Annually, around 1 million cases of CRC are diagnosed, with 50% death rate, representing 8% of cancer-related deaths worldwide. The implementation of early diagnosis screening programs contributed to the reduction of the incidence and the mortality rates. The combination of chemotherapeutic regimens (5-FU, oxaliplatin and irinotecan) and targeted drugs (EGFR and VEGF inhibitors) have improved patients' prognosis. However, the combination of some types of chemical and biological therapies failed in the fourth line treatment and resistance to targeted therapies is the major limit in CRC.
Under these circumstances, we have conducted an RNAi screening aiming to identify new targetable oncogenes. Starting from the studies conducted by the Cancer Genome Atlas that identified a list of amplified genes in CRC, we analyzed putative genes that their Knockdown could negatively influence cell viability in different CRC cell lines, indicating the possible involvement in CRC. After a careful bioinformatic analysis I found that one gene called AP3M2 could be a successful candidate. AP3M2 (adaptor related protein complex 3 subunit mu 2) encodes for the neuronal Mu subunit of the heterotetrameric adaptor-related protein complex 3 (AP-3), which recognizes tyrosine-based sorting signals within the cytoplasmic domains of transmembrane cargo proteins and is involved in the biogenesis of lysosome-related organelles.
Deletion of this gene in mice was associated with susceptibility to drug-induced seizures and fewer synaptic vesicles. In the other hand, AP3M2 amplicons were observed in the primary tumor and maintained in at least two passages of breast cancer xenograft. AP3M2 is also in the list of genes in recurrent amplicons associated with reduced survival in breast cancer.
The new "druggable" gene was found to be altered in 6% of CRC patients and was associated with reduced survival rate according to the TGCA data. Our experiments confirmed it to be overexpressed in colon cancer patient tissues and cancer cell lines and proved to be a potent oncogene by promoting the colorectal cancer cell lines viability, adhesion and colony formation. Interestingly, AP3M2 levels affects the expression of other CRC associated protein such as P62, ATG7and ARF6 involved in autophagy and influenced ROS levels in colorectal cancer cell lines.
