=1) or early T-cell precursor ALL (n=1), respectively. No SUZ12 or EED mutations have been identified so far. Remission material was available for all patients with missense mutations to test whether the mutation was acquired or inherited. Hereby, a somatically acquired EZH2 mutation was revealed in a 3-year-old boy diagnosed with c-ALL in 1982 and treated within the standard-risk group of the German ALL-VII/81 trial. He achieved complete remission but relapsed four years after initial treatment (late relapse). A second complete remission was not achieved under relapse treatment within the German ALL-REZ BFM 85 study and the patient died four weeks after relapse. The underlying mutation was a heterozygous missense mutation in EZH2 exon 17 that caused an amino acid change of aspartic acid to histidine at position 664 within the SET domain. No remission or constitutional material was available for the patient harboring a heterozygous frameshift deletion in EZH2 exon 6, however, analysis of serial samples at diagnosis and relapse revealed the same EZH2 mutant clone. Furthermore, this mutation has not been described as a polymorphism in large single nucleotide polymorphism databases so far. The patient was a 10-year-old girl diagnosed with pro-B-ALL in 1984 and treated within the high-risk group of the German ALL-VII/81 trial. She achieved complete remission but relapsed and died 6 months after start of treatment (very early relapse). In summary, although mutations of PRC2 components seem to be rare in childhood ALL our findings indicate that genetic defects in PCR2 might contribute to the disease in individual cases associated with a poor prognosis.