Researchers at the Yale School of Public Health investigating why do right-sided and left-sided colon cancers differ, why are right-sided ones more dangerous and why do women get more of them are centring their research on metabolites and their role in human health, particularly in colorectal cancer.
People with right-sided colon cancer (RCC) have a 20 percent greater chance of dying than people with left-sided colon cancer (LCC). Subdividing further, studies have reported that left-sided cancers were split almost evenly between males and females (52 to 48 percent, respectively), but the more deadly right-sided type affected women much more - females account for 62 percent of RCCs, males just 38 percent.
Dr Caroline Johnson, Assistant Professor of Epidemiology (Environmental Health Sciences) at Yale School of Public Health and a member of the Cancer Prevention and Control Research Program at Yale Cancer Center, began researching colorectal cancer as a research fellow at the National Cancer Institute and has continued that focus in her lab at Yale School of Public Health.
She explained that metabolites are molecules formed or used during the process of metabolism. They can be produced by diet, hormones, genetic or environmental factors, and the microbiome.
"Metabolites give us information about biological processing within a sample or within a tissue," she said. "Tumours have metabolic activity and make metabolites that can produce energy for cell growth."
In the first such study ever performed, she and her colleagues used metabolomics and mass spectrometry to explore the marked differences between right-sided colon cancer in males and females. First, they extract metabolites from a tumour sample by liquefying and centrifuging it. The proteins sink, the metabolites float. The researchers ran the metabolites through a mass spectrometer to get a survey of everything in the sample - perhaps 20,000 variables - which can be separated into various groups. They look for patterns and connections by putting the metabolites into a pathway analysis that reveals their links and shared networks.
"We're able to pinpoint what genes and enzymes might be altered or upregulated or downregulated in relation to the disease," added Johnson.
The researchers discovered that colorectal cancer cells on the right side generate metabolites that enable more aggressive growth in women, than in men. They also determined that the colons of males and females produce different metabolites. That insight led them to identify a distinct metabolic phenotype common among women with right-sided colon cancer.
"This phenotype produces energy in a different way than happens in males," she explained.
The biggest difference is a large increase in asparagine, a common amino acid found in most proteins.
"So, we looked at the gene that produces asparagine, which is asparagine synthetase (ASNS) and then we looked at many cancer databases to see if females with right-sided colon cancer have a higher expression of that gene and lower survival."
The data showed a correlation in women but not in men.
"Asparagine might be involved in tumour growth in females," said Johnson. When right-sided colon tumours become starved for energy, they look for new sources. In women, asparagine production goes into overdrive to help the tumour increase its uptake of the amino acids and fatty acids that fuel cell growth.
"It's called 'metabolic rewiring, almost like you rewire the electricity supply in your house. The tumour suddenly switches to using different nutrients to survive in a different situation."
Johnson's discovery of asparagine's role opens therapeutic possibilities. Drugs that remove asparagine from circulating blood, depriving tumour cells of fuel, are already being used against acute lymphocytic leukaemia. One of Johnson's next studies will test whether the drugs might be effective against RCCs. Using mice with colon cancer, she will remove the asparagine synthetase gene using the gene editing tool CRISPR, and then will feed the mice diets high or low in asparagine to monitor the effects on tumour growth. She isn't sure that removing asparagine will be as effective on RCCs as it is on acute lymphocytic leukaemia, since the colon is a more complex environment, including the microbiome. For that reason, she is also working on tracking the biology back toward the source before asparagine is activated.
Her research indicates that mutant KRAS genes, which are implicated in several cancers including colon cancer, play a role in asparagine metabolism. at connection is now her lab's primary focus.
"We've seen that mutant KRAS could be regulating the production of asparagine," she said. "We've also seen that women with right-sided colon cancer, if they express mutant KRAS, have a much poorer survival. That observation was quite striking, and I don't think it has been reported before. So, we're trying to see if we can prevent asparagine production by targeting mutant KRAS better. We're also looking at why mutant KRAS is regulated differently in women than in men, and what different hormones are involved. It's a very complex signalling pathway. We need to look a little bit further upstream at that oncogene."
Johnson notes that the connection between mutant KRAS and asparagine could have therapeutic implications. A patient with early-stage colon cancer and elevated levels of asparagine could be treated more aggressively, perhaps with hormones. The phenotype Johnson identified among women with RCC occurs in menopausal and older women.
"We know that oestrogen and oestrogen receptor beta are protective against colon cancer," she said. "Maybe this protection is lost approaching menopause or in menopause. We're looking at how oestradiol and the oestrogen receptor beta might be regulating this mutant KRAS pathway. In these patients, hormonal therapies might work. But that's a future direction for our research.