Familial and Acquired SH2B3 mutations in ALL

Abstract 1326

Acute lymphoblastic leukemia (ALL) is a heterogeneous disease in which multiple genetic alterations contribute to the malignant transformation of lymphoid progenitor cells. Although the pathogenesis of ALL is considered to be multifactorial, few environmental factors have been conclusively associated with increased risk. In addition, although recent genome-wide association studies of childhood ALL have suggested that common variation at some genetic loci may predispose to ALL, the risk associated with these variants seems to be modest. Here we hypothesized that leukemia development in the context of rare familial inherited disorders may indicate a tumor suppressor activity for the underlying genetic defect present in these kindreds. In this context, we studied a pediatric B precursor ALL female patient of Eastern European Ashkenazi Jewish background with a history of remote consanguinity, short stature and mild developmental delay. Her family history is significant for a brother with similar growth retardation and mild developmental defects, neonatal anemia, thrombocytopenia and elevated white cell count, who was diagnosed with chronic hepatitis of probable autoimmune origin and Hashimoto's autoimmune thyroiditis. A second male sibling was healthy. We proposed that an underlying autosomal recessive genetic defect resulting in dysregulation of the development and function of the immune system could contribute to the concurrence of a lymphoid neoplasia and an autoimmune disorder in two siblings in this family. We performed homozygosity mapping and linkage analysis in the family. Whole exome sequencing of each of the two affected siblings identified a homozygous frameshift c.671insGGCCCCG p.Asp231fs mutation in the SH2B3 gene. Dideoxynucleotide sequencing of SH2B3 confirmed that both affected siblings were homozygous for this mutation, while the unaffected brother and both parents were carriers. Analysis of 2,000 normal Ashkenazi Jewish adults identified 1 SH2B3 c.671insGGCCCCG p.Asp231fs carrier and no homozygous individuals. The SH2B adaptor protein gene 3 (SH2B3) encodes an adaptor protein involved in the negative regulation of the JAK STAT signaling pathway. Western Blot analysis of immortalized lymphoblastoid cell lines from the two affected siblings and an unrelated normal control showed complete loss of SH2B3 protein expression in SH2B3 mutant cells. Consistently, analysis of JAK STAT signaling in these cells showed increased JAK2 and STAT3 phosphorylation compared with controls. Moreover, SH2B3 mutant lymphoblastoid cells showed increased proliferation and accelerated cell cycle kinetics, which was particularly pronounced in low serum conditions. Consistently, expression of wild type SH2B3 in these cells impaired cell growth and decreased JAK-STAT signaling. Analysis of a cohort of 167 additional ALLs including 119 B-precursor ALLs and 48 T-ALLs identified a homozygous c.1279insCTGTTGCCGTGTGC p.Val426fs SH2B3 mutation in a B-precursor ALL sample and a homozygous nonsense c.908C>A p.Ser303* SH2B3 mutation in a pediatric T-cell ALL. Notably, analysis of normal (remission) DNA demonstrated the somatic origin of the c.908C>A p.Ser303* SH2B3 mutation. In addition, our sequence analysis of SH2B3 in ALL identified two additional heterozygous germline single nucleotide variants resulting in amino acid substitutions (c.557G>T p.Ser186Ile and c.232G>A p.Glu78Lys). Expression of each of these variants in SH2B3 null cells resulted in partial correction of the increased cell growth phenotype and JAK-STAT signaling associated with SH2B3 loss compared with wild type controls. Notably, heterozygous somatic mutations in SH2B3 have been observed in patients with myeloproliferative neoplasms and heterozygous germline SH2B3 alleles have been linked to essential erythrocytosis, suggesting that the hypomorphic SH2B3 p.Ser186Ile and p.Glu78Lys variants found in our analysis may represent rare leukemia predisposing SH2B3 alleles. Overall these results link the loss of SH2B3 with the pathogenesis of a familial developmental delay, autoimmunity and leukemia predisposition syndrome and demonstrate a tumor suppressor role for SH2B3 in the pathogenesis of B-precursor and T-cell ALL.