Correlation between deletion of the CDKN2 gene and tyrosine kinase inhibitor resistance in adult Philadelphia chromosome-positive acute lymphoblastic leukemia

In the pre-TKI era, the prognosis of BCR-ABL-positive B-ALL has been shown to be extremely unfavorable with 7 years of OS <50 % [10]. Recently, loss of tyrosine kinase receptor in Ph-positive cells was found to result in the development of ALL [11]. The development of TKIs has also been reported to markedly improve the outcome of Ph-positive ALL [12‐14]. However, there exists enormous variation in response to chemotherapy with the heterogeneity of biological and clinical outcomes in leukemia patients [15‐17]. CDKN2A/B deletion is one of the most common genetic mutations involved in leukemogenesis in Ph-positive ALL cells characterized by recurrent genetic abnormalities along with BCR-ABL fusions [18]. CDKN2A/B genes usually remain undetected in hematopoietic stem cells and begin to activate following blood cell differentiation process in response to potential oncogenic stress [19‐21]. The unchanged epigenetic inactivation of CDKN2A/B(INK4-ARF) in differentiated cells will result in an inappropriate self-renewal capacity and leads to malignant transformation. Our current results reported the frequency of CDKN2 deletion as 32.6 % (44/135) in adult Ph-positive B-ALL patients and agreed with the previous observed incidence of CDKN2 deletion in adult patients with the BCR-ABL fusion gene (29 %) [5]. Our results showed that no difference occurred between CDKN2 deletion patients (44/135) and wild-type patients with regard to sex, age, and induction complete remission rate. CDKN2 deletion carriers demonstrated greater white blood cell count, enhanced rates of hepatosplenomegaly, an upregulation of CD20 expression, and a higher relapse rate. It is well known that accelerated tumor cell proliferation could occur as a result of CDKN2A/B deletion due to the direct removal of tumor suppressors and activation of tumor growth factors such as MDM2 and CDK4/6. This may explain why CDKN2 deletion patients present with higher WBC counts, and hepatosplenomegaly is an indicator of a higher tumor load. Currently, research regarding the effect of CD20 in Ph-positive ALL patients has been controversial. Some reports suggested that CD20 expression is not an adverse factor [22, 23]. But, many studies found that expression of CD20 is associated with an increased incidence of a relapse [24], and CD20 upregulation is frequent in patients who suffered later from a relapse [25]. Thus, CD20 expression had an adverse effect on the prognosis in patients diagnosed with B-cell acute lymphoblastic leukemia [26, 27]. For B-lymphoblastic leukemia patients who are Ph-negative, the expression of CD20-positive patients has shown some significant benefit from rituximab therapy, especially towards younger aged patients [28, 29]. Ph-positive ALL cohorts have been prescribed with a similar type of therapy, and these patients most commonly undergo monoclonal antibody therapy. Nevertheless, the adverse expression of CD20 in this particular subtype of ALL has yet to be determined and requires further examination [30]. In our study, the adult Ph-positive ALL patients with CDKN2 deletion had a higher rate of CD20 expression (at a level of at least 20 %), and these patients with CD20 expression had an inferior OS and DFS than the patients without CD20 expression. Hence, CD20 expression may be a cause of the inferior OS and DFS. Together, CDKN2 deletion patients who exhibit a higher CD20 expression could possibly benefit from rituximab treatment. The efficacy of rituximab combination with chemotherapy and its association with the improved survival of patients with ALL needs further research.

Results from the current study revealed that a shorter survival time and a higher recurrence rate was observed in Ph-positive ALL patients with CDKN2 deletion. Previous studies reported similar poor survival as a significant clinical outcome and observed a short relapsed period within 1 year in all of the adult BCP-ALL patients concurrent with the BCR-ABL fusion gene and CDKN2 deletion [5, 31]. In previous studies, CDKN2 deletion was not found in CML-CP patients, but had been detected in part of CML-BC- and Ph-positive ALL patients [32, 33]. Commonly, the CDKN2 gene cluster was silenced in CML-CP progenitors. However, during the process of differentiation, the CDKN2 gene undergoes a series of epigenetic changes in response to BCR-ABL-induced oncogenic signals and consequently stimulates p53 which degrades initial tumor cells via apoptosis. On the other hand, the abnormal progenitors which sustain deletions of the CDKN2 gene acquire an intrinsic self-renewing ability and eventually contribute to the transition to CML lymphoid blast crisis [34]. Previous studies in mouse models, which injected mice with Arf−/− or Arf+/− p210 (BCR-ABL)-positive pre-B cells, demonstrated an aggressive and a higher dose imatinib resistance model [35, 36]. Williams and his team suggested that the BCR-ABL fusion gene and CDKN2 deletion interferes with the tumor suppressor network of Rb and p53, thereby accelerating the self-renewal of leukemia initial cells and enhancing its resistance to the drug [37]. p16(INK4a) or p14(ARF) which are transfected into primary blast cells from CML in blast crisis (CML-BC) and Ph-positive ALL could result in the inhibition of cell proliferation and an increase in cell apoptosis and could further promote sensitivity to imatinib [38]. Recent studies in Ph-positive ALL mouse models showed the attenuation effect of silenced CDKN2 to targeted BCR-ABL kinase inhibitors. Also, CDKN2 inactivation contributes to the prolonged survival of leukemia-initiating cells within the hematopoietic stem cell (HSC) environment and gives rise to the formation of malignant clone cells containing drug-resistant BCR-ABL kinase mutations [35, 36]. Typically, BCR-ABL mutations represent drug resistance in Ph-positive ALL patients, but the deletion of CDKN2 in Ph-positive ALL patients further exacerbates the disease condition and eliminates the favorable outcome of targeted therapy. Together, these mechanisms may explain why additional imatinib or dasatinib therapy presents a poor prognosis in Ph-positive ALL patients with CDKN2 deletion.