[1240] A Bone Marrow Morphologic and Immunohistochemical Study of Polycythemia Vera (PV) with Exon 12 Mutations

MA Lakey, JD Hoyer, A Tefferi, A Pardanani, PL Nguyen, TL Lasho, CA Hanson. Mayo Clinic, Rochester, MN

Background: The diagnosis of PV requires the integration of clinical features, laboratory findings (including serum erythropoietin (Epo) levels), bone marrow (BM) morphology, and JAK2 analysis. The JAK2V617F (exon 14) is found in 95% of PV. Mutations in exon 12 have also been described in PV. These 2 mutations account for virtually all cases of PV. A thorough BM study of PV patients with exon 12 mutations has not been done.
Design: We identified 7 PV patients with exon 12 mutations. JAK2V617F studies were done using an allele-specific PCR analysis. JAK2 exon 12 mutation screening was accomplished by using PCR primers to amplify exon 12 of JAK2 followed by DNA sequencing.
Results: Three previously recognized point mutations of exon 12 were found in 6 patients; 1 patient had a novel exon 12 mutation. All patients had low Epo. CBC showed an erythrocytosis in all patients; the Hgb ranged from 18.3-22 g/dl (med=19.4 g/dl). Neutrophilia and/or basophilia were seen in 4 cases; 2 patients had thrombocytosis. The BMs were hypercellular (45-100%; med=90%). An erythroid hyperplasia was present in all cases with M:E ratios ranging from 1:1 to 1:5 (med=1:2). Erythroid maturation was left-shifted in 5 cases, and granulocytic hyperplasia was present in 4 cases. Reticulin fibrosis was not increased except in 1 patient who evolved into post-polycythemic myelofibrosis. No prominent clusters of large, bizarre megakaryocytes that characterize JAK2V617F positive PV were seen in the exon 12 cases. A spectrum of megakaryocytes was identified; small to medium-sized forms predominated over larger forms. Atypical megakaryocytic lobulation and abnormal megakaryocytic chromatin distribution were identified in all cases. Loose clusters of megakaryocytes could be found but were subtle in all cases. Immunohistochemical stain for CD61 confirmed the above findings.
Conclusions: The BMs from patients with exon 12 mutations show an erythroid hyperplasia and lack the prominent clusters of large, bizarre megakaryocytes that characterize classic JAK2V617F positive PV. However, subtle megakaryocytic atypia can be identified. Because these cases lack the classic myeloproliferative morphology, the BMs from patients with exon 12 mutations may be initially difficult to diagnose. Clinically suspected PV with low serum Epo and absent JAK2V617F together with the BM finding of erythroid hyperplasia and subtle megakaryocytic atypia/clustering should prompt an evaluation for an exon 12 mutation.
Category: Hematopathology

Tuesday, March 10, 2009 2:00 PM

Platform Session: Section D, Tuesday Afternoon