Discussion
IKZF1 is composed of eight exons and encodes the transcription factor
Ikaros [9]. IKZF1 alterations can occur frequently
in germline cells and somatic cells, and it has been studied
extensively. IKZF1 alterations in somatic cells are found in
approximately 15% of pediatric patients with ALL[9] patients, but the alternations increase in
prevalence among adult patients with ALL. The most common types of IKZF1
alterations are referred to as focal deletions, such as the whole gene
and exons 4–7 deletions, resulting in loss-of-function in Ikaros,
whereas IKZF1 mutations were less reported, including missense
mutations, nonsense mutations, and frameshift mutations[2]. IKZF1 deletions are important molecular
genetic makers in the development and relapse in ALL and attract
significant enough attention, until Mullighan et al. first described the
IKZF1 deletions in patients with BCR-ABL1 positive ALL[12] in 2008. Overall, the prevalence of IKZF1
deletions is 10% -30% in pediatric patients with B-ALL who are
BCR-ABL1 negative [10], and can be as high as
56.8%-70.6% and 27%-68% in pediatric patient with B-ALL who are
BCR-ABL1 positive or who have Ph-like ALL. These patients are often
associated with high WBC counts, older age, poor chemotherapy responses,
higher risks of relapse, and insensitivity to TKIs[13, 14]. In contrast, among ALL subtypes with
good prognosis, the prevalence of IKZF1 deletions is low, accounting for
3%–6% and 15% of ETV6-RUNX1 and hyperdiploid cases[10, 15, 16]. Thus, for a long time, IKZF1 gene
deletions have been established to be an independent adverse prognostic
factor for patients with ALL [3, 17]. Therefore,
screening for IKZF1 deletions is very important in recognizing high-risk
patients and initiating risk-adapted chemotherapy for pediatric patients
with ALL.
Most of the published data on IKZF1 deletions have been generated using
MLPA analyses, SNP arrays, or multiplex PCR assays. Previous results
have shown that RQ-PCR based on ALB as a control gene is a reliable and
sensitive method for detecting IKZF1 deletions[5], and there is a good correlation between
quantification of MRD and the level of IKZF1 deletions[5, 17, 18]. In this study, we conducted a
quantitative analysis of IKZF1 deletions in pediatric patients with
BCP-ALL to assess the impact of different levels of IKZF1 deletions on
prognosis. The level of IKZF1 deletions in our patients ranged from 0.01
to 243.76%, with 60.8% of them being in the IKZF1 deletions of
< 1% group. The rate of IKZF1 deletions in patients with ALL
who were ETV6-RUNX1 positive or hyperdiploid were account for 10.8% and
14.9%, respectively, which is consistent with previous observations.
Overall, 13.5% of the patients were BCR-ABL1 positive but patients with
IKZF1 deletions of ≥ 1% had a significantly higher BCR-ABL1 positive
rate (31.0%) than those with IKZF1 deletions of < 1%
(2.2%). In contrast, all patients who were ETV6/RUNX1 positive had
IKZF1 deletions of < 1%. Wu et al. [5]have reported that 90% of adult patients with IKZF1 deletions had
levels of IKZF1 deletions of ≥ 1%, and the rate of IKZF1 deletions in
patients who were BCR-ABL1 positive was 68.8%. These results differ
from ours. One reason for this may be that patients who are in the
high-risk group and BCR-ABL1 positive are more likely to had high levels
of IKZF1 deletions; therefore, in our study fewer pediatric patients
with ALL were in the high-risk group and were BCR-ABL1 positive than
adult patients.
In our study, the percentages of patients whose age at onset was ≥10
years old, had WBC ≥ 50×109/L at initial diagnosis,
and in high-risk group were significantly higher in the IKZF1 deletions
of ≥1% group than in the IKZF1 deletions of <1% group. In
contrast, there were no differences in other clinical and laboratory
characteristics between the two groups. Various clinical trials have
shown that IKZF1 deletions are associated with an older age at
diagnosis, a higher WBC count, higher levels of MRD after induction, and
a higher risk of relapse, which corresponded with our study[2, 13, 15, 20-22]. We found no difference in MRD
at 15 and 33 days between the two groups of patients with different
levels of IKZF1 deletions, whereas the rate of relapse/refractory in
IKZF1 deletions of ≥1% group was significantly higher than in the IKZF1
deletions of <1% group, suggesting that there was more HR ALL
in the IKZF1 deletions of ≥1% group.
Almost all clinical studies have demonstrated that IKZF1 deletions are
an independent adverse prognostic factor in both pediatric patients with
BCP-ALL who are either BCR-ABL1 positive or negative, especially in
combination with an early MRD response. In children with BCR-ABL1
negative ALL who had been treated using the BFM-2000 protocol[15], the 5-year EFS was 69% for patients with
IKZF1 alterations vs. 85% for patients without the IKZF1 alterations.
Patients with IKZF1 alterations also had a higher incidence of relapse
(21% vs. 10%). Similar poor outcomes in pediatric patients with IKZF1
deletions were observed in a Japanese pediatric Ph-negative B-ALL cohort[23], the AEIOP-BFM 2000 cohort[24], and the IC-BFM-2002 cohort[25]. Of note, the ETV6-RUNX1 subtype should be
ruled out in patients with Ph-negative B-ALL who also have IKZF1
alterations because they have favorable outcomes despite the presence of
IKZF1 alterations [26]. Therefore, the prognosis
is still poor for BCR-ABL1–positive and Ph-like patients who have IKZF1
deletions, even if they are treated with TKIs. In AALL0622, a Children’s
Oncology Group (COG) phase II trial, patients with ALL who were Ph+ and
also had IKZF1 deletions were treated with dasatinib plus intensive
chemotherapy and were found to have inferior 5-year EFS and OS compared
to those patients with ALL who were Ph+ but had wild type IKZF1 (52%
vs. 82%, P = 0.04 and 80% vs. 100%, P = 0.04,
respectively) [27].Our results also showed that
the 3-year OS and 3-year EFS in the IKZF1 deletions of ≥1% group were
both significantly poorer than the 3-year OS (79.3% vs. 97.7%,P =0.022) and 3-year EFS (62.4% vs. 83.2%, P =0.019) in
the IKZF1 deletions of <1% group. Similar results were found
in BCR-ABL1 negative patients in corresponding different levels of IKZF1
deletions group. Therefore, the prognosis is worse in patients with
higher percentage of IKZF1 deletions, regardless of BCR-ABL1 positive or
negative. Because there was only one BCR-ABL1 positive patient in the
IKZF1 deletions of <1% group, we could not compare the effect
of different levels of IKZF1 deletions on the prognosis of BCR-ABL1
positive children. Multivariate analysis showed that the level of IKZF1
deletions of ≥ 1% and CNSL were independent risk factors of EFS. The
level IKZF1 deletions of ≥1% group is likely to be a poor prognosis
factor of OS, although no significant statistical difference was
indicated due to a limited sample size. In the future, studies on the
effect of different levels of IKZF1 deletions on prognosis with large
cohorts of pediatric patients with BCP-ALL should be carried out. In
addition, pediatric BCP-ALL patients with the level IKZF1 deletions of
≥1% may be stratified into high-risk groups due to poor prognosis and
receive intensive chemotherapy.
Our study has several limitations. First, this was a single-center study
with a small sample size. Second, due to the limitation of sample size,
comparison between subgroups could not be performed.
In conclusion, pediatric BCP-ALL patients with IKZF1 Δ2-8/ALB deletions
of <1% had a good prognosis, whereas patients with IKZF1
Δ2-8/ALB deletions of ≥1% had a poor outcomes, and these patients
always had other risks such as older age, higher WBC count, with
positive for the BCR/ABL1 fusion gene, and a higher relapse rate.
Conflicts of interest : The authors have no conflicts of
interest to declare.