Germline Calr exon 9 Variant and Somatic JAK2 V617F Mutation in a Patient with Polycythemia Vera Associated with Late Presentation and Severe Clinical Phenotype

Introduction

The discoveries of JAK2 V617F and CALR mutations have significantly improved our understanding of MPN and its subtypes. Somatic JAK2 V617F mutation is present in almost all patients with polycythemia vera (PV) and in 50% to 60% of those with essential thrombocythemia (ET) or primary myelofibrosis (PMF).Somatic CALR mutations are generally absent in PV but their frequencies are approximately 20-25% in ET and PMF. Initial reports suggested that these variants were mutually exclusive. However subsequently co-occurrence has been reported and it is suggested but not yet confirmed that this may confer a distinct phenotype. The role of germline variation is unknown. We report here the first occurrence of a germline CALR variant in association with a somatic JAK2 V617F mutation.

Case report

A 73-year old Malay gentleman was noted to have an abnormal cardiac rhythm and function during admission for herniorrhaphy. On further questioning, he reported a 2 month history of reduced cognitive function, short term memory loss, occasional urinary incontinence and involuntary movements. Splenomegaly was noted and blood test showed Hb 205 g/L, WCC 33.5 x 10⁹/L and platelet 304 x 10⁹/L. Cardiac investigations revealed an anteroseptal myocardial infarction requiring an angioplasty and stent insertion. Targeted Ion Torrent high-throughput sequencing and fragment analysis showed a somatic JAK2 V617F mutation and a germline 3bp in-frame deletion c.1213_1215delGAG (p.Glu405del) in exon 9 of CALR.

Discussion

Although somatic CALR exon 9 mutations are mostly found in patients with ET and PMF who are JAK2-negative, coexistence of somatic JAK2 V617F and CALR exon 9 alterations have been reported previously, mostly in patients with ET. To the best of our knowledge, this is the first case of a patient with PV presenting with a somatic JAK2 V617F mutation and a germline CALR variant, Glu405del, comprising a 3 bp in-frame deletion in exon 9. According to existing databases, this CALR variant is a previously undescribed germline variant not found in the 60,000 exomes on the ExAC database; but one instance of an overlapping p.Glu405_Asp408del was reported (frequency of 2 x 10^-5). Although it is at present of unknown clinical significance, it is predicted by in silico modeling as being likely pathogenic: MutationTaster predicted both splice-site and protein feature changes, including loss of an endoplasmic reticulum export motif. Unlike the "classical" CALR-positive patients who present at a younger age with a higher platelet count, this patient presented incidentally at the age of 73 with a normal platelet count but a rather severe clinical phenotype with significant thrombosis requiring surgical intervention. Further studies are required to examine in detail the molecular signatures in this group of patients and whether they represent a distinct group of patients with clinical phenotypes which are different from the "classical" cases.