LNK Mutation Studies In Chronic- and Blast-Phase Myeloproliferative Neoplasms and JAK2 Mutation-Negative Erythrocytosis

JAK2 mutations are found in approximately 99% of patients with polycythemia vera (PV) and 60% of those with essential thrombocythemia (ET) or primary myelofibrosis (PMF). It is currently assumed that other mutations relevant to JAK signaling contribute to the pathogenesis of JAK2 mutation-negative myeloproliferative neoplasms (MPN). The same might hold true for some cases of “idiopathic erythrocytosis” associated with subnormal serum erythropoietin level (sEpo). LNK is a plasma membrane-bound adaptor protein whose function includes inhibition of wild-type and mutant JAK2 phosphorylation. LNK exon 2 mutations were recently described in two patients with JAK2V617F-negative ET or PMF. Both mutations involved the LNK pleckstrin homology (PH) domain; one was a 5 base-pair deletion and missense mutation leading to a premature stop codon (603_607delGCGCT; 613C>G) and the other a missense mutation (622G>C; glutamic acid to glutamine substitution; E208Q).

i) To estimate the prevalence of LNK mutations in chronic and blast phase MPN

ii) To determine if LNK mutations are mutually exclusive of other MPN-associated mutations

iii) To test the hypothesis that LNK mutations might contribute to the pathogenesis of JAK2 mutation-negative PV or otherwise unexplained erythrocytosis

LNK mutation analysis was performed on bone marrow or blood cells using modified primers for amplifying the SH2 and PH domains, using previously published methods (Oh et al. Blood First Edition Paper, prepublished online April 19, 2010; DOI 10.1182/blood-2010-02-270108).

LNK mutation studies were performed in 172 patients; paired chronic-blast phase samples were analyzed in 26 cases. Diagnoses in the 172 study patients were as follows: 78 chronic-phase MPN (39 JAK2V617F-positive PV, 25 PMF and 14 ET) enriched for TET2, IDH, JAK2V617F, or MPL mutations, 61 blast-phase MPN (41 blast-phase PMF, 11 blast-phase PV and 9 blast-phase ET), 25 JAK2V617F-negative PV and 8 “unexplained erythrocytosis” associated with subnormal sEpo and negative for Epo receptor (EPOR) mutations.

Ten novel heterozygous LNK mutations, all but one affecting exon 2 in the PH domain, were identified: 6 missense mutations involving codons 215, 220, 223, 229 and 234, one nonsense mutation involving codon 208 (622G>T leading to glutamic acid to a stop codon substitution; E208X), one synonymous mutation involving codon 208, and 2 deletion mutations involving exon 2 (685-691_delGGCCCCG) or exon 5 (955_delA). LNK mutations were most frequent in blast-phase MPN, occurring in 6 (9.8%) of the 61 informative patients; chronic-phase sample analysis in 4 of these revealed the same mutation in only one case. Mutant LNK was detected in chronic-phase samples only in 2 additional patients with blast-phase MPN. Among the 8 LNK-mutated blast-phase MPN cases, 7 had blast-phase PMF. The clinical phenotype in the remaining 2 LNK-mutated patients was that of an isolated erythrocytosis without overt features of PV. JAK2V617F was documented in 3 and IDHR140Q in 1 LNK-mutated patients.

LNK mutations i) target an exon 2 ‘hot spot' in the PH domain spanning residues E208-D234, ii) might be most prevalent in blast-phase PMF iii) are not mutually exclusive of JAK2 or IDH mutations and iv) might be part of the “missing link” in the pathogenesis of JAK2 mutation-negative “idiopathic” erythrocytosis.

No relevant conflicts of interest to declare.