Identification of Novel LNK Mutations In Patients with Chronic Myeloproliferative Neoplasms and Related Disorders

Dysregulated JAK-STAT signaling in chronic myeloproliferative neoplasms (MPNs) has primarily been attributed to activating mutations in tyrosine kinases. However, JAK-STAT activation can be demonstrated in some patients lacking JAK2 or MPL mutations, suggesting alteration of other regulatory elements in this pathway. One regulator of JAK-STAT signaling is LNK (SH2B3), an adapter protein that contains a proline-rich N-terminal dimerization domain (Pro/DD), a pleckstrin homology (PH) domain (plasma membrane localization), and an SH2 domain. LNK binds to cytokine receptors (e.g. MPL, EPOR) and JAK2 via its SH2 domain, inhibiting downstream STAT activation and providing critical negative feedback regulation. LNK^-/- mice exhibit features consistent with an MPN phenotype. We recently reported the first human disease-related LNK mutations in two JAK2 V617F-negative MPN patients (Oh et al, Blood, Aug 12, 2010). One patient with primary myelofibrosis (PMF) exhibited a 5 base-pair (bp) deletion and missense mutation (DEL) leading to a premature stop codon and loss of the PH and SH2 domains. A second patient with essential thrombocythemia (ET) was found to have a missense mutation (E208Q) in the PH domain. Both mutations conferred aberrant JAK-STAT signaling in cell lines and primary patient samples, indicating that loss of LNK negative feedback regulation contributes to MPN pathogenesis. We now report the results of a comprehensive screen of a large cohort of MPN, overlap myelodysplastic syndrome (MDS)/MPN, and post-MDS/MPN acute myeloid leukemia (AML) patients for LNK mutations.

A total of 341 samples were sequenced (Table 1; polycythemia vera (PV)=34, erythrocytosis=7, ET=61, PMF=75, post-PV/ET MF=25, MPN-U=7, chronic myelomonocytic leukemia (CMML)=71, juvenile myelomonocytic leukemia=20, MDS/MPN=8, MDS with fibrosis=2, refractory anemia with ring sideroblasts and thrombocytosis=4, idiopathic hypereosinophilic syndrome/chronic eosinophilic leukemia=4, systemic mastocytosis=4, and post MDS/MPN AML=19). A deep sequencing approach (Illumina multiplexing system) was used to evaluate 84 samples, in which all exons of LNK were sequenced. For the remainder of the samples, direct sequencing was performed on exon 2, the region containing the previously reported DEL and E208Q mutations.

After excluding variants previously reported in SNP databases, a total of 11/341 (3.2%) patients were found to have non-synonymous mutations, including 3/61 (4.9%) ET, 3/75 (4.0%) PMF, and 5/71 (7.0%) CMML patients (Table 1). Each of the mutations localized to exon 2 of LNK, implicating this region as a possible mutational hotspot. This included the aforementioned patients with the DEL and E208Q mutations, which were confirmed by deep sequencing. In two other patients, sequencing of DNA from cultured skin fibroblasts DNA indicated that the mutations were germline. For the remaining seven patients, germline analysis is currently ongoing. In one patient with CMML, a 1 bp deletion leading to a frameshift and premature stop codon was identified (Q72fs). This mutation localized to the Pro/DD, likely resulting in a complete loss of LNK function. Interestingly, this patient who is wild type for the JAK2 and RAS genes, also carries a heterozygous CBL mutation (C396Y), suggesting that LNK and CBL mutations may have cooperative effects. Four patients (one with PMF, three with CMML) were found to have a missense mutation (S186I) at a highly conserved residue in the Pro/DD. The previously reported E208Q mutation was also found in one patient with ET and one patient with CMML. None of the 81 patients known to be JAK2 V617F-positive exhibited somatic LNK mutations, suggesting that LNK mutations may provide an alternative basis for JAK-STAT activation in the absence of JAK2 V617F.

Missense and deletion mutations of the LNK gene occur at a low frequency in MPNs and MDS/MPNs and segregate predominantly in exon 2. Further analysis of post-MPN AML samples (represented at a low frequency in the current cohort) and other subtypes of acute and chronic myeloid malignancies is warranted to better characterize the disease spectrum of LNK mutations and whether they are mutually exclusive of JAK2 V617F. We are currently investigating whether loss of negative feedback regulation of JAK-STAT signaling is related to haploinsufficiency of LNK or dominant negative effects of the mutant protein.

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No relevant conflicts of interest to declare.