Abstract
Activating transcription factor 4 (ATF4) is a crucial mediator of the integrated stress response and a negative regulator of RET tyrosine kinase receptor in medullary thyroid carcinoma (MTC). However, the impact of genomic abnormalities in the ATF4 locus on MTC pathogenesis and response to tyrosine kinase inhibitor therapy remains unknown. Here, we evaluated ATF4 copy number variation and protein levels, with overall survival and response to TKIs in a clinical cohort of fifty-nine sporadic primary MTC. We assessed the somatic RETM918T mutation by sequencing, ATF4 copy number by a real-time polymerase chain reaction, and ATF4 protein levels using immunohistochemistry. This MTC cohort comprised 45 (76%) stage IV patients with a median follow-up of 100 months (interquartile range: 58-134 months). Somatic RETM918T was present in 23/57 (40%) tumors. Mono-allelic (36%; 21/59) and bi-allelic (5%; 3/59) loss of ATF4 was identified and was associated with low ATF4 protein expression (0-20%). Kaplan-Meier curves highlight low ATF4 protein or ATF4 loss alone had a significant negative impact on median survival compared to high protein expression (P<0.001) or diploid ATF4 (P=0.011), respectively. The combination of somatic RETM918T and low ATF4 protein levels further decreased overall survival. Both allelic loss and protein reduction were associated with worse overall survival (HR=3.79, 4.06 +RETM918T , and HR=10.64, 11.66 +RETM918T , respectively). Additionally, all 4 of the 11 patients treated with TKIs with a progressive disease by RECIST had low tumor ATF4 protein, with the two partial responder's tumors having high ATF4 protein. These findings suggest that ATF4 may predict response to tyrosine kinase inhibitors, serve as a prognostic marker for personalized care, and a therapeutic target in MTC.