A Novel KIT INDEL Mutation in Acute Myeloid Leukemia With t(8;21)(q22;q22); RUNX1-RUNX1T1

Dear Editor, KIT (also known as c-KIT) is a receptor tyrosine kinase expressed on various cell types [1]. KIT mutations are detected in approximately 5% of patients newly diagnosed with acute myeloid leukemia (AML) and in 20-30% of patients with core-binding factor AML [2, 3]. KIT mutations are predominantly presented as one or two amino acid substitutions affecting codon 816. The most frequent mutation type, D816V, is detected in about 40% of cases with AML, where KIT is mutated [4]. In AML with t(8;21), the presence of KIT mutation at codon 816 is associated with a high white blood cell (WBC) count at diagnosis, a high incidence of extramedullary leukemia, and high risk of relapse during the course of the disease [5]. Other less frequent types of KIT mutation are D816Y, D816H, and D816I. KIT mutations present a prognostic significance. The National Comprehensive Cancer Network categorized AML with either t(8;21) or inv(16)/t(16;16) combined with KIT mutations as the intermediate-risk group [6]. We report a novel INDEL mutation in KIT codon 816, discovered in a woman with AML with t(8;21) (q22;q22); RUNX1-RUNX1T1. A 35-yr-old woman was admitted for purpura in both lower limbs. The initial complete blood cell count indicated: WBC count, 20.1×10/L; hemoglobin, 8.2 g/dL; and platelet count, 15×10/L. A peripheral blood smear revealed leukocytosis with 45% of blasts. The bone marrow (BM) aspirate smear and cytochemical stains showed an increased number of myeloblasts (68%) and other myeloid precursors (Fig. 1A). The 197 bpsized transcript of RUNX1 exon 6/RUNX1T1 exon 2 was detected in a multiplex PCR analysis of BM aspirates. In the cytogenetic study, the karyotype was 46,XX,t(8;21)(q22;q22) [18]/46,XX[2]. The patient was diagnosed with AML with t(8;21) (q22;q22); RUNX1-RUNX1T1. In the screening test for KIT mutations using a real-time allele-specific PCR kit (Real-Q C-KIT; BioSewoom, Seoul, Korea), a mutation was detected, which was not the D816V, D816Y, or D816H type (Fig. 1B). Conventional sequencing was performed to identify the mutation. Unlike the common mutation types presenting one or two ambiguous bases in codon 816, the sequencing result showed five consecutive ambiguous bases through codon 815–817 (Fig. 1C). Allele separation by T-vector cloning using pGEM-T Easy Vector Systems (Promega, Madison, WI, USA) and additional conventional sequencing were performed and a missense INDEL mutation NM_000222.2:c.2445_2449delAGACAinsGTTAC (p.D816_ I817delinsLL) was confirmed. To the best of our knowledge, this mutation has not yet been reported. Three in-silico prediction tools were utilized to predict the effect of the mutation on the KIT protein function and stability (Table 1). Using SIFT (Sorting Tolerant From Intolerant, version 1.03,