Hypoplastic Myelodysplastic Syndrome: Clinical, Histopathological and Molecular Characterization

Introduction and Aims

Hypoplastic Myelodysplasic Syndrome (h-MDS) has distinct features compared to normo- or hypercellular MDS, with a higher response rate to immunosuppressive therapy and more favorable prognosis. However, to date, reproducible diagnostic criteria for h-MDS have not been clearly defined, making differential diagnosis from other bone marrow failure (BMF) syndromes challenging. Based on a large and well-annotated consecutive cohort of patients with BM hypocellularity from two tertiary centres, we aimed to delineate the clinical, histopathological and molecular features of h-MDS.

Methods

Of 1262 consecutive adult patients analyzed at King's College Hospital, London, UK and IRCCS Policlinico San Matteo & University of Pavia, Italy: 533 patients had a hypocellular BM, including 205 MDS patients with BM cellularity ≤25%, 77 MDS with reduced age-adjusted BM cellularity, 139 with Aplastic Anemia (AA), 97 with a diagnosis of Idiopathic Cytopenia of Undetermined Significance (ICUS) and 15 with congenital BMF (c-BMF) and 729 with normo- or hypercellular MDS (n-MDS).

Results

Comparison of clinical features and outcome of patients with h-MDS as defined by a BM cellularity ≤25% (n=205) and those with a reduced age-adjusted cellularity (n=77) did not detect any significant differences. Therefore, a reduced age-adjusted BM cellularity was adopted as a criterion to define h-MDS (n=282). Compared with patients with n-MDS, those with h-MDS were significantly younger, displayed higher levels of hemoglobin (p=.01), lower neutrophil and platelet counts and lower percentage of BM blasts, ring sideroblasts (RS) and CD34+cells as determined by immunohistochemistry (all p<.001). No significant difference was observed in the distribution of chromosomal abnormalities. A significantly higher prevalence of PNH clones was observed in h-MDS (24% vs 2%, p<.001). Those patients with h-MDS showed a significantly lower risk of leukemic evolution compared with n-MDS (4-year cumulative incidence 14% vs 26%, p=.003). Comparing h-MDS with AA, h-MDS showed a significantly higher percentage of RS (p=.004), BM blasts, dysmegakaryopoiesis and dysgranulopoiesis (p<.001), BM fibrosis (p=.002), clusters of CD34+ cells and chromosomal abnormalities (p<.001). H-MDS patients showed a significantly higher risk of leukemic evolution compared with AA (4-year cumulative incidence 14% vs 0%, p<.001).

Targeted sequencing of 24 commonly mutated myeloid genes was performed on 358 patients (73 h-MDS, 45 AA and 240 n-MDS). Twenty-five patients with h-MDS (34%) showed one or more somatic mutation (median 1; range 1-4). Comparing h-MDS to n-MDS revealed a significantly lower number of mutations per subject and variant allele frequency (VAF) (p<.001). Moreover, a significantly lower number of mutations per patient and VAF were observed in AA compared to h-MDS (p=.031 and p=.003, respectively). Thirty-seven patients with a hypocellular BM had a peripheral blood telomere length (TL) ≤1^st percentile and/or a germline mutation consistent with a diagnosis of c-BMF, leading to an estimated prevalence of unidentified c-BMF of 13% in cases without robust morphological evidence of MDS.

Based on these findings, we defined a diagnostic score (h-score), including variables with the highest specificity for MDS: dysmegakaryopoiesis, dysgranulopoiesis, BM fibrosis, clusters of CD34+ cells, RS 2-14%, BM blasts 2-4% (score 1); RS≥15% and BM blasts ≥5% (score 2). By applying ROC analysis, a cut off value of 2 was associated with the highest specificity for MDS (0.98). Notably, 71% of patients with a hypoplastic BM and low h-score (0-1) had no evidence of clonal disease by cytogenetic or mutation analyses or had a mutation pattern consistent with age-related clonal hematopoiesis. These patients showed a significantly better OS and lower risk of leukemic evolution (P<.001).

Conclusions

Integration of cyto-histological and genetic features in patients with a hypocellular BM identifies two distinct groups, one with clinical and genetic features highly consistent with a clonal disease with high risk of leukemic evolution, and one with features more consistent with a non-malignant bone marrow failure. Furthermore, we identified relevant enrichment of patients with previously undiagnosed c-BMF.