Abstract: PB2023
Type: Publication Only
Background
The role of the KIT M541L variant in patients with hypereosinophilia (HE) is controversial. On the one hand, this variant is a recognised inherited single nucleotide polymorphism (c.1621 A>C; rs3822214) with a minor allele frequency of 0.08 in the ExAC database and classified as benign/likely benign on ClinVar. On the other hand, it has been suggested that KIT M541L increases the sensitivity of the KIT receptor to stem cell factor (Foster R et al., Br J Dermatol. 2008;159:1160-9) and may be somatically acquired in imatinib responsive CEL-NOS patients negative for PDGFRα/β abnormalities (Iurlo A et al., Oncotarget. 2014;5:4665-70). Consequently it has been suggested that HES patients should be screened for KIT M541L, as positive cases may benefit from imatinib treatment.
Aims
We aimed to (i) compare the KIT M541L allele frequency between patients referred for investigation of HE and normal healthy controls (ii) investigate the variant allele frequency (vaf) to determine if KIT M541L mutations may be acquired somatically and (iii) investigate the KIT M451L status in cases negative for PDGFRα/β abnormalities who responded to imatinib.
Methods
We screened healthy controls (n=214) and patients referred for investigation of FIP1L1-PDGFRA negative HE (n=220) for KIT M541L using an amplification refractory mutation system (ARMS) PCR designed to amplify allele specific products of different sizes, and able to detect KIT M541L down to 5% vaf. Fishers exact two tailed test was used to compare the allele frequency between the control and HE groups. Digital droplet PCR (ddPCR) was used for patients heterozygous for KIT M541L by the ARMS assay to determine whether the KIT M541L mutation burden was close to 50% (consistent with a constitutional polymorphism) or <50% (suggestive of a somatic mutation). We also studied pre-treatment DNA from 3 patients with hypereosinophilic syndrome who were treated with imatinib (400 mg/day) and showed normalization of eosinophil counts at a median of 0.8 months (0.4-5.0) after treatment for a duration of 13.6 months (range, 3.7-44.8).
Results
Forty two (19%) of HE cases tested positive for KIT M541 compared to 38 (18%) of healthy controls. The KIT M541L allele frequency was no different between cases and controls (0.095 versus 0.098; P=0.91). Of the 42 KIT M541L heterozygous HE cases, 40 had sufficient DNA for analysis by ddPCR. The mean allele burden was 50.4% (range 48.3%>56.0%), consistent with all instances being constitutional. None of the three imatinib responders tested positive for KIT M541L prior to treatment.
Conclusion
Whilst we cannot exclude the possibility that KIT M541L may be acquired somatically in very rare cases, we conclude that there is no clinical value in screening for this variant on a routine basis for patients with HE or HES.
Session topic: 15. Myeloproliferative neoplasms - Biology
Keyword(s): kit, Hypereosinophilic syndrome
