Clinical implications of c-maf expression in plasma cells from patients with multiple myeloma

Multiple myeloma (MM) is a type of monoclonal plasma cell dysplasia among hematological malignancies, often leading to renal insufficiency, bone damage, anemia, and other terminal visceral damage. MM often has a relatively long period of benign pre-disease state called ‘monoclonal gammopathy of undetermined significance’ (MGUS). Approximately 1% of patients with MGUS progress to MM every year [1, 2]. There is significant heterogeneity in the clinical features and prognosis of MM patients. Diagnostic clinical and biochemical staging systems are commonly used in the evaluation of MM. However, MM patients at the same stage may still have different disease progression [2, 3]. Cytogenetic abnormalities have become increasingly important in prognostic risk stratification and management of MM [4–7]. Conventional karyotype analysis and fluorescence in situ hybridization (FISH) are common methods for the analysis of cytogenetic abnormalities in MM. Compared to conventional karyotype analysis, which has a real lower detection rate of abnormal karyotypes, the higher detection rate of FISH make it to be a primary approach in cytogenetic analysis [4, 8].

However, there are many cell surface molecules and intracellular molecular markers that abnormally expressed in malignant plasma cells, which are closely related to disease prognosis [9–13]. Studies have shown that the use of immunohistochemistry (IHC) to detect the expression of cancer-related genes, such as fibroblast growth factor receptor 3 (FGFR3), the tumor suppressor gene p53, and transcription factor c-maf can be an alternative method for the prognostic analysis of MM [14–17]. In this study, we utilized immunohistochemistry to examine the expression of the transcription factor c-maf, the cellular homolog of v-maf, in MM patients in China to retrospectively analyze its correlation with disease characteristics, treatment efficacy, and patient survival.

Cytogenetic changes are important factors involved in the incidence and development of MM and are closely associated with drug resistance. Furthermore, cytogenetic changes affect the prognosis of MM and thus are considered as an important tool for the decision-making process in clinical treatment. Chromosomal translocations in the immunoglobulin heavy chain (IgH) locus are the common chromosomal abnormalities in MM [2, 4, 6], with the most common types including t(4;14), t(11;14), and t(14;16). These result in the structural hyperactivation of fibroblast growth factor receptor 3 (FGFR3), cyclin D1, and the transcription factor c-maf, thereby regulating the proliferation, apoptosis, and biological behaviors of MM cells [5, 14, 15].

The presence of t(14;16) chromosomal abnormality in MM patients has been generally considered an indicator of poor prognosis [4–7, 20, 21]. A study showed that MM patients with t(14;16) have a median survival of only 16 months, while those without t(14;16) have a median survival of 41 months [22]. Most of clinical guidelines for the diagnosis and treatment of myeloma consider t(14;16) as one of the major criteria of high-risk patients [2, 5–7]. However, some researchers have offered alternative explanations and do not consider the prognostic significance of t(14;16) [23, 24]. They suggested that the poor prognosis of MM might actually be associated with the presence of +1q21 and del(17p13) [24]. In addition, the incidence of t(14;16) in MM is quite low and only present in approximately 1.0–4.6% of advanced MM patients [20, 22–25]. Data from recent studies in China also showed that the incidence of t(14;16) was only 0.8–6.7% among newly diagnosed MM patients [26–28], and the detection rate of t(14;16) was 1.2% in recently diagnosed MM patients [26]. A recent study from Hong Kong showed the absence of t(14;16) among 40 MM cases [29]. Translation of t(14;16) has also been observed in patients with MGUS, with the detection rate of approximately 5% [1, 30], suggesting that the chromosomal translation may be involved in the early stages of the disease; however, its impacts on disease prognosis remains controversial.

In this study, we used IHC to detect c-maf protein expression in the bone marrow plasma cells of MM patients and evaluated the impact of c-maf protein expression on the pathology and prognosis of MM. Among the 128 MM patients, 39 (30.5%) displayed high c-maf expression, which was concordant with the findings of previous studies [17, 31]. Chang et al. [14–17] studied the expression of p53, FGFR3, and c-maf protein in MM cells and found that the incidence of high c-maf protein expression in MM cells was significantly higher than the t(14;16) chromosomal abnormality. In addition, among the 73 MM patients examined by Chang et al., 22 expressed c-maf, while only 4 patients displayed the t(14;16) chromosomal translocation as detected by FISH [17]. Hurt et al. [31] used RT-PCR to detect the expression of c-maf mRNA in MM cell lines and plasma cells in the bone marrow of MM patients and showed that 46% (13/28) of the cell lines had high c-maf expression, whereas only 6 of the cell lines harbored the t(14;16) abnormality. In addition, up to 50% of MM patients (13/26) had high c-maf expression in the bone marrow cells, suggesting that 16q23 and 14q32 translocations were not the only cause of high c-maf expression. All these results demonstrated that the expression of c-maf is significantly high in MM patients, highlighting the importance of immunohistochemistry-based examination of c-maf expression. Of note, the correlations between c-maf mRNA and c-maf protein expression and t(14;16) currently still remain controversial [14, 17, 31–33]. Rasmussen et al. [32] and Natkunam et al. [33] showed that approximately 5% of the bone marrow plasma cells of MM patients expressed c-maf mRNA and protein, with a similar detection rate as that of t(14;16) and significant correlation with t(14;16). However, it is unclear whether the inconsistent findings are due to differences in the evaluation of IHC and FISH.

The t(14;16) chromosomal abnormality of myeloma cells indicates poor prognosis of MM. However, the relationship between c-maf gene expression and MM characteristics and prognosis had not been extensively explored. Our results suggested that the expression of c-maf protein was associated with serum albumin and β2-microglobulin levels in MM patients. In addition, the expression of c-maf protein in MM cells did not have any significant correlation with treatment response, PFS, and OS of MM patients. Nevertheless, the sub-group analysis demonstrated that patients who received non-bortezomib-based and thalidomide-based regimens or traditional chemotherapy displayed less VGPR or better treatment response if they had high levels of c-maf expression that if they were negative for c-maf expression, suggesting that c-maf-positive patients exhibited some degree of drug resistance, which could be overcome with bortezomib.

Therefore, further studies are necessary to identify the relationship between the expression of the transcription factor c-maf and the t(14;16) chromosomal translocation, as well as the relationship between the abnormal expression of c-maf gene and disease characteristics and prognosis of MM to clarify the significance of c-maf in the course of MM. Nevertheless, studies have shown that c-maf may be involved in the regulation of the cell cycle and promotion of growth, survival and angiogenesis of MM cells and facilitation of interactions between MM cells and tumor microenvironment [31, 34]. Thus, c-maf may be a potential therapeutic target for MM.

In this study, it was shown that the rate of expression of the transcription factor c-maf in MM patients was higher than the incidence of translocation of chromosomes 14 and 16 as reported earlier. The positive expression of c-maf may be correlated with hypoproteinemia and elevated serum β2-microglobulin levels in MM patients. Moreover, the c-maf-negative patients had better remission, especially with non-bortezomib-based treatments although c-maf expression displayed no significant correlation with the prognosis of MM patients.