Consequences of Oncogene and Tumor Suppressor Gene Targeting for Human Cancer Development and Therapeutic Efficacy

Abstract

Finding genes mutated at high frequencies in multiple human cancers uncovers the importance of these genes in the advancement of the disease. Understanding the effects of such mutations on tumor biology is an important and clinically relevant task. p53 and PTEN are the most mutated tumor suppressors and PI3K is one of the most mutated oncogenes in human malignancy. Mismatch repair proteins are also positioned as key regulators of neoplastic transformation such that, when lost, lead to tumorigenesis for 15% of all malignancies. All of these genes are implicitly involved in cell processes such as cell cycle arrest, apoptosis, and the maintenance of genome stability. However, each protein has a unique action in preventing the tumorigenic process, and when mutated, may cooperate or may lie in divergent paths towards malignancy. An examination of the phenotypic effects of alterations in p53, PI3K, and PTEN in cancer progression and therapeutic efficacy was achieved through the deletion of p53, the constitutive activation of PI3K, or the deletion of PTEN via somatic cell gene targeting in the non-tumorigenic cell line, MCF-10A. Deletion of p53 eliminated canonical p53 functions as well as produced p53-/- clones with varying phenotypes due to chromosomal instability and with treatment-schedule dependent responses to DNA damage. The knockin of two "hotspot" PIK3CA mutations allowed for growth factor-independent cell proliferation partly mediated by GSK3β. Moreover, GSK3β inhibitors Lithium Chloride or SB216763 were selectively toxic for PIK3CA mutant clones. The loss of PTEN in MCF-10A cells allowed for increased resistance to anoikis and increased sensitivity to DNA damage. Alternatively, the role of mismatch repair deficiency brought about by loss of MLH1 was examined by the targeted correction of MLH1 in HCT116 cancer cells, recapitulating cancer gene therapy. However, the heterogeneous nature of tumor cell pools or cancer cell compensatory mechanisms allowed for mismatch repair deficiency to be maintained in one corrected clone. In conclusion, the model systems reviewed herein will allow for a better understanding of specific downstream effects of these alterations as well as the consequences of potential tumor-specific responses cancer treatment options such as chemotherapy or cancer gene therapy.