Mutation specific immunohistochemistry is highly specific for the presence of calreticulin mutations in myeloproliferative neoplasms.

The identification of somatic calreticulin (CALR) mutations can be used to confirm the diagnosis of a myeloproliferative disorder in Philadelphia chromosome-negative, JAK2 and MPL wild type patients with thrombocytosis. All pathogenic CALR mutations result in an identical C-terminal protein and therefore may be identifiable by immunohistochemistry. We sought to test the sensitivity and specificity of mutation specific immunohistochemistry for pathogenic CALR mutations using a commercially available mouse monoclonal antibody (clone CAL2). Immunohistochemistry for mutant calreticulin was performed on the most recent bone marrow trephine from a cohort of patients enriched for CALR mutations and compared to mutation testing performed by polymerase chain reaction (PCR) amplification followed by fragment length analysis. Twenty-nine patients underwent both immunohistochemistry and molecular testing. Eleven patients had CALR mutation, and immunohistochemistry was positive in nine (82%). One discrepant case appeared to represent genuine false negative immunohistochemistry. The other may be attributable to a 12 year delay between the bone marrow trephine and the specimen which underwent molecular testing, particularly because a liver biopsy performed at the same time as molecular testing demonstrated positive staining in megakaryocytes in extramedullary haematopoiesis. All 18 cases which lacked CALR mutation demonstrated negative staining. In this population enriched for CALR mutations, the specificity was 100%; sensitivity 82-91%, positive predictive value 100% and negative predictive value 90-95%. We conclude that mutation specific immunohistochemistry is highly specific for the presence of CALR mutations. Whilst it may not identify all mutations, it may be very valuable in routine clinical care.

Polyploidy in myelofibrosis: analysis by cytogenetic and SNP array indicates association with advancing disease.

BACKGROUND: Myelofibrosis occurs as primary myelofibrosis or as a late occurrence in the evolution of essential thrombocythaemia and polycythaemia vera. It is the rarest of the three classic myeloproliferative neoplasms (MPN). Polyploidy has only rarely been reported in MPN despite the prominent involvement of abnormal megakaryocytes. The use of peripheral blood samples containing increased numbers of haematopoietic progenitors has improved the output from cytogenetic studies in myelofibrosis and together with the use of single nucleotide polymorphism arrays (SNPa) has contributed to an improved knowledge regarding the diverse genetic landscape of this rare disease. RESULTS: Cytogenetic studies performed on a consecutive cohort of 42 patients with primary or post ET/PV myelofibrosis showed an abnormal karyotype in 24 cases and of these, nine showed a polyploid clone. Six of the nine cases showed a tetraploid (4n) subclone, whereas three showed mixed polyploid subclones with both tetraploid and octoploid (4n/8n) cell lines. The abnormal clone evolved from a near diploid karyotype at the initial investigation to a tetraploid karyotype in follow-up cytogenetic analysis in four cases. In total, six of the nine polyploid cases showed gain of 1q material. The remaining three cases showed polyploid metaphases, but with no detectable structural karyotypic rearrangements. Three of the nine cases showed chromosome abnormalities of 6p, either at diagnosis or later acquired. SNPa analysis on eight polyploid cases showed additional changes not previously recognised by karyotype analysis alone, including recurring changes involving 9p, 14q, 17q and 22q. Except for gain of 1q, SNPa findings from the polyploid group compared to eight non-polyploid cases with myelofibrosis found no significant differences in the type of abnormality detected. CONCLUSIONS: The study showed the use of peripheral blood samples to be suitable for standard karyotyping evaluation and DNA based studies. The overall profile of abnormalities found were comparable with that of post-MPN acute myeloid leukaemia or secondary myelodysplastic syndrome and cases in the polyploidy group were associated with features of high risk disease. The above represents the first documented series of polyploid karyotypes in myelofibrosis and shows a high representation of gain of 1q.