FLT3

Primary Author(s)*

Dr Kay Weng Choy PhD, FRACP, Monash Health

Synonyms

Fms Related Tyrosine Kinase 3, Fms-like Tyrosine Kinase 3

Genomic Location

Cytoband: 13q12.2

Genomic Coordinates:

chr13:28,577,411-28,674,729 (GRCh37/hg19)

chr13:28,003,274-28,100,592 (GRCh38/hg38)

Cancer Category/Type

Acute myeloid leukemia (AML)

Gene Overview

FLT3 is a member of the type III receptor tyrosine kinase family that regulates hematopoiesis [1]. The receptor is activated by binding of the FLT3 ligand to the extracellular domain, which leads to homodimer formation in the plasma membrane and consequently autophosphorylation of tyrosine residues in the receptor; the activated receptor kinase then phosphorylates and activates multiple cytoplasmic effector molecules involved in proliferation and differentiation of hematopoietic cells in the bone marrow [2]. FLT3 contains five functional domains: an immunoglobulin-like extracellular domain, a transmembrane domain, a juxtamembrane domain (JMD), an interrupted tyrosine kinase domain (TKD), and a small C-terminal domain [3].

FLT3 mutations occur in about one-third of patients with AML [3]. In-frame duplications of 3 to >400 base pairs, also known as internal tandem duplications (ITDs), are the most common mutations in FLT3 and they occur in up to 30% of adult patients with de novo AML [3,4,5,6]. About 70% of FLT3-ITDs occur in the JMD and about 30% in the TKD [3]. The JMD inhibits activation of the receptor by steric hindrance, preventing the TKD from assuming an active conformation; presence of an ITD causes loss of this inhibitory effect, resulting in activation of the TKD.

Differences in expression levels [measured using the FLT3-ITD-to-wild-type (WT) allelic ratio] have prognostic implications [3,7,8,9]. It is commonly agreed that a high FLT3-ITD-to-wild-type allelic ratio is a negative prognostic factor (regardless of cytogenetics); the 2017 European Leukemia Net (ELN) guidelines defined 0.5 as the cut-off between low and high allelic ratios [10]. FLT3-ITD remains relevant as a prognostic factor even after intensive chemotherapy and/or stem cell transplant [3]. FLT3 testing was historically viewed as being purely prognostic; however, with the advent of FLT3 inhibitors, it will likely be considered as both prognostic (clinical outcome) and predictive (treatment benefit) [3]. The second most common type of FLT3 mutations in AML are those within the TKD (occurring in up to 14% of adult patients with AML) [3]. The majority are point mutations within the activation loop (e.g., residues D835, I836, Y842) of the TKD2, and within the TKD1 (e.g., residues N676, F691) [3,11]. As a result of amino acid substitutions, changes in the activation loop favor the active kinase confirmation. The prognostic significance of FLT3-TKD mutations is controversial and may depend on additional mutations and cytogenetics [3].

FLT3-ITD and FLT3-TKD mutations are common in patients with AML with normal karyotype but they can also be associated with abnormal karyotype, such as t(15;17)/PML-RARA and core binding factor (CBF) AML. FLT3-ITD is frequently associated with t(6;9)(p23;q34.1) abnormalities [12]. The prognosis for patients with FLT3 mutations can be affected by the presence or absence of additional mutations. Patients who are FLT3-ITD negative (FLT3-ITD-) or FLT3-ITD low and positive for nucleophosmin 1 mutations (NPM1+) have a “favorable” prognosis; patients who are FLT3-ITD- (or FLT3-ITDlow) with NPM1-WT have an intermediate prognosis [10]. On the other hand, patients who are FLT3-ITDhigh with NPM1-WT have a “poor” prognosis and are less likely to achieve complete remission (CR) with induction chemotherapy than patients with other FLT3/NPM1 combinations [3,13].

Allogeneic hematopoietic stem cell transplant (alloHSCT) is generally recommended for patients with FLT3-ITD mutations in first complete remission (CR1), provided they are eligible for transplant therapy and have a suitable donor. Among patients with FL3-ITD mutations in CR1, those who undergo alloHSCT have significantly better outcomes (e.g., prolonged survival and decreased risk of relapse) than those who receive chemotherapy alone. Despite this, FLT3-ITD remains a poor prognostic factor (high relapse rate and short relapse-free and overall survival) after alloHSCT and chemotherapy. Patients with FLT3-ITD AML may benefit from the use of FLT3 tyrosine kinase inhibitors as maintenance therapy to prevent relapse following alloHSCT. Given its prognostic and predictive values, it is argued that FLT3 testing should be performed in all AMLs regardless of cytogenetics [3]. If the FLT3-ITD-to-WT allelic ratio is used for risk stratification, harmonisation of FLT3 testing is essential to ensure that comparable results are achieved [3].

Common Alteration Types

In-frame duplications of 3 to >400 base pairs, also known as internal tandem duplications (ITDs), are the most common mutations in FLT3 and they occur in up to 30% of adult patients with de novo AML [3,4,5,6]. About 70% of FLT3-ITDs occur in the JMD and about 30% in the TKD. See Figure in [3].

The second most common type of FLT3 mutations in AML are those within the TKD (occurring in up to 14% of adult patients with AML) [3]. The majority are point mutations within the activation loop (e.g., residues D835, I836, Y842) of the TKD2, and within the TKD1 (e.g., residues N676, F691) [3,11]; activating mutations caused by insertions (e.g., insertion of glycine and serine between residues S840 and N841) and deletions have also been found in TKD [3].

Additional FLT3 point mutations that have been found in patients with AML include mutations within the extracellular domain (e.g. T167, V194, D324, Y364, and V491), transmembrane domain (e.g., I548, V557), JMD (e.g., V579, E598), TKD1 (e.g., A680, M737), and TKD2 (e.g., V816, A814, T784). See Figure in [3].

Copy Number Loss	Copy Number Gain	LOH	Loss-of-Function Mutation	Gain-of-Function Mutation	Translocation/Fusion
EXAMPLE: X	EXAMPLE: X	EXAMPLE: X	EXAMPLE: X	EXAMPLE: X	EXAMPLE: X

Internal Pages

Not applicable.

External Links

Put your text here - Include as applicable links to: 1) Atlas of Genetics and Cytogenetics in Oncology and Haematology, 2) COSMIC, 3) CIViC, 4) St. Jude ProteinPaint, 5) Precision Medicine Knnowledgebase (Weill Cornell), 6) Cancer Index, 7) OncoKB, 8) My Cancer Genome, 9) UniProt, 10) Pfam, 11) GeneCards, 12) GeneReviews, and 13) Any gene-specific databases.

EXAMPLES

FLT3 by Atlas of Genetics and Cytogenetics in Oncology and Haematology - detailed gene information

FLT3 by COSMIC - sequence information, expression, catalogue of mutations

FLT3 by CIViC - general knowledge and evidence-based variant specific information

FLT3 by Precision Medicine Knowledgebase (Weill Cornell) - manually vetted interpretations of variants and CNVs

FLT3 by Cancer Genetics Web - gene, pathway, publication information matched to cancer type

FLT3 by OncoKB - mutational landscape, mutation effect, variant classification

FLT3 by My Cancer Genome - brief gene overview

FLT3 by UniProt - protein and molecular structure and function

FLT3 by Pfam - gene and protein structure and function information

FLT3 by GeneCards - general gene information and summaries

References

EXAMPLE Book

1. Arber DA, et al., (2008). Acute myeloid leukaemia with recurrent genetic abnormalities, in World Health Organization Classification of Tumours of Haematopoietic and Lymphoid Tissues, 4th edition. Swerdlow SH, Campo E, Harris NL, Jaffe ES, Pileri SA, Stein H, Thiele J, Vardiman JW, Editors. IARC Press: Lyon, France, p117-118.

EXAMPLE Journal Article

1. Li Y, et al., (2001). Fusion of two novel genes, RBM15 and MKL1, in the t(1;22)(p13;q13) of acute megakaryoblastic leukemia. Nat Genet 28:220-221, PMID 11431691.

Notes

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