Evaluation of Bone Marrowmmorphology in Addition to JAK2 Allele Burden Is Essential for Assessing Disease Status in Recombinant Interferon Alpha-Treated Polycythemia Vera Patients

Introduction

Polycythemia vera (PV) is a myeloproliferative neoplasm (MPN) associated with the JAK2 V617F mutation. Recombinant interferon alfa-2b (rIFNα-2b) or pegylated rIFN2α (pegrIFNα-2a) treatment in PV is effective in inducing hematologic and molecular responses. Reduction in JAK2^V617Fallele burden (%V617F) has been used increasingly as a surrogate marker of disease response to treatment and as a unique criterion for discontinuation of therapy. To date, no studies have evaluated the relationship between changes in %V617F and established indicators of disease activity such as bone marrow cellularity and degree of fibrosis. Having observed persistent marrow hypercellularity or progressive fibrosis in PV patients despite a significant reduction in %V617F, we therefore systematically examined the correlation of %V617F with marrow morphology. We also assessed immunohistochemical expression of pSTAT5 as a marker of JAK2 activation.

Methods

The diagnosis of PV was based on the WHO criteria which include presence of erythrocytosis and/or an increased Cr⁵¹ red cell mass, JAK2^V617F mutation and typical bone marrow findings such as hypercellularity due to panmyelosis and megakaryocyte morphology consistent with PV. Patients with two marrow examinations and quantitative JAK2 levels measured at a median of 1.2 months from the biopsy date were eligible for inclusion. The JAK2^V617Fallele burden in peripheral blood samples was determined by pyrosequencing. Clinical, hematologic and molecular responses were graded according to ELN criteria. Marrow fibrosis was scored according to the WHO three-tiered semi quantitative grading system. Immunohistochemical staining for p-STAT5 was performed on clot sections in a subset of patients.

Results

We identified 15 patients for inclusion. The median %V617F at the time of the initial biopsy was 62.2% (18.2-100%), which decreased to a median of 33% (0-93.7%) on subsequent evaluation (p=0.02). Patients were initially treated with peg rIFNα-2a 45-90 mcgm weekly (n=13) or rIFNα-2b (n=2), 1x10⁶units thrice weekly, and gradually increased based upon response and tolerance. The median duration of treatment between biopsy was 4.2 years (0.8 - 6.6). All patients achieved either a complete (CHR) (n=8) or a partial (PHR) (n=7) hematologic response. Of these 15 patients, 3 achieved a complete molecular response (CMR), 4 a partial molecular response (PMR), and 8 patients no molecular response (NMR). We observed no correlation between clinical and molecular response: of the 8 patients who achieved CHR, 2 achieved a CMR and 6 did not. There was no correlation between clinical response and fibrosis or cellularity; of 8 patients who achieved CHR, 5 showed persistent/increased hypercellularity and 6 had persistent/increased fibrosis.

We also found no correlation between marrow morphologic changes and molecular response. Of the 3 patients with CMR, all had persistent hypercellularity (range: 60-90%) and increased/persistent fibrosis. Among the 4 patients who achieved a PMR, 2 had increased cellularity and 2 decreased cellularity. All had increased or persistent fibrosis. Among the 8 patients with NMR, 2 had an increase in marrow cellularity, 2 no change, and 4 an insignificant decrease. Fibrosis increased in 2, was unchanged in 3, and decreased in 3. Thus, there were no significant correlations between changes in %V617F and cellularity (κ=0.02) and fibrosis (κ=0.04). Immunohistochemical staining for p-STAT5 expression in megakaryocyte nuclei showed no correlation between degree of positivity and %V617F.

Conclusions

Although we observed a statistically significant decrease in %V617F after treatment with rIFNα, there were no parallel changes in marrow morphology. In all patients with CMR, we observed persistent or worsening marrow disease, suggesting to us a need for continued therapy. In patients who achieved either CHR or PHR, we observed persistent hypercellularity and persistent/progressive fibrosis. Our results question the use of JAK2^V617F allele burden as the sole criterion for discontinuation of rIFNα therapy. The prognostic significance of failing to attain a morphologic response in patients who have achieved a hematologic and/or molecular response remains unresolved. Resolution of these issues with longer follow-up is important, as premature discontinuation of rIFNα may allow the pathogenic mechanisms of the disease to progress unfettered.