The secondary KIT mutation p.Ala510Val in a cutaneous mast cell tumour carrying the activating mutation p.Asn508Ile confers resistance to masitinib in dogs

Abstract

Background: Gain-of-function mutations in KIT are driver events of oncogenesis in mast cell tumours (MCTs) affecting companion animals. Somatic mutations of KIT determine the constitutive activation of the tyrosine kinase receptor leading to a worse prognosis and a shorter survival time than MCTs harbouring wild-type KIT. However, canine MCTs carrying KIT somatic mutations generally respond well to tyrosine kinase inhibitors; hence their presence represents a predictor of treatment effectiveness, and its detection allows implementing a stratified medical approach. Despite this, veterinary oncologists experience treatment failures, even with targeted therapies whose cause cannot be elucidated. The first case of an MCT-affected dog caused by a secondary mutation in the tyrosine kinase domain responsible for resistance has recently been reported. The knowledge of this and all the other mutations responsible for resistance would allow the effective bedside implementation of a deeply stratified and more effective medical approach. Case presentation: The second case of a canine MCT carrying a different resistance mutation is herein described. The case was characterised by aggressive behaviour and early metastasis unresponsive to both vinblastine-and masitinib-based treatments. Molecular profiling of the tumoural masses revealed two different mutations; other than the already known activating mutation p.Asn508Ile in KIT exon 9, which is tyrosine kinase inhibitor-sensitive, a nearly adjacent secondary missense mutation, p.Ala510Val, which had never before been described, was detected. In vitro transfection experiments showed that the secondary mutation did not cause the constitutive activation by itself but played a role in conferring resistance to masitinib. Conclusions: This study highlighted the importance of the accurate molecular profiling of an MCT in order to improve understanding of the molecular mechanism underlying tumourigenesis and reveal chemoresistance in MCTs for more effective therapies. The detection of the somatic mutations responsible for resistance should be included in the molecular screening of MCTs, and a systematic analysis of all the cases characterised by unexpected refractoriness to therapies should be investigated in depth at both the genetic and the phenotypic level.

Histological classification and expression of markers of canine mast cell tumors

“…It is important to consider that c-kit mutations and abnormal expression of c-kit are related to increase in vitro cell proliferation of MCTs [ 27 ]. c-kit is a reliable immunohistochemical marker, and a positive relationship is expected between the presence of c-kit mutations and a higher grade of canine MCT [ 8 , 9 ]. The findings of the present study show that the expression of p53 was similar to that of c-kit, indicating that p53 expression can also be used to estimate the prognosis of canine MCTs.…”

“…For Amagai et al [38], the analysis of c-kit labeling in a single tumor region is probably insufficient. The presence of the c-kit mutation in primary MCTs may indicate a high probability of metastases, suggesting that c-kit labeling indicates tumor progression [8].…”

“…The c-kit proto-oncogene product is used as a marker for the diagnosis and prognosis of MCT. Measurement of the expression of the c-kit proto-oncogene by immunohistochemistry is a valuable method for the detection of MCT [8][9][10]. Another likely cause of MCT is mutation of the tumor suppressor gene tumor protein 53 (TP53), which regulates an extensive network protecting the integrity of the genome from damage and encodes phosphoprotein 53 (p53).…”