The AML Hub uses cookies on this website. They help us give you the best online experience. By continuing to use our website without changing your cookie settings, you agree to our use of cookies in accordance with our updated Cookie Policy

KIT mutations in pediatric, core binding factor (CBF), acute myeloid leukemia (AML) 

Aug 14, 2019

Core binding factor (CBF)* acute myeloid leukemia (AML) is present in 20–30% of pediatric cases and is associated with favorable outcomes. The co-occurrence of KITmutations, found in 20–40% of patients, has been reported to be a negative prognostic factor in some studies, however, other contradictory studies reported no association between KITmutation and clinical outcome. 1Whilst the European Leukemia Network (ELN) 2017 guidelines recognize that CBF AML with KITmutations appear to be associated with a poorer prognosis, they do not recommend patients are assigned to a different genetic risk category. 2

To better determine whether KITmutations are of prognostic significance in CBF AML, Katherine Tarlock, Fred Hutchinson Cancer Research Centerand Seattle Children’s Hospital, Seattle, US, and colleagues conducted a study, recently published in Clinical Cancer Research . 1They aimed to:

  • Identify the functional impact of distinct KITmutation subsets
  • Analyze the response of different KITmutants to tyrosine kinase inhibitors (TKIs) in vitro
  • Define the prognostic impact of KIT +CBF AML using data from a phase III trial, COG AAML0531 ( NCT01407757)

* CBF AML is associated with the cytogenetic abnormalities translocation (t)(8;21)(q22;q22) and inversion(16)(p13;1q22)/t(16;16)(p13;q22)

Background 1

The KITproto-oncogene encodes a transmembrane glycoprotein type III receptor tyrosine kinase (RTK). KITmutations cause significant oncogenic events as they affect RTK activity. In some malignancies with KITmutations, such as gastrointestinal stromal tumors, melanomas, and mastocytosis. TKIs have proven effective and have become standard-of-care for first-line therapeutic approaches.

Study design and patient characteristics 1

  • Evaluated two KITmutations:
    • Immunoglobulin G (IgG) domain - exon 8 (E8)
    • Tyrosine kinase domain (TKD) - exon 17 (E17)
  • Used data from the phase III study COG AAML0531 (NCT01407757) to analyze patient outcomes by mutation status ( KIT + vs KIT -) and mutation location (E8 vsE17)
    • Study compared the use of the CD33-targeted antibody-drug conjugate, gemtuzumab ozogamicin (GO) in combination with conventional chemotherapy, to conventional chemotherapy alone
    • Of the total patient population (N= 1,022), 247 eligible patients were identified with CBF AML, of whom 205 had available bone marrow (BM) or peripheral blood specimens for KITanalysis
    • KITmutations were identified in 63 patients (31%). These were located in:
      • E8 only: 22 (35%)
      • E17 only: 32 (51%)
      • Both E8 and E17: 6 (10%)
      • Alternative exons: 3 (5%)
    • Outcomes of patients with KIT +(n= 63) were compared to those who were KIT -(n= 142)
      • No significant differences in baseline characteristics between two groups and no difference in treatment assignment
      • KIT +patients had a higher percentage of BM blast cells and peripheral blasts cells

Results 1

Growth characteristics of transduced cells in vitro

  • HEK293 cells transduced with E8 mutations in the IgG domain (D419G, D419del) showed:
    • No KIT phosphorylation
    • No interleukin-3 (IL-3) independent proliferation when transduced into Ba/F3 cells
  • HEK293 cells transduced with E17 mutations in the TKD (D816V, N822K) showed:
    • Autonomous receptor phosphorylation
    • IL-3 independent proliferation in Ba/F3 cells

Response of transduced cells to TKI in vitro

  • Ba/F3 cells (E17 variants): uniquely susceptible to the TKIs, dasatinib and crenolanib
    • Both dasatinib and crenolanib provided an apoptotic effect
    • Supports E17 mutations as therapeutic targets in AML
  • Ba/F3 cells (parental or KIT -): no response to TKIs with IC 50s > 10,000 nmol/L
    • Cells transduced with E8 KITmutations were insensitive to the cytotoxic effects of TKIs
  • These results support the investigation of using TKIs in patients with E17, but not E8, CBF AML

Clinical analysis

The prognostic significance of KITmutations was analyzed by comparing outcomes between patients with KIT +and KIT -CBF AML ( Table 1).

  • Five-year overall survival (OS) was comparable between both groups
  • There was an overall trend for decreased event-free survival (EFS) and disease-free survival (DFS), and increased relapse rate (RR) in KIT +patients
  • In patients who received traditional chemotherapy, those with KITmutations had inferior EFS, DFS and a higher RR
  • When comparing outcomes by location of KITmutation and treatment type ( Tables 2–4)
    • E8 KIT +patients: no improvement in outcome when treated with GO
    • E17 KIT +patients: DFS and RR were comparable to KIT -patients indicating a trend towards improved outcomes in this subset of patients. Indicates GO abrogated the negative impact of E17 KITmutation

In the Tables below, statistically significant results are indicated in a boldfont.

Table 1.Comparison of outcomes to evaluate the impact of KITmutations

 

KIT +(%)

KIT -(%)

pvalue

Five-year OS

78 ± 12

81% ± 7

0.905

Patients in CR

DFS

55 ± 14

73 ± 8

0.040

RR

43 ± 14

21 ± 8

0.005

Treatment with conventional chemotherapy only

EFS

44 ± 20

70 ± 11

0.021

DFS

43 ± 20

69 ± 12

0.034

RR

57 ± 21

28 ± 12

0.013

Table 2.Comparison of outcomes in patients with E17 KITmutations, overall and by treatment

 

KIT +(%)

KIT -(%)

pvalue

E17 KIT +versus KIT -

 

 

 

OS

72 ± 17

81 ± 7

0.478

DFS

51 ± 18

73 ± 8

0.027

RR

46 ± 19

21 ± 8

0.007

E17 KIT +versus KIT -treated with GO

DFS

67 ± 24

78 ± 11

0.396

RR

27 ± 24

13 ± 9

0.207

E17 KIT +versus KIT -treated with conventional chemotherapy

DFS

35 ± 25

69 ± 12

0.021

RR

65 ± 27

28 ± 12

0.010

Table 3.Comparison of outcome in E17 KIT +by treatment type

 

GO (%)

Conventional chemotherapy

pvalue

DFS

67 ± 24

35 ± 25

0.161

RR

27 ± 24

65 ± 27

0.062

Table 4.Comparison of outcomes in patients with E8 KITmutations, overall and by treatment

 

KIT +(%)

KIT -(%)

pvalue

E8 KIT +versus KIT -

 

 

 

OS

91 ± 12

81 ± 7

0.302

DFS

60 ± 22

73 ± 8

0.305

RR

40 ± 23

21 ± 8

0.072

E8 KIT +versus KIT -treated with GO

OS

100 ± 0

77 ± 10

0.092

DFS

64 ± 29

78 ± 11

0.456

RR

36± 31

13 ± 9

0.082

Advantages and limitations of analysis 1

Advantages

  • Patients were homogenously treated with the same chemotherapy backbone
    • Provides an insight into distinct functional properties of KITmutations in context of GO treatment
  • Large scale analysis: allowed investigation by location of KITmutation
  • Investigated CD33-targeted therapy in KIT +CBF AML
    • Provides an insight into the potential therapeutic efficacy of anti-CD33 agents in this population

Limitations

  • Prognostic impact of KITmutations may be influenced by therapy (inclusion of GO and no GO patients)
  • A larger cohort size would be required to definitively confirm these results

Conclusions 1

In vitro:

  • KITmutations in E17 were associated with aberrant KITphosphorylation, and a higher sensitivity to KIT-directed TKIs in vitro
  • KITmutations in E8 had no functional impact

In vivo

  • Patients with E17-mutated CBF AML form a high-risk subgroup
  • Despite a lower CD33 expression than other AML subgroups, this study supports the use of CD33-targeted, intensive treatment for the E17 KIT +subgroup
  • Adding a TKI to pediatric CBF AML treatment may be an important strategy
    • TKIs have been shown to be safe when combined with intensive chemotherapy
  1. Tarlock K. et al., Functional Properties of KIT Mutations Are Associated with Differential Clinical Outcomes and Response to Targeted Therapeutics in CBF Acute Myeloid Leukemia. Clin Can Res. 2019 Jun 10. DOI: 10.1158/1078-0432.CCR-18-1897
  2. Döhner H. et al., Diagnosis and management of AML in adults: 2017 ELN recommendations from an international expert panel. Blood. 2016 Nov 28. DOI: 10.1182/blood-2016-08-733196