AURKA and SRC alterations in neuroendocrine tumors of the small intestine

Abstract

Neuroendocrine tumors (NETs) are rare neoplasias of hormone-producing cells largely emerging from the gastroenteropancreatic system. They represent the leading malignancy of the small intestine. Due to their long asymptomatic progression SI-NETs are diagnosed late and most commonly in a metastatic stage. This limits the feasibility of radical surgery, the only curative therapy. Aside from local procedures to reduce tumor load somatostatin analogs and interferons are applied for symptomatic treatment. The development of specific immunotherapy requires an understanding of causal genetic modifications. Loss of chromosome 18 is an early event in cancer development and is thus considered the most important finding in gastrointestinal NETs. Based on comparative genomic hybridization (CGH) analyses determining further chromosomal alterations, Banck et al. conducted a whole exome sequencing (WES) study using SI-NET samples of 48 patients. They confirmed several amplifications including chromosome 20 and associated genes that facilitate enhanced cell growth and reduced apoptosis, such as AURKA and SRC. This thesis is focused on fluorescence in situ hybridization (FISH) analysis of these two genes and provides insight into their protein expression via immunohistochemistry. Comprising 217 SI-NET samples of 135 patients the study is fairly representative. Both genes were characterized by signal gains. AURKA gains were demonstrated in 24% and SRC gains in 25% of the studied cases in accordance with the results of Banck et al. FISH attested slightly higher copy numbers compared to sequencing. For the most part these gains are ascribed to amplification of major chromosomal segments (polysomy) but a few samples also featured specific gene amplifications. Moreover, this analysis yielded a distribution pattern of copy number gains between primary tumor and metastases. A significant correlation between copy number gains in primary tumors and loss of chromosome 18 was demonstrated for AURKA and SRC (p-value: 0.028 for AURKA and 0.027 for SRC). Since particularly high copy numbers were most notably detected in advanced tumors in UICC stages IIIB and IV (p-value: 0.020 for AURKA and 0.002 for SRC) these alterations are most likely related to tumor invasion and dissemination. Protein analysis proved medium to high expression in 78% for AURKA and 97% for SRC. There were considerably higher levels of both proteins in primary tumors than in metastases. In contrast to their encoding genes, AURKA and SRC seem to be more involved in tumor emergence than in spreading. Even though increased expression at molecular and protein levels are both factors in SI-NET growth, there is no direct interrelation of these two. Instead, elevated protein levels potentially result from intensified transcription, translational up-regulation or reduced degradation. In this context, the EGF-signaling pathway plays an important role in the control of both proteins. AURKA / SRC have already been targeted by inhibitory agents such as AURKA inhibitors Alisertib or Danusertib and SRC inhibitors Bosutinib or Dasatinib in several cancer types. Future studies could therefore test these candidates for immunotherapy in non-resectable SI-NETs.