Clinical Course and Significance of the Novel FLT3-Y842C Mutation in a Patient with AML Treated with PKC412 Monotherapy.
Authors: Kindler T. , Breitenbuecher F. , Kasper S. , Estey E. , Giles F. , Feldman E. , Ehninger G. , Schiller G. , Klimek V. , Nimer S. D. , Gratwohl A. , Choudhary C. , Mueller-Tidow C. , Serve H. , Gschaidmeier H. , Cohen P. S. , Huber C. , Fischer T. 
Published: April 2020

Abstract
We recently identified a novel mutation (Y842C) within the tyrosine kinase domain of FLT3 in a patient treated with PKC410 monotherapy (ASH 2003, # 4681). Here, we present follow up studies including the clinical course of the patient and frequency analysis in 110 patients with AML. In addition, we characterized the novel mutation using overexpression of FLT3-Y842C in 32D cells.
AML M2 was diagnosed in a 63 year old, male patient in 1993. After having experienced his second relapse upon standard therapy the patient was refractory to alemtuzumab treatment. Due to reduced performance status the patient was not eligible to standard chemotherapy and was enrolled into a phase II trial investigating PKC412. On conventional FLT3 mutation analysis the patient was considered to be FLT3 wild-type. Upon 8 and 29 days of treatment complete clearance of PB blast counts and BM blast infiltration was observed, respectively. Daily substitution of G-CSF resulted in transient recovery or the patients ANC′s. Since the patient showed an excellent clinical responsiveness, we reasoned whether the patient may have a yet unidentified FLT3 mutation. Sequence analysis revealed a novel point mutation in exon 21 of FLT3 (Y842C). Protein analysis of primary AML blasts showed constitutive FLT3 tyrosine-phosphorylation, ex vivo treatment with PKC412 caused significant inhibition of FLT3 and STAT5 activation. Further, in vivo analysis of FLT3 tyrosine-phosphorylation during the course of PKC412 treatment showed complete suppression of FLT3 activation within 8 days.
Overexpression of FLT3-Y842C in 32D cells resulted in constitutive activation of FLT3 and STAT5 as well as in factor independent proliferation. Treatment with PKC412 caused inhibition of FLT3 tyrosine-phosphorylation, factor independent growth and apoptotic cell death. To further investigate the clinical significance of the novel Y842C mutation, the tyrosine kinase domain of FLT3 was investigated in 110 patients with AML using sequence analysis. Altogether, the novel mutation Y842C was identified in 2 patients, FLT-ITD in 22 patients and D835 in 7 patients, respectively.
It is interesting to note that the recently described crystal structure of FLT3 reveals a critical role for Y842 in regulating the switch from the closed to the open (=active) conformation of the FLT3 activation loop. Since our data is consistent with the concept that the Y842C mutation results in constitutive activation of FLT3, it is tempting to speculate that the exchange of tyrosine for cysteine at position 842 disrupts the autoinhibited state of the FLT3 activation loop. Given that the novel mutation described here could only be identified by direct sequencing, it is likely that the number of mutations in this region of FLT3 is currently underestimated. Thus, extended sequence analysis of this mutational hotspot may be helpful in further defining the spectrum of TKI-sensitive FLT3 mutations in AML.