Three weeks after her 40th birthday, Julie McDonald had a colonoscopy. When she walked into the doctor’s office afterwards, there were photos lined up on the desk.
“I knew something wasn’t right,” she says.
Julie was diagnosed with stage three bowel cancer. While she had experienced some mild symptoms, the news still came as a shock.
“I didn’t have any belly aches or bleeding. There was nothing out of the ordinary, other than I felt a bit constipated every now and then,” she says.
As a young woman, Julie didn’t fit the ‘usual profile’ of someone with bowel cancer (also referred to as colorectal cancer). But her grandfather had died from the disease, and she uncovered more incidences while researching her family tree.
“If I hadn’t discovered a family history, I probably would have fobbed off the symptoms as something else,” she says. “I wouldn’t have gone to the doctor until things were really bad, and by then it might have been too late.”
Bowel cancer rates have doubled in young people
People over the age of 50 have a higher risk of developing colorectal cancer, but the prevalence of the disease has been rapidly increasing in young people, who now make up one in nine new diagnoses.
Rates in people aged between 20 and 39 have more than doubled from 2001 to 2021, making it the most common cause of cancer-related death in that age group.
Researchers don’t know what’s causing these high numbers yet, but it could be linked to what’s going on in the gut.
“The timeframe at which early-onset colorectal cancer is increasing suggests it is unrelated to hereditary causes, which would take generations to surface,” explains Associate Professor Daniel Buchanan, who leads the Colorectal Oncogenomics lab at the University of Melbourne Centre for Cancer Research.
“A potential cause of this increasing incidence is related to changes in our gut microbiome. Over the last few decades our diet, lifestyle and environmental factors have changed, which can alter the type of bacteria as well as the balance between good and bad bacteria that live naturally in our gut,” Associate Professor Buchanan says.
“This phenomenon is known as the birth cohort effect, which refers to the shared variations a group of people born around the same time experience as a result of the common behaviours and environmental factors they have been exposed to.”
The microbiome is a complex community of microorganisms including bacteria, fungi, and viruses that occur naturally in and on our bodies. A healthy balance can help regulate our digestion and immune systems. When something goes wrong, it can trigger disease.
Associate Professor Buchanan’s group have just released a study analysing the role certain bacteria in the microbiome may contribute to colorectal cancer development.
“We looked at three different types of bacteria that occur in the gut, each of which produces a different genotoxin that can cause DNA damage and inflammation in the colon,” he explains.
“While most studies have analysed stool samples, we wanted to see if the bacteria were present at the scene of the crime, in the tumour itself.”
The role of bacteria
The study found at least one of the three bacteria in anywhere from six to ten per cent of the colorectal cancer cases they looked at. One strain was more likely to occur in patients who were diagnosed with early-onset colorectal cancer.
“We found a specific variant of the bacteria Escherichia coli that causes DNA damage by producing a genotoxin called colibactin – we can measure this specific DNA damage in the tumour,” says Associate Professor Buchanan.
“It’s the first time a non-genetic biomarker for the cause of colorectal cancer has been identified – meaning we can now link the cause of cancer back to this bacteria.”