Childhood Virus Linked To Bladder Cancer Decades Later

BKPyV infection is often asymptomatic or mild during childhood, but may leave a lasting impact on cancer risk.

A virus that 95% of adults carry from childhood infections may be initiating bladder cancer decades later, according to research from the University of York and collaborating institutions.

BK polyomavirus (BKPyV) infects nearly everyone during childhood and stays dormant in kidney tissue throughout life. Scientists have now uncovered how this common infection can trigger cancer-linked DNA damage in healthy bladder cells, even though the virus itself disappears from the tumor.

The research, published in Science Advances, solves a medical mystery: bladder tumors show clear signs of viral damage yet rarely contain any actual virus.

How Your Immune System’s Defense Becomes a Cancer Risk

When the dormant virus reactivates and sheds into urine, bladder cells detect the threat and launch a defense by releasing interferons. These signals activate enzymes called APOBEC3A and APOBEC3B, which scramble DNA to stop viruses from replicating. But the enzymes lack precision. Healthy cells next to the infection site get caught in the crossfire and suffer DNA damage too.

Researchers call this “transmutagenesis” because mutations happen in bystander cells rather than in the infected cells themselves.

Using human bladder tissue cultures and organ models, researchers watched infected cells get isolated and pushed out through the bladder’s surface, a defense mechanism called apical extrusion. Around this zone, neighboring cells cranked up production of APOBEC3 enzymes and started accumulating DNA mutations identical to those found in bladder cancer patients.

The team used a very sensitive DNA reading method that can detect real, permanent mutations rather than just temporary DNA lesions.

Cells with higher APOBEC3 activity picked up noticeably more of these mutations than cells with lower activity. When researchers disabled the APOBEC3A and APOBEC3B genes, those modified cells stopped accumulating mutation patterns during infection, confirming the enzymes cause the DNA damage.

The research team also tested whether blocking the interferon signaling pathway could prevent APOBEC3 activation. They used drugs called JAK inhibitors that stop interferon signals from reaching their targets. When cells were pretreated with these inhibitors before viral infection, APOBEC3A levels stayed low, preventing the enzyme buildup that drives mutation accumulation.

Why Bladder Tumors Test Negative for the Virus

A large share of mutations in many bladder cancers carry these distinct patterns, though the proportion varies widely between patients. Yet most tumors contain no detectable virus.

The explanation: infected cells trigger damage in neighbors, then get eliminated by the bladder’s defenses. The damaged bystander cells survive and accumulate more mutations each time the virus reactivates over the years. Eventually they develop into tumors with no trace of what started the process.

A biopsy from a patient with BK virus bladder inflammation showed exactly this pattern, with APOBEC3-positive cells surrounding but not touching cells that contained viral proteins.

Researchers also analyzed 348 muscle-invasive bladder cancer samples from a large patient database. On average, 36% of mutations in these tumors were APOBEC3-type, though the range varied widely from 8% to 74% between patients. Neither the total number of these mutations nor their specific character differed between bladder cancer subtypes, suggesting this mechanism operates across different forms of the disease.

The team found substantial differences between people in how their cells respond to infection. Some people’s cells mainly produced APOBEC3A, others made more APOBEC3B, and many had mixed responses. These individual differences appeared in mutation patterns and reflect the variety seen in actual bladder cancer tumors, suggesting genetics or environment could affect someone’s cancer risk from this virus.

Could Bladder Cancer Become Preventable?

BKPyV already causes problems in people with weakened immune systems, including kidney damage in transplant recipients and bladder inflammation in bone marrow transplant patients. These groups have higher bladder cancer rates too.

But the new research shows the virus’s role extends beyond immunocompromised populations. BKPyV appears in urine samples from healthy people, and detection rates climb with age.

“A large proportion of urothelial carcinomas may be preventable by antiviral intervention,” the authors write. No vaccines or antiviral drugs exist yet for BKPyV, but scientists think such treatments might eventually reduce bladder cancer risk, though this idea remains untested. Transplant patients would especially benefit since they face both immediate viral complications and long-term cancer risk.

Screening could help too. Because BKPyV appears in urine years or decades before cancer develops, testing for viral reactivation might identify high-risk individuals who need closer monitoring or preventive treatment.

Unlike viruses that stick around in tumor cells, BKPyV operates through hit-and-run tactics, creating lasting damage before vanishing. That makes the cancer connection trickier to spot but potentially simpler to prevent by blocking the virus before it damages cells.

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