FAM83 proteins: Fostering new interactions to drive oncogenic signaling and therapeutic resistance

Abstract

The FAM83 proteins were recently identified as novel transforming oncogenes that function as intermediaries in EGFR/RAS signaling. Using two distinct forward genetics screens, the Bissell and Jackson laboratories uncovered the importance of the FAM83 proteins in promoting resistance to EGFR tyrosine kinase inhibitors and therapies targeting downstream EGFR signaling effectors. The discovery of this novel oncogene family using distinct genetic screens provides compelling evidence that the FAM83 proteins are key oncogenic players in cancer-associated signaling when they are overexpressed or dysregulated. Consistent with a role in oncogenic transformation, the FAM83 genes are frequently overexpressed in diverse human cancer specimens. Importantly, ablation of numerous FAM83 members results in a marked suppression of cancer-associated signaling and loss of tumorigenic potential. Here, we review the current knowledge of the FAM83 proteins' involvement in cancer signaling and discuss the potential mechanisms by which they contribute to tumorigenesis. Both redundant activities shared by all 8 FAM83 members and non-redundant activities unique to each member are highlighted. We discuss the promise and challenges of the FAM83 proteins as novel points of attack for future cancer therapies.

Keywords: EGFR; FAM83; oncogene; resistance.

Figure 1. FAM83 proteins

FAM83 members are characterized by a common N-terminal Domain of Unknown Function 1669 (DUF1669), but have vastly different C-terminal regions.

Figure 2. Distinct genetic screens identifying FAM83A and FAM83B

A. The Bissell Laboratory utilized a phenotypic reversion screen to identify for genes that confer resistance the EGFR tyrosine kinase inhibitor AG1478 in breast cancer. HMT3522 T4-2 cells were infected with a retroviral cDNA library. Infected cells were grown in 3D laminin-rich basement membrane. In the absence of AG1478, all T4-2 cells form disorganized, proliferative colonies. In the presence of AG1478, the cells will “revert” to form polarized, acinar structures that resemble non-transformed cells. AG1478-resistant cells that continue to form disorganized, proliferative colonies are isolated, and the cDNA conferring AG1478 resistance was identified as FAM83A. B. The Jackson Laboratory utilized an insertional mutagenesis screen to identify genetic changes that promote the transformation of HMEC. Non-transformed HME1 cells (which can be transformed by a single genetic event, such as mutant RAS) were infected with Validation-Based Insertional Mutagenesis lentiviruses. Following VBIM infection and integration, a CMV promoter is randomly inserted throughout the genome, modifying transcription of nearby genes (most commonly by causing high-level expression full-length, truncated, or anti-sense mRNAs). VBIM-infected cells were plated into soft agar to identify mutants capable of anchorage-independent growth, a hallmark of transformed cells. Identified mutants were validated by adding Cre, which removes the VBIM insertion via loxP sites in the construct, and the VBIM insertion site was mapped to the FAM83B gene.

Figure 3. FAM83 family of proteins promotes ErbB receptor signaling

The ErbB signaling network controls normal cell growth, survival and proliferation. Ligand-mediated activation of the ErbB family of receptor tyrosine kinases results in receptor dimerization, autophosphorylation of the receptors and activation of downstream signaling effectors. ErbB receptors activate RAS/MAPK and PI3K/AKT/mTOR pathways and Phospholipase D (PLD), among others. PLD generates Phosphatidic acid (PA) that enhances RAF recruitment to the membrane and also activates mTOR signaling. In many cancers, activating mutations in RAS, PI3K and AKT drive transformation by inappropriately activating the MAPK and PI3K/AKT pathways. The novel FAM83 (FAMily with sequence similarity 83) family of signaling proteins have emerged as important therapeutic targets as they are overexpressed in many cancers and they function as key intermediates in EGFR, MAPK and PI3K/AKT signaling.

Figure 4. Normal Tissue Expression Levels of FAM83

Analysis of the human proteome across most major tissue types showing expression of FAM83A-H members and oncogenes EGFR and RAS alongside beta-actin. The tissue types include the following: Blood and Immune (Serum, Plasma, Monocyte, Peripheral Blood mononuclear cells, Platelet, Lymph node, Tonsil, Bone marrow stromal cell, Bone marrow mesenchymal stem cell), Nervous (Brain, Fetal Brain, Frontal cortex, Cerebral cortex, Cerebrospinal fluid, Spinal Cord, Retina), Muscoskeletal (Heart, Fetal Heart, Bone, Colon muscle), Internal (Oral epithelium, Nasopharynx, Nasal respiratory epithelium, Esophagus, Stomach, Cardia, Fetal gut, Colon, Rectum, Liver, Fetal liver, Liver secretome, Kidney, Spleen, Lung, Lung Alveolar lavage, Adipocyte, Synovial fluid, Amniocyte), Secretory (Vitreous humor, Saliva, Salivary gland, Thyroid, Adrenal, Breast, Milk, Pancreas, Pancreatic Juice, Islet of Langerhans, Gallbladder, Prostate, Urine, Urinary Bladder, Skin, Hair Follicle, Placenta), Reproductive (Uterus, Cervix, Ovary, Fetal Ovary, Testis, Fetal Testis, Seminal vesicle).