Early T-cell precursor leukemia: a subtype of high risk childhood acute lymphoblastic leukemia

Abstract

Acute lymphoblastic leukemia includes T-cell acute lymphoblastic leukemia (T-ALL) and B-cell acute lymphoblastic leukemia (B-ALL). In children, T-ALL usually has a worse prognosis than B-ALL, although childhood T-ALL prognoses have improved remarkably. The varying outcomes among T-ALL cases suggest that an unrecognized biological heterogeneity may contribute to chemo-resistance. Deep exploration of T-lymphocyte development in recent years has found a subgroup of patients with a phenotype that resembles early T-cell precursor, which confers a much poorer prognosis than any other form of T-ALL. This novel subtype of T-ALL was called early T-cell precursor acute lymphoblastic leukemia (ETP-ALL). Flow cytometry data from T-ALL patients enrolled in Shanghai Children’s Medical Center between July 2002 and October 2010 were assessed according to Dr. Campana’s protocol. Among total 89 T-ALL cases, 74 cases had enough immunophenotype data available to differentiate between ETP (CD1a^-, CD8^-, CD5^dim, at least one marker of stem cell or myeloid lineage) and non-ETP. From these 74 subjects, 12 ETP-ALL cases (16.2%) were identified. The event-free survival (EFS) rate at 66.8 months was 11.1%±10.1% for ETP-ALL and 57.6%±5.6% for non-ETP-ALL (P=0.003). The overall survival rates were 13.3%±11.0% for ETP-ALL and 64.7%±6.3% for non-ETP-ALL (P=0.002). Our findings demonstrate that early T-cell precursor leukemia is a very high-risk subtype of acute lymphoblastic leukemia with poor prognosis.

Keywords

acute lymphoblastic leukemia / early T precursor / prognosis

Introduction

Acute lymphoblastic leukemia (ALL) is the most common childhood malignant clonal expansion of leukemic cells. T-cell acute lymphoblastic leukemia (T-ALL) is a subtype of ALL that accounts for 10%-15% of childhood and 25% of adult ALL cases. With wider use of intensive chemotherapy, the prognosis for childhood T-ALL has improved remarkably: nearly 60% of patients can currently be cured [1]. Further improvement in treatment outcome would require increased stratification of patients so alternative therapy could be introduced as early as possible. Features of clinical presentation, such as high leukocyte count and older age at diagnosis, are now considered unreliable predictors of outcome in ALL patients treated with intensive chemotherapy [2,3]; cell marker profiling has led to controversial conclusions about prognostic significance [3-6]. More recent studies have provided insights into the genetic abnormalities underlying T-ALL development, some of which seem to relate with prognosis [7-11].

Dario Campana’s group at St. Jude Children’s Research Hospital in Memphis, Tennessee used gene expression profiling to identify a subset of T-ALL with cells that resembled early T cell precursors (ETPs). ETPs are a subset of thymocytes that immigrated from the bone marrow to the thymus and retained multilineage differentiation potential [12-14]. These ETP-ALL cases had distinctive immunophenotype markers (lack of CD1a and CD8, weak CD5 expression, one or more stem-cell or myeloid markers present). Patients with this form of leukemia had high risk of remission failure and hematological relapse. To confirm this in a Chinese population, the immunotypes of T-ALL cases from Shanghai Children’s Medical Center were reviewed.

Methods

Patients and treatment

Patients with T-ALL who were enrolled from July 2002 to October 2010 at Shanghai Children’s Medical Center were reviewed. T-ALL diagnosis was confirmed by morphological and cytochemical evaluation of bone marrow smears as well as immunophenotype and cytogenetic criteria. Among total 89 cases, 74 cases were qualified for determination of ETP-ALL or non-ETP-ALL based on the documented immunotype data. Patients received a chemotherapy protocol of XH-ALL-99 [15] if enrolled before May 1st, 2005 or SCMC-ALL-2005 [16] if enrolled after May 1st, 2005. All 74 T-ALL phenotype cases in this study were cytoplasmic CD3 or surface CD3 positive. ETP-ALL cases were identified by distinctive immunophenotype features: CD1a^–, CD8^–, CD5^dim, and one or more stem-cell or myeloid markers such as CD13, CD33, CD34, CD117, or HLA-DR .

Evaluation of therapeutic effects

Complete remission (CR) was defined as the presence of all of the following: less than 5% blasts in bone marrow; no leukemic blasts in peripheral blood (PB), recovery of PB neutrophil count to at least 1.5 × 10⁹ /L, hemoglobin of at least 90 g/L, a platelet count of at least 100 × 10⁹ /L, and no evidence of extramedullary leukemia. Those who failed to obtain CR after two courses of induction therapy were regarded as cases of induction failure. Relapse was defined as the presence of at least one of the following: recurrence of more than 10% leukemic cells in bone marrow or of any leukemic cells in PB or extramedullary sites. The treatment outcome was evaluated in terms of event-free survival (EFS). EFS duration was calculated from the day of diagnosis to the date of the first event (induction failure, relapse, death, or the occurrence of a second malignancy) or the date of final follow-up. The median follow-up duration was 66.8 months.

Statistical analysis

Overall survival (OS), EFS, and relapse rate were evaluated with the Kaplan-Meier curve. The significance was tested with the Mantel-Haenszel (log-rank) test. t-test was used to compare the difference in mean peripheral leukocyte count between ETP-ALL and non-ETP-ALL. All statistical analysis was performed using SPSS 17.0 software. The level of significance was set at 0.05.

Results

Among the 74 confirmed cases of T-ALL, the EFS rate was 50%±6% and the OS rate was 56.6%±6.1%. In total, there were 13 relapses, 5 chemotherapy related deaths, and 9 remission failures. One subgroup of 25 patients received chemotherapy according to the XH-ALL-99 protocol [15] and the other subgroup of 49 patients received chemotherapy according to the SCMC-ALL-2005 protocol [16]. The EFS rate of these subgroups were 41%±10% and 54%±7.5%, respectively (P = 0.302).

According to Campana’s protocol [14], we identified 12 ETP-ALL cases (16.2%) from the total 74 confirmed T-ALL patients, where 4 were in the XH-ALL-99 cohort and 8 were in the SCMC-ALL-2005 cohort. The distribution of ETP cases between these two cohorts was similar (16.3% vs. 16%, P = 0.97). Fig. 1 shows the flow data of an ETP-ALL case that displayed a distinctive immunophenotype: CD1a^–, CD8^–, CD5^dim and CD33⁺.

Three of the 12 ETP-ALL patients and 6 of the 62 non-ETP-ALL patients failed to obtain complete remission after the first course of therapy. The complete remission rate for ETP-ALL was 75%, whereas the complete remission rate for non-ETP-ALL was 90.3% (P = 0.15).

The predicted 5-year overall survival rate was 13.3%±11.0% for patients with ETP-ALL, and 64.7%±6.3% for patients with non-ETP-ALL (P = 0.002). Follow-up at median time 66.8 months found the EFS rate for ETP-ALL to be 11.1%±10.1% and the EFS for non-ETP-ALL to be 57.6%±5.6% (P= 0.003). 4 (3 in bone marrow, 1 in testis) of the 12 ETP-ALL patients relapsed while 9 (8 in bone marrow, 1 in CNS) of the 62 non-ETP-ALL patients relapsed (P = 0.12), as seen in Fig. 2. When we combined the resistant cases and the relapse cases, we found that 7 of the 12 ETP-ALL patients (58.3%) responded poorly to chemotherapy or relapsed, denoting a poor outcome. The percentage of non-ETP-ALL patients with poor outcomes was 24.2% (P = 0.025).

The peripheral leukocyte count at diagnosis is commonly accepted to be the prognostic factor that predicts poor outcomes in most cases of ALL. However, in ETP-ALL the white blood cell count (16.8×10⁹±18.1×10⁹ /L) is usually lower than that of typical T-ALL (125.8×10⁹±107×10⁹ /L) (P = 0.003). This difference might be the reason that peripheral leukocyte count at diagnosis does not seem to be an important prognostic factor for T-ALL. When we analyzed the entire T-ALL cohort, patients with higher leukocyte counts (≥50×10⁹ /L) appeared to have better prognoses than those with lower leukocyte counts. Conversely, when we analyzed the non-ETP-ALL cohort exclusively, higher leukocyte counts seemed to indicate a poor prognosis, although the P values for these analyses were not statistically significant.

Discussion

ETP-ALL is a subset of T-ALL that was first described by Dr. Dario Campana of Memphis, Tennessee. It has a distinctive immunophenotype and is the T-ALL subset with the worst prognosis in patients treated with contemporary chemotherapy protocols. The similarity of the ETP-ALL gene expression profile to normal early T cell precursors (early immigrants from the bone marrow to the thymus) suggests that ETP-ALL is the clonal expansion of T cells at an early stage.

Remission quality is an important early indicator of treatment response. Good remission quality usually predicts good prognosis. Our results demonstrated that first course remission rate for ETP-ALL was lower than for other T-ALLs (75% vs. 90.3%), although there was no statistical significance, which may be due to the limited number of cases we studied. Relapse rate, overall survival rate, and EFS rate reveal the long-term outcome of treatment. The ETP immunophenotype was also a very important prognostic factor for our T-ALL cohort. The relapse and remission failure rates for ETP-ALL were significantly higher than those for typical T-ALL (P = 0.025). In addition, the 5-year overall survival rate for ETP-ALL was lower compared with the rate for non-ETP-ALL (P = 0.002), and the event-free survival rate for ETP-ALL was poorer than that of typical T-ALL (P = 0.002).

If we took both ETP and non-ETP groups into account, our data also indicated that high peripheral leukocyte counts at diagnosis did not predict a poorer outcome, contradicting the widely accepted belief about this negative prognostic factor for ALL in children [17,18]. This discrepancy may be due to the lower leukocyte count in ETP-ALL compared to counts in non-ETP-ALL. When we analyzed non-ETP-ALL exclusively, a higher leukocyte count seemed to indicate a more negative prognosis; however, this result was not significant.

The poor outcomes for ETP-ALL when treated with current standard intensive chemotherapy indicate the need for alternative approaches to be explored. Most recently, whole-genome sequencing of ETP-ALL revealed that this leukemia may be caused by the activating mutations on genes regulating cytokine receptors and RAS signaling, histone-modifying genes, and inactivating molecular lesions that disrupt hematopoietic development. The mutational spectrum was found to be similar to myeloid neoplasm. Moreover, the global transcriptional profile of ETP-ALL was similar to that of normal hematopoietic stem cells and myeloid leukemia [19]. However, ETP-ALL represents a neoplasm of a less mature hematopoietic progenitor or stem cell where arrest at a very early maturational stage allows the capacity for myeloid differentiation to be retained. This observation raises the possibility that treatment regimens used to treat acute myeloid leukemia (AML) had potential for treating T-ALL. Unfortunately, there was one case in our T-ALL cohort with a distinctive ETP-ALL phenotype that failed to respond to the AML protocol (consisting of daunorubicin, cytarabine, and etoposide) or prednisone (data not shown). Nevertheless, the genetic basis of activating mutations on cytokine receptors, RAS signaling, and JAK signaling does indicate that targeted therapies inhibiting these pathways might benefit ETP-ALL patients. Alternatively, it might be possible to cause ETPs to become responsive to conventional lymphoid-cell-directed therapy by repressing genetic programs that prevent transformed ETPs from entering T cell differentiation pathways.

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