A new study has, for the first time, settled the debate about how a mutated protein present in half of all human cancers drives tumor growth. The discovery will not only facilitate a rethink of the way cancer is treated but may lead to new and improved therapeutics.
Normally, the TP53 gene provides instructions for making the protein p53, which plays a critical role in repairing or, if it’s unable to be repaired, destroying damaged cellular DNA. It’s a tumor suppressor, regulating cell division and preventing cells from proliferating too quickly or in an uncontrolled way.
p53 mutations are found in half of all human cancers and can be triggered by environmental factors, such as exposure to UV radiation, or are inherited. However, there are conflicting views about whether mutation creates loss- or gain-of-function in p53. Loss-of-function would mean the protein doesn’t regulate the cellular responses that prevent tumor development and growth; gain-of-function would lead to a supercharged protein that assists cancer cells in surviving and proliferating.
In a new study, researchers from Australia’s Walter and Eliza Hall Institute (WEHI), in collaboration with the University of Trento in Italy, have settled the debate, finding, for the first time, what traits of the mutant p53 protein are critical for driving cancer growth.
“If you look at all cancers of humanity, about 50% of them have a mutation in p53,” said Gemma Kelly, co-corresponding author of the study. “Specific cancers, like those of the pancreas, lung and breast, commonly have defects in these proteins. Our findings have transformed our understanding of these mutations, which helps rethink how they can be targeted in the development of new cancer treatments.”
The researchers used CRISPR/Cas9 technology to genetically inactivate 12 distinct mutant TP53 genes reported to engender gain-of-function activities in human cancer cell lines. They found that the removal of mutant p53 had no impact on the in vitro survival or proliferation of the cancer cell lines tested. Nor did it affect mitochondrial content or activity or intracellular levels of reactive oxygen species (ROS); increased cellular metabolism and high intracellular levels of ROS are hallmarks of cancer.
The gain-of-function effects of mutant p53 have been reported to help cancer cells adapt to stresses such as a lack of nutrients or generate resistance to anti-cancer agents. When the researchers starved the TP53-deleted cells of nutrients and exposed them to various chemotherapy drugs, they observed that sustained expression of mutated p53 was not required for the cancer cells to adapt.
Xenografting human and mouse cancer cell lines and patient colon-cancer-derived organoids in immunodeficient mice – considered the gold standard model for studying human tumor growth – revealed that the removal of mutant p53 did not impair tumor growth or metastasis. Moreover, restoring p53’s normal functions, lost when the protein mutates, reduced cancer growth in pre-clinical models.
“Our study has provided the first evidence to show that it is actually the loss-of-function that impacts cancer growth,” Kelly said. “We found no evidence of gain-of-function contributing to cancer growth.”
To extend beyond their study, the researchers mined the data within the Cancer Dependency Map (DepMap), an ongoing project to identify cancer-related genes, to examine the effect of deleting mutant TP53 on the growth and survival of 391 diverse types of human cancer cell lines. This showed “unequivocally” that removing the mutant TP53 did not affect the growth of any of these cancer cell lines.
“Having these tools at my disposal allowed me to assess 157 different p53 mutations,” said Zilu Wang, the study’s lead author. “The mutations I looked at basically account for at least 90% of human cancers with defects in p53, which will provide crucial insight when informing the development of new anti-cancer strategies.”
The researchers say their findings could prevent hundreds of millions of dollars being wasted on developing ineffective drugs.
“There is research underway that is working towards finding the first therapeutics to target gain-of-function traits,” said Andreas Strasser, the study’s other corresponding author. “Our findings indicate there is no further merit in this therapeutic avenue and the focus needs to shift to restoring the mutant protein’s lost function and normal tumor suppressor ability.”
The study was published in the journal Cancer Discovery.
Source: WEHI