In the present study, in patients with Ph+ALL who had achieved CR by imatinib-based therapy and subsequently received allo-HSCT in their first CR, the major BCR–ABL
subtype revealed significantly unfavorable prognostic impact on NRM, and consequently on OS and DFS (). During the pre-imatinib era, several groups reported the relationship between the clinical outcome and BCR–ABL
subtypes in patients with Ph+ALL. German Multicenter Adult ALL Study Group reported a trend toward poor OS for patients with major BCR–ABL
(19% OS for the minor and 3% for the major at 3 years, P
Gruppo Italiano Malattie Ematologiche dell' Adulto also reported that minor BCR–ABL
was an independent prognostic factor favorably affecting the 5-year OS and DFS (P
=0.008 and P
=0.02, respectively), although response rates to the induction therapy were similar in both groups.16
Of note in their study, none of 14 patients with major BCR–ABL
transcript who underwent HSCT (8 allogeneic and 6 autologous) survived in CR, whereas, among 22 patients with minor BCR–ABL
, 6 of 12 who received allo-HSCT and 2 of 10 who received autologous HSCT survived in CR.
In this imatinib era study, patients with major BCR–ABL
transcript showed significantly unfavorable OS rates, compared with those with minor BCR–ABL
transcript. Among 100 patients registered into the JALSG Ph+ALL 202 study, three patients died from chemotherapy-related toxicity during induction therapy and all of them expressed minor BCR–ABL
transcript. Additionally, among 40 patients who did not receive HSCT in their first CR, OS of 7 patients with major BCR–ABL
transcript was not inferior to that of 33 patients with minor BCR–ABL
=0.254) (Supplemental figure S1
). Therefore, the unfavorable clinical impact of major BCR–ABL
transcript might be specific in the setting of allo-HSCT. Then the question arises: How the BCR–ABL
subtype influenced the prognosis after allo-HSCT?
As shown in , MRD status and the period from diagnosis to HSCT were not significantly different among patients with major or minor BCR–ABL
transcript. As the cause of NRM after allo-HSCT, high incidence of graft failure (22%) was observed in patients with major BCR–ABL
(), and to predict NRM, transplantation-specific comorbidity index (HCT-CI) is reportedly useful.17
In the present study, 54 of 60 patients could be evaluable for this scoring system, but we found no difference in HCT-CI scores between major and minor BCR–ABL
Biological heterogeneities between major and minor BCR–ABL
transcripts may have influenced NRM of HSCT. Juric et al.18
performed a comprehensive analysis of the gene expression profiles in 37 BCR–ABL
-positive adult ALL. They identified the genes overexpressed (PILRB, STS-1, SPRY1) or underexpressed (TSPAN16, ADAMTSL4) in ALL with minor BCR–ABL
transcript, relative to ALL with major BCR–ABL
, and constructed a gene expression- and interaction-based outcome predictor, consisting of 27 genes, which correlated with OS, independent of age and WBC count at presentation. Zheng et al.19
spotlighted the role of the reciprocal ABL–BCR
fusion proteins, derivative chromosome 9 (der 9) -associated p96ABL–BCR
fusion proteins. They indicated that p96ABL–BCR
fusion proteins regulated the different expression of genes involved in the maintenance of stem-cell capacity. However, even if the biological heterogeneity would affect the clinical outcome of patients, the following question would arise: Could pre-existing aberrant gene translocation before allo-HSCT affect the prognosis of patients after transplantation? An inspiring report from Kreil et al.20
verifies a function of p40ABL–BCR
fusion protein in the setting of allo-HSCT. They developed a DNA-based deletion screen, and investigated 339 patients with chronic phase CML and detected der (9) deletions in 59 (17%) patients. Of these, 21 spanned the ABL–BCR
junction and 38 were centromeric or telomeric of the breakpoint. Patients with ABL–BCR
junction-spanning deletions (p40ABL–BCR
deficiency) had poorer survival, compared with patients without deletions.20
More interestingly, this tendency was most distinctive in the setting of allo-HSCT where bone marrow was replaced by normal stem cells from healthy donor.20
Deletions that did not span the ABL–BCR
junction were associated with improved survival, compared with patients without deletions. From these, one could speculate that p40ABL–BCR
has an important role on the stem-cell re-constitution after allo-HSCT in patients with BCR–ABL-
positive leukemia, and that, even when the patient's bone marrow was replaced by normal donor stem cells, a deficiency of this protein induced by imatinib-combined chemotherapy could contribute to the relatively high incidence of graft failure (22%) in patients with major BCR–ABL
transcript as observed in our present study.
Investigation of transplant outcome of Ph+ALL patients who expressed minor BCR–ABL transcript and der (9) deletion would be helpful to evaluate clinical relevance of p96ABL–BCR. However, to our knowledge, there is no report focusing on the BCR–ABL subtypes and der (9) deletions in patients with Ph+ALL. In our present study, three patients who had der (9) deletions were all positive for minor BCR–ABL transcript and alive at the last known date of follow-up. Further investigation for clinico-biological effects of not only BCR–ABL but also ABL–BCR transcripts will be needed to clarify the prognostic relevance of BCR–ABL subtypes after allo-HSCT in patients with Ph+ALL.
We categorized two patients with both major and minor BCR–ABL
transcripts into the major BCR–ABL
transcript group. Several investigators who studied Ph+ALL with both BCR–ABL
transcripts have reported that the level of minor BCR–ABL
transcript was consistently low, such as only one transcript per 100 cells with major BCR–ABL
Fujimaki et al.22
studied four patients with Ph+ALL with both transcripts before and after allo-HSCT, and reported that PCR negativity for minor BCR–ABL
was documented in all cases 1–2 months before PCR negativity for major BCR–ABL.
Taking these preceding studies into consideration, we believe our categorization of the two patients would be justified.
In the present study, negative MRD before HSCT resulted in significantly lower relapse rate after HSCT (). Some investigators reported that MRD before HSCT served as a powerful predictor of lower relapse rate and better DFS.4, 23, 24
Therefore, prospective monitoring of MRD may potentially identify patients at risk of relapse, although the implications of different transcript levels and increments require validation within each therapeutic context or clinical study.4
These issues highlight the need for the standardization and harmonization of methodologies used for BCR–ABL
quantification in Ph+ALL.4
Employment of highly sensitive methods such as nested PCR or of normalization by total ABL
transcripts may make clear the predictive value of MRD analysis for the prognosis after HSCT.25
To our knowledge, this is the first report on the clinical impact of the BCR–ABL
subtypes on the outcomes of patients with Ph+ALL after allo-HSCT, analyzing results of a substantial number of patients with a sufficient follow-up period. However, the strength and limitations of our study need to be considered. The strength lies in the relatively large sample size, if not sufficient, and relatively homogenous population, as all patients received a uniform imatinib-combined chemotherapy regimen (JALSG Ph+ALL202)12
and underwent allo-HSCT in their first CR. These facts gave us a better estimation of the endpoints, and also added statistical power to the analyses. Our limitations are the presence of residual confounding factors, both known and unknown, and insufficient number of patients in each different prognostic factor. Among the known factors, difference in transplantation procedure, including pre-transplant conditioning regimens, should be noted. In this study, conditioning regimens and GVHD prophylaxis were determined by each institution. However, the small number of patients per institution and the changes of the conditioning regimens themselves within the same institution inevitably rendered the analysis on these factors impossible.
We have no comparative clinico-biological data in patients with Ph+ALL transplanted during the pre-imatinib era, and were unable to evaluate whether BCR–ABL subtype has a prognostic impact during that time. Further study should be undertaken to evaluate the prognostic value of BCR–ABL subtypes both in pre- and post imatinib eras.
The treatment strategy for Ph+ALL in the imatinib era, especially for Ph+ALL with major BCR–ABL transcript, should be reconsidered, and additionally, not only allo-HSCT but also second generation tyrosine kinase inhibitors need to be incorporated. Further study would be warranted to determine the clinical impact of BCR–ABL transcripts on the outcome of allo-HSCT in this disease.