Serum CD44 levels predict survival in patients with low-risk myelodysplastic syndromes

Abstract

Myelodysplastic syndromes (MDS) represent a heterogeneous group of clonal hematopoietic stem cell disorders that are preferentially diagnosed in the elderly. Aberrant expression of the adhesion receptor CD44 correlates with poor prognosis in various neoplasms. To evaluate the prognostic impact of CD44 in MDS serum levels of soluble CD44 standard (solCD44s) were measured in 130 MDS patients (median age 68 years) using an enzyme-linked immunosorbent assay (ELISA). solCD44s levels were significantly elevated in MDS patients as compared to those of healthy donors (p < 0.001) and were found to correlate with distinct FAB and WHO subtypes. The highest levels of solCD44s were found in patients with CMML, in RAEB and in patients with MDS transformed into secondary acute myeloid leukaemia (AML). In univariate analysis elevated levels of solCD44s (cut-off level > 688.5 ng/ml) correlated significantly with shorter overall survival in MDS patients (12 versus 39 months; p < 0.001). In multivariate analysis solCD44s displayed prognostic significance independent of the International Prognosis Scoring System (IPSS). To test for refined prognostication, IPSS risk groups were split into two separate categories based on the solCD44s levels. Using this approach, MDS patients with a shorter survival were identified both in the IPSS low-risk (p = 0.037) and in the IPSS intermediate-1-risk group (p = 0.015). The CD44s-adjusted IPSS defines a cohort of MDS patients with unfavorable prognosis, which might be helpful in risk stratification and in therapeutic algorithms.

Introduction

Myelodysplastic syndromes (MDS) constitute a heterogeneous group of myeloid neoplasms with ineffective hematopoiesis, peripheral cytopenias and the risk of progression to acute myeloid leukaemia (AML) [1], [2], [3], [4]. MDS are preferentially diagnosed in the elderly. The median age at diagnosis is 70+ in epidemiological studies (72 years in the Düsseldorf registry and 76 in the Tyrol registry). The incidence of MDS increases dramatically with advanced age revealing age specific incidences of 9, 25, and 31/100.000/year for the age groups 60–70, 71–80, and 80+, respectively [5]. The large and increasing proportion of elderly MDS patients and the availability of more and more treatment options, imposes an urgent need to develop strategies and algorithms for optimal management and treatment. Of several prognostic models, the International Prognostic Scoring System (IPSS) is the most widely utilized and incorporates bone marrow blast cell count, cytogenetic abnormalities and the degree of cytopenia [6]. However, it is still difficult to predict survival in individual patients. Thus, several refinements of the IPSS have been suggested, such as the integration of elevated serum lactate dehydrogenase (IPSS+LDH) [7], transfusion requirements (WHO-IPSS) [8], distinct cytogenetic aberrations [9] or patient-associated factors such as comorbidities or functional capacities [5], [10]. A scoring model including low platelets, anemia, advanced age, bone marrow blasts and cytogenetics was recently proposed to overcome the limitations of the IPSS and to stratify patients with low-risk disease [11].

Adhesive interactions including cell–cell and cell–matrix interactions are essential in the pathogenesis of various types of solid and hematological malignancies [12]. The adhesion receptor CD44 comprises a family of transmembrane glycoproteins generated by alternative splicing, resulting in proteins that range in size from 80 to 200 kDa [13]. The most widely expressed form is called the “standard” 80–90 kDa form (CD44s), whereas “variant” isoforms CD44 (CD44v) display a molecular weight of 160–200 kDa and a restricted expression pattern. Evidence that CD44 may be involved in the regulation of early stages of normal hematopoiesis has been suggested by its expression in primitive clonogenic cells and a marked decrease in mature cells after addition of anti-CD44 monoclonal antibodies to long-term marrow cultures (LTC) [14]. Very high levels of CD44 have been reported on both LTC-initiating cells and granulopoietic colony-forming cells. In contrast, primitive erythropoietic progenitors express low CD44 levels [15]. During maturation lineage-associated changes in CD44 were described, displaying a CD44 down-regulation in mature myeloid and erythroid cells [16]. Moreover, CD44 is relevant for B-cell differentiation and function [17].

CD44 mediates the binding of normal progenitors and of leukemic cells to hyaluronan, and its activity has been associated with an unfavorable clinical course in acute myeloid leukaemia [18], [19]. An aberrant expression of CD44 isoforms resulting in an unfavorable clinical course was demonstrated in several types of hematological malignancies including non-Hodgkin's lymphoma [20], B-cell chronic leukaemia [21] and multiple myelomas [22]. In myelodysplastic syndromes immunophenotypic characterization of myelopoiesis revealed elevated expression of CD44 with an increase in the immature myeloid compartment, whereas refractory anemia showed significantly decreased expression of CD44 on gated myeloid cells [23].

CD44 can be shed from the cellular surface by active proteolytic processes and is subsequently released as a soluble molecule (solCD44) [24], [25]. CD44 shedding may have important effects on tumor cell behaviour as CD44-dependent interactions with extracellular matrix and signalling pathways are regulated [26]. Actually, active cleavage of CD44 was demonstrated in several types of malignant cells [27], [28], [29] and elevated serum solCD44 levels correlate with unfavorable prognosis in several types of human tumours [26]. Analyses in small cohorts have demonstrated elevated solCD44 levels in MDS [30], [31]. Aim of this study was to analyze serum levels of soluble CD44 standard isoform (solCD44s) in an expanded group of MDS patients and to evaluate its relevance in the prognostication of MDS patients.