Prognostic irrelevance of ring sideroblast percentage in World Health Organization–defined myelodysplastic syndromes without excess blasts

Ring sideroblasts (RS) are erythroid precursors with iron laden mitochondria forming a perinuclear ring, and are commonly seen in patients with myelodysplastic syndromes (MDS).^1,2  The presence of ≥ 15% RS constitutes the operational diagnosis of MDS-RS.¹  Refractory anemia with ring sideroblasts (RARS) forms the prototypical MDS subcategory with RS but the latter can also be seen in other MDS subcategories including refractory cytopenias with multilineage dysplasia (RCMD), MDS-unclassifiable (MDS-U), and refractory anemia with excess blasts (RAEB-1/2). Outside the context of MDS, RS are seen in other myeloid malignancies (eg, myeloproliferative neoplasms)^3,4  and nonclonal conditions such as excess alcohol use, lead toxicity, copper or pyridoxine deficiency, isoniazid therapy, and hereditary sideroblastic anemias associated with severalgerm line mutations involving δ-aminolevulinate synthase 2 (ALAS2),⁵  solute carrier family 25 member 38 (SLC25A38),⁶  glutaredoxin-5 (GLRX5),⁷  and the ATP-binding cassette sub-family B member 7 (ABCB7).⁸  Most recently, spliceosome mutations were shown to be prevalent in MDS with a relatively specific association between MDS-RS and mutations involving splicing factor 3B subunit 1 (SF3B1).^9-12  The pathogenetic contribution of SF3B1 mutations in MDS-RS is not clear although up-regulation of ALAS2 and down-regulation of ABCB7 have been reported in RARS.^13,14 

Among the different categories of MDS-RS without excess blasts (< 5%), RARS is generally believed to have the best survival rate with the lowest risk of leukemic transformation.¹  The rationale behind assigning the 15% threshold in defining RARS is not clear and the overall clinical relevance of quantifying bone marrow (BM) RS%, especially if one was to account for the presence or absence of multilineage dysplasia has not been systematically studied. In the current study, we examined the phenotypic and prognostic relevance of RS%, in general, and the “15%” RS% threshold, in particular, in the context of MDS without excess blasts by identifying cases defined by the presence of ≥ 1% RS and < 5% blasts in their BM.

After approval by the Mayo Clinic institutional review board, Mayo Clinic databases and cell banks were queried to identify patients with MDS without excess blasts (< 5%) and ≥ 1% RS. All study patients were required to have undergone BM examination and cytogenetic evaluation at diagnosis. Pathology slides, including iron stains, were centrally re-reviewed (by C.A.H. and J.M.H.) to accurately quantify BM RS percentage and confirm World Health Organization morphologic diagnosis.¹  To assess the phenotypic and prognostic impact of RS%, the specific variable was evaluated both as a continuous and categorical parameter; the latter was accomplished by stratifying patients into 4 categories based on the percentage of RS (< 5%, 5%-14%, 15%-50%, and > 50%).

Detailed analysis of clinical parameters and cytogenetic findings was performed to risk stratify patients according to the International Prognostic Scoring System (IPSS) and the revised IPSS (IPSS-R).¹⁵  Patients were also screened for JAK2, MPL, IDH, and SF3B1 mutations, using previously described methods.^10,16-18  All analyses included parameters obtained at the time of initial diagnosis. Standard statistical methods were used for parameter comparison and survival curves were prepared by the Kaplan-Meier method and compared by the log-rank test. Cox proportional hazard regression model was used for multivariable analysis. P values less than .05 were considered significant. The Stat View (SAS Institute) statistical package was used for all computations.

The current study included 200 Mayo Clinic patients (median age 71 years; 70% males) with MDS without excess blasts and ≥ 1% RS. Among them, 56 (28%) displayed < 5% RS, 32 (16%) displayed 5%-14% RS, 79 (40%) displayed 15%-50% RS, and 33 (17%) displayed > 50% RS. Table 1 outlines the presenting clinical and laboratory features and subsequent events in all 200 study patients stratified by RS percentage. IPSS-R risk distributions were: 20% very good, 57% good, 11% intermediate, 5% poor, and 9% very poor. Thirty-four (17%) patients were red blood cell transfusion–dependent at presentation. Fifty-six patients (28%) met the World Health Organization (WHO) criteria for RARS whereas the remaining 144 patients were classified as RCMD (n = 130) or MDS-U (n = 14). The median (range) RS% in these 3 morphologic groups were 36% (18%-70%), 8% (1%-78%), and 5% (1%-75%), respectively. Abnormal karyotype was detected in 91 (45%) patients, including 41 (21%) with high-risk abnormalities. Mutational frequencies were 58% for SF3B1, 9% for IDH2, 3% for JAK2V617F, 2% for IDH1, and 1% for MPL (Table 1).

In a univariate analysis, RS% as a continuous variable correlated directly with age (P = .02), platelet count (P < .01), bone marrow cellularity (P < .01), red blood cell transfusion dependency (P = .01), and presence of mutant SF3B1 (P = .04) and inversely with hemoglobin level (P < .01) and presence of multilineage dysplasia (P < .01) or high-risk karyotype (P < .01; Table 1). These associations were also apparent during comparison of patients with < 5% vs 5%-14% vs 15%-50% vs > 50% RS (Table 1). In other words, patients with lower RS% were more likely to be classified as RCMD and carry a high-risk karyotype, as opposed to those with higher RS% where RARS and mutant SF3B1 are more than-represented. Accordingly, IPSS-R risk distribution was significantly affected in favor of patients with higher RS%. Of note, there were no significant differences between the RS% groups in either leukocyte count or IDH mutational frequency (Table 1).

At a median follow-up of 33 months, 156 (73%) deaths and 24 (12%) leukemic transformations were documented. Median survivals were 63 months for MDS with > 50% RS, 43 months for MDS with 15%-50% RS, 35 months for MDS with 5%-14% RS, and 14 months for MDS with < 5% RS (P = .005; supplemental Figure 1, available on the Blood Web site; see the Supplemental Materials link at the top of the online article). However, multivariable analysis did not identify RS%, either as a continuous or categorical variable, to significantly affect overall or leukemia-free survival (Table 2). Instead, independent predictors of overall survival included WHO histologic category and IPSS-R risk category (Table 2). Identifying the WHO histologic category as one of the independent indicators of overall survival emphasizes the importance of recognizing multilineage dysplasia in bone marrows that have increased ring sideroblasts so as to accurately distinguish RCMD from RARS.

Molecular markers have taken center stage in MDS pathogenesis and prognosis.^19,20  Specifically, somatic mutations involving IDH1, TP53, EZH2, ETV6, ASXL1, DNMT3A, or RUNX1 have been associated with poor survival and SF3B1 with better survival.^20-23  However, the prognostic impact of these mutations, after adjusting for karyotype, BM blast percentage and presence or absence of multilineage dysplasia remains unclear. For example, we have recently demonstrated the lack of morphology-independent prognostic value for SF3B1 mutations in MDS, an observation recently confirmed by others.^10,24  In the current study, we show a direct correlation between RS% and mutant SF3B1 in patients with MDS without excess blasts and ≥ 1% RS, including a higher mutational frequency in patients with > 50% RS compared with 15%-49% RS. In addition, patients with higher RS% were less likely to display multilineage dysplasia or high-risk karyotype, which adequately explains the spurious appearance of a favorable prognostic impact from higher RS%. Consistent with this observation, we show that RS% per se does not influence either overall or leukemia-free survival in the context of MDS where the separate consideration of RARS is endorsed by the WHO. The results of the current study underscore the importance of accounting for multilineage dysplasia and karyotype in prognostic studies involving MDS without excess blasts and also suggest the limited practical value of quantifying BM RS.

Contribution: M.M.P., N.H.S., and A.T. designed the study, contributed patients, collected data, performed the statistical analysis, and wrote the paper; T.L.L. participated in study design, primer design, and sequence analysis; J.M.H. reviewed histopathology; R.A.K. reviewed cytogenetic information; R.P.K. reviewed cytogenetic information; C.A.H. reviewed histopathology; and all authors approved the final draft of the paper.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Ayalew Tefferi, MD, Mayo Clinic, 200 First St SW, Rochester, MN 55905; e-mail: tefferi.ayalew@mayo.edu.