Prognostic Implications Of PRAME Expression Levels In Myelodysplastic Syndromes

The preferentially expressed antigen of the melanoma (PRAME) gene was first identified in melanoma tissue as a tumour-associated antigen recognized by autologous cytotoxic T cells against a melanoma surface antigen. While expression level of PRAME is quite low in most of normal tissues of human, including bone marrow normal CD34+ cells, except for testis, PRAME overexpression is observed in various human cancers, including hematological malignant disorders. Although clinical and biological significance of PRAME expression in human cancers has not been fully elucidated, it has been demonstrated that high PRAME expression is associated poor clinical outcomes in a number of solid cancers. Association between PRAME expression and clinical outcomes in hematological malignancies has not been clarified. In acute myeloid leukemia, high expression of PRAME was shown to be associated with worse progression-free survival or overall survival by some research groups but not by other groups. Our previous study demonstrated that inhibition of PRAME expression in leukemic cells results in cell cycle arrest and induction of apoptosis, suggesting oncogenic function of PRAME expression in leukemogenesis. Clinical significance of PRAME expression in myelodysplastic syndromes (MDS) also has not been fully elucidated. In the present study, we examined PRAME expression of bone marrow cells in MDS patients to clarify clinical significance of PRAME expression.

Bone marrow samples of MDS patients were used for analysis. Samples were taken at the time of diagnosis with written informed consent from patients. Total RNA extraction, cDNA synthesis and quantitative real-time RT-PCR by the TaqMan probe method using an ABI 7500 real-time PCR system (Applied Biosystems) with co-amplification of the endogenous control gene, human GAPDH (Applied Biosystems), were performed. Expression levels were obtained using the standard curve method in each experiment, after normalization with the GAPDH gene for each sample in duplicate wells. The human PRAME primer-probe sets were from Applied Biosystems (assay ID: Hs00196132_m1). Data including patients’ demographic, disease status, medical history, clinical and laboratory findings, and outcome, were collected from medical records and laboratory data base.

A total of 111 MDS patients, 68 males and 43 females with median age of 69 years (range: 20-91 years) were included in the present study. They were classified as RCUD (n=15), RCMD (n=61), RARS (n=8), RAEB-1 (n=15), and RAEB-2 (n=12) according to WHO classification. Based on the IPSS, they were categorized in four risk groups, low risk (n=29), intermediate-1 risk (n=55), intermediate-2 risk (n=23), and high risk(n=4). Expression level of PRAME was varied among the MDS patients analyzed. Median value of relative PRAME expression level of 111 MDS patients and that of 19 control subjects were 0.073 and 0.07, and there was no significant difference in distribution of expression level of PRAME. However, when we compared expression patterns of PRAME among five WHO-subtypes, statistical difference was observed (P=0.0116). Relative PRAME expression level in WHO-subtypes with high blast counts (RAEB-1 and RAEB-2) was significantly higher than that in WHO-subtypes with less blast counts (RCUD, RCMD, RARS) (median value: 0.44 vs. 0.05, P=0.0012). To investigate prognostic implication of PRAME expression in MDS, we analyzed impact of PRAME expression on overall survival (OS). Based on PRAME expression level, 111 patients were divided into two categories, ‘high expression group’ (above median value) and ‘low expression group’ (below median value). Kaplan-Meier analysis demonstrated that high expression group showed significantly poorer overall survival than low expression group (P=0.0165). The estimated 5-year OS rates in high expression group and low expression group were 63.5% and 78.4%, respectively.

The present study demonstrated that high PRAME expression is associated with poorer clinical outcome, indicating that PRAME expression could be a useful prognostic marker in MDS. Biological significance of PRAME expression in MDS is unclear. Expression level of PRAME was higher in WHO-subtypes with high blasts counts, suggesting that PRAME may play role in disease progression in MDS. Further studies should be necessary to clarify clinicopathological and biological significance of PRAME expression in MDS.