4. Discussion
So far survival analysis has not been performed to evaluate the impact
of EVI1 mRNA expression in ALL. In the current study, basing on the EVI1
transcript detected by RQ-PCR in our cohort and further validating by
RNA-seq data from TARGET database, we demonstrated that low EVI1
expression at diagnosis predicted poor outcomes in pediatric Ph-negative
BCP-ALL patients receiving chemotherapy only.
EVI1 gene encodes an oncogenic transcription factor and aberrant EVI1
expression has been found in various human myeloid malignancies,
including AML, MDS, and CML.6 High EVI1 expression
predicted poor outcomes in adult and pediatric AML patients as well as
ICR-AML patients.9-13 EVI1 could induce MDS in the
mouse model, and it was a poor prognostic marker for MDS
patients.24,25 In CML patients, high EVI1 expression
was related to blast crisis.26 Compared to AML, the
prognostic role of EVI1 in ALL remains obscure.
ALL is the most common cancer and the most frequent cause of death from
cancer among children. It is a heterogeneous disease with various
genetic abnormalities.27 Although the outcomes of
pediatric BCP-ALL have improved greatly in the past decades, relapse
remains occur in the minority leading to death.28Therefore, searching for new prognostic markers is still necessary in
order to guide appropriate treatment selection and improve the overall
outcomes. Therefore, the prognostic impact of EVI1 expression in
pediatric BCP-ALL needs to be evaluated.
EVI1 expression patterns were evaluated firstly. Several studies have
showed EVI1 transcript levels in ALL which tested by PCR or
microarray.14,15,18 T-ALL patients and cell lines were
found to have no detectable EVI1 transcript by
RT-PCR.16 Whereas, Su G et al performed RQ-PCR and
found that T-ALL had significantly higher EVI1 levels than BCP-ALL.
Furthermore, they showed that the overall EVI1 transcript levels were
significantly higher in ALL than AML.15 We also found
the similar result in pediatric Ph-negative BCP-ALL patients. That is,
they generally had significantly higher levels than adult ICR-AML
patients which we reported previously,13 and great
variation existed among patients.
The cutoff value for patient grouping usually affected the final
comparison result greatly. As for the cutoff value of EVI1 transcript,
both the upper limit and the mean±standard deviation values of NBM were
ever used in previous studies.13,15 Furthermore, some
studies arbitrarily determined.9,14 In the current
study, quartile grouping implied that low EVI1 expression was related to
low RFS rate, and ROC curve was used to determine the optimal cutoff
value.
The survival analysis in our cohort showed that low EVI1 expression
independently predicted low RFS and OS rate. The TARGET data for
validation showed the similarly negative impact of low EVI1 expression
on OS rate though the P value is no less than 0.05. However, a
previous in vitro and mice transplantation study demonstrated that EVI1
contributes to the leukemogenic potential and apoptosis resistance of
ALL cells, which implied the poor prognostic role of
EVI1.14 One reason for this contradictory result is
that protein function is usually cell context-specific. Therefore, the
in vitro result might be different from that in vivo and cell line could
not represent primary leukemic cells completely. Another reason might be
the EVI1 level. Just as illustrated by our quartile grouping result, the
2nd to 4th quartile group had similar RFS. Therefore, so-called low or
high EVI1 expression must be clearly defined and the in vitro study
should correspond to clinical meaningful value in order to clarify the
function of EVI1 in ALL.
The percentage of patients belonging to low EVI1 expression were
different in our cohort and TARGET cohort for validation, 18.2% and
72.9%, respectively. It might be caused by different case composition.
The TARGET ALL Pilot project included comprehensive genomic profiles of
nearly 200 high-risk, clinically annotated, BCP-ALL patient cases from
Children’s Oncology Group (COG), most of which experienced an early bone
marrow relapse (within 4 years of initial diagnosis).
A significant correlation between high EVI1 expression and MLL
rearrangement has been demonstrated in both adult and pediatric AML
patients.9-12,29 However, we found an association
between low EVI1 expression and MLL rearrangement in pediatric BCP-ALL
patients. Furthermore, TARGET data and previous reports did not show
this relationship in BCP-ALL patients.15,18 These
inconsistent results reflected that the association between MLL
rearrangement and EVI1 expression were related to leukemia type.
Difference in the partner of MLL fusion and all enrolled patients
received chemotherapy only in our cohort might also be the reasons. In
addition, both our cohort and TARGET data showed that patients with
TEL-AML1 fusion gene had high EVI1 expression, and its mechanism remains
to be studied.