(From the December 2014 Research Now)
Researchers have uncovered a new opportunity for therapeutic intervention in cancers such as metastatic pediatric rhabdomyosarcoma by teasing out how two alternative forms of p53-suppressing proteins function when encoded under stress. The potential therapeutic aims vary based on tumor type, but better understanding the various roles of these proteins’ variants may allow clinician-scientists to begin the search for techniques to manipulate or induce the p53-suppressing proteins.
In healthy cells, the p53 pathway prevents the perpetuation of damaged cells bearing mutations. This helps suppress tumor growth, but p53 levels must remain low to avoid over-aggressive cell killing. Proteins called MDM2 and MDMX work to inhibit p53 and keep its levels in check in normal cells. In cancer cells, however, MDM2 and MDMX are sometimes overexpressed, leading to tumorigenesis by totally blocking p53 and enabling uncontrolled cell proliferation.
Collaborative research now suggests that MDM2 and MDMX splice variants — alternative proteins transcribed by mRNA when reading the genome — can actually modulate the p53 pathway in a number of ways, even upregulating or activating it. The work was led by Dawn S. Chandler, PhD, principal investigator in the Center for Childhood Cancer and Blood Diseases in The Research Institute at Nationwide Children’s Hospital. According to Dr. Chandler, p53 upregulation caused by an alternative MDM2 or MDMX protein may be beneficial for a number of pediatric cancers, including metastatic rhabdomyosarcoma, in which overexpression of MDM2 and MDMX lead to tumor growth.
“It is important to understand the exact nature of the interaction between MDM2, MDMX and p53, since in the context of cancer it becomes important to activate p53 in order to destroy the cancerous cells,” says Dr. Chandler, senior author on the paper, published in August in PLoS One. “Our work provides evidence that alternative, short forms of MDM2 and MDMX play important roles in the activation of the p53 pathway by opposing the functions of their full-length counterparts.”
Now that the researchers are aware of the opposing activities of the normal MDM2 and MDMX proteins and their counterparts produced when cells are under stress, Dr. Chandler believes there is a clear opportunity to use this information to activate the p53 cell death pathway in cancer cells.
The protein variants are generated by a tightly controlled process called pre-mRNA splicing. Once they gain an understanding of the mechanisms by which the splicing of MDM2 and MDMX is regulated, Dr. Chandler says, it will be possible to manipulate this process with the aid of molecules called antisense oligonucleotides, or ASOs. These molecules can be designed to target the splicing and expression of specific genes without affecting the activity of other genes.
“This strategy has been successfully used to manipulate splicing and relieve symptoms in several disorders, including spinal muscular atrophy,” Dr. Chandler says. “In cancers where p53 upregulation is therapeutic, ASOs specifically targeting MDM2 and MDMX can be used to modulate their splicing and generate the short forms. This would aid p53 activation and lead to cancer cell death.”
Although Dr. Chandler’s team has a long research path ahead of them, clinical oncologists already recognize the potential of such work.
“This research is groundbreaking, because it opens doors to the study and development of targeted therapeutic strategies for tumors, such as rhabdomyosarcoma, that harbor aberrantly spliced isoforms of MDM2 and MDMX,” says Bhuvana A. Setty, MD, an oncologist at Nationwide Children’s. “These also provide additional targets for development and use of some existing therapeutic agents that target these particular mRNAs.”
Dr. Chandler’s team is currently working to identify the mechanisms of the splicing process that dictate the stress-induced formation of MDM2 splice variant MDM2-ALT1. They are also designing ASOs that can interfere with the functions of these proteins and enable the researchers to titrate the ratios of normal and variant forms of MDM2. They are examining the role of the variant proteins in mouse models, as well.
“Taken together, MDM2 and MDMX splicing manipulation present valuable opportunities for anti-cancer therapies. However, the status of the p53 pathway in the tumors will be a crucial determinant of the rationale and design of the splicing alteration strategies,” Dr. Chandler says. “Some cancers harbor mutations in p53 itself that actually promote tumor growth. In these cases, the expression of MDM2 and MDMX short splice forms would prove detrimental.”
Dr. Chandler’s team also relies on the work of other cancer scientists to help unravel the biology at play in a wide range of tumors, so that her therapeutic strategy, if effective, can be applied to the appropriate types of cancers.
“Understanding the basic biology and pathways that mitigate the development and proliferation of malignancies allows investigators to move these findings into translational and, subsequently, clinical research,” Dr. Setty says. “These advances can affect patients’ overall treatment and prognosis. Basic science research such as this is critical in paving the way for new strategies for treatment and management of cancers.”
Jacob AG, Singh RK, Comiskey DF Jr, Rouhier MF, Mohammad F, Bebee TW, Chandler DS. Stress-induced alternative splice forms of MDM2 and MDMX modulate the p53-pathway in distinct ways. PLoS One. 2014 Aug 8, 9(8):e104444.