A Common Gene Variant Once Blamed for Cancer Turns Out to Guard Against It

Back in 1998, Boris Pasche found something odd buried in a stretch of human DNA that most people will never think about. A tiny deletion, three little repeats gone, in a gene that helps cells decide when to stop growing. It looked like trouble. And for the better part of three decades, that is exactly what nearly everyone assumed it was.

The variant has a name only a geneticist could love: TGFBR1*6A. Roughly 14 percent of us carry it, often without the faintest idea, because the thing has a habit of hiding from the very machines built to find genes like it.

Here is the twist. A study published on 9 June in Cancer Communications reports that this supposed villain has been miscast all along. Far from raising the risk of colorectal cancer, carrying 6A appears to lower it. In some people, dramatically so. The gene that scientists once eyed as a possible culprit now looks more like a quiet bodyguard, and the reversal has been a long time coming.

To understand why it took so long, you have to appreciate how slippery this particular bit of code is. The 6A variant sits in a region thick with guanine and cytosine, the G and C of the genetic alphabet, and that chemistry makes the stretch a nightmare to read. The standard tools of modern genetics, the GWAS chips and the sequencing platforms that hoover up DNA by the millions of letters, tend to skate right past it. “This mutation has often been overlooked by genome-wide association study chips, which cannot detect TGFBR1*6A, and is commonly missed by next-generation sequencing platforms due to the complexity of the region,” says Allan Johansen, the postdoctoral fellow at the Barbara Ann Karmanos Cancer Institute who led the work.

A mouse built to settle the question

So the team did something laborious. They built a mouse to carry the human version of the gene, swapping out a chunk of the rodent’s own DNA for either the common 9A form or the rarer 6A. Then they crossed those animals with a strain prone to sprouting intestinal polyps, and waited to see what grew.

The mice carrying 6A grew far less. After twelve weeks, animals with the variant had about 46 percent fewer polyps than their counterparts, and the polyps they did have were smaller and less menacing, mostly low-grade growths rather than the aggressive sort. None of the 6A carriers developed outright carcinoma. “This model revealed that mice carrying the TGFBR1*6A allele developed fewer polyps and adenocarcinomas compared to those with two copies of the TGFBR1 allele,” says Pasche, who is now president and chief executive of Karmanos and first stumbled across the variant during a postdoc in Joan Massague’s lab in the late 1990s.

Mice are one thing, of course. People are another. To see whether the effect held up in humans, the researchers turned to the Colon Cancer Family Registry, a vast collection of genetic and medical data spanning families across the US, Canada and Australia. They pulled germline DNA for some 3,374 individuals and, crucially, did the hard statistical work of adjusting for genetic ancestry, a step several earlier studies had skipped. That omission, the team reckons, may be one reason the variant’s true colours stayed hidden for so long.

The closer the kin, the stronger the shield

The human numbers told a remarkably similar story. Across all participants, 6A carriers had a modestly reduced risk of colorectal cancer. But when the analysis narrowed to siblings, brothers and sisters who share much of their genetic background, much like the mice, the protection roughly doubled, to a 44 percent reduction in risk. And in people with familial adenomatous polyposis, a brutal inherited condition that carpets the colon with hundreds of precancerous growths and, untreated, leads almost inevitably to cancer by around 40, the effect was larger still: an 84 percent drop. For that group, a variant elsewhere described as low-penetrance starts to look like a powerful shield indeed.

It is worth keeping the caveats in view. The familial polyposis result rested on a small group, just 27 cases and ten controls, so the precise figure should be taken with a pinch of salt. The authors are careful to say the work needs replicating, and that the findings, drawn largely from people of European ancestry, may not transfer cleanly to other populations. Whether the protective signal extends to those of Asian, African or Hispanic background remains, for now, an open question.

Still, the convergence is hard to wave away. When a humanised mouse and a registry of human families point in the same direction, with almost eerily matched numbers, you tend to sit up. Pasche has lived with this puzzle for the better part of his career, watching the early hint of a risk gene fade, then invert. “Early studies suggested that TGFBR1*6A may act as a colorectal cancer susceptibility allele; however, subsequent research by other groups did not confirm this association,” he says. The vindication, when it came, ran in the opposite direction from where he started.

What lingers is a slightly uncomfortable thought about everything else we might be missing. If a variant carried by one in seven people could duck the genetic dragnet for twenty-odd years, simply because its chemistry made it awkward to read, how many other hard-to-sequence stretches of the genome are quietly shaping who gets sick and who doesn’t? The 6A story suggests the answer is more than we’d like to think, and that some of the most interesting biology may be sitting exactly where the machines find it hardest to look.

DOI / Source: TGFBR1*6A and Risk for Colorectal Cancer, Cancer Communications (2026)

Frequently Asked Questions

If I carry this gene variant, am I protected from colorectal cancer?

Not in any guaranteed sense. Carrying TGFBR1*6A appears to lower the odds rather than remove them, and the size of the benefit seems to depend heavily on context, with siblings of patients and people with a rare inherited polyp condition showing the strongest effect. Most carriers have no idea they have it, since the variant routinely slips past standard genetic tests. Whether it ever becomes something doctors screen for will depend on larger studies confirming the pattern.

Why did scientists spend years thinking this variant raised cancer risk?

The earliest work in the late 1990s flagged it as a possible susceptibility gene, but later studies failed to back that up, and the picture stayed muddy for decades. Part of the problem was statistical: many studies did not adjust for genetic ancestry, which can quietly distort results. Once the team accounted for that and built a matched mouse model, the signal flipped from harm to protection.

How does a gene variant actually shield the colon?

TGFBR1 helps relay a chemical signal that tells cells when to stop dividing, and the 6A version transmits that signal a little differently from the common form. In mice carrying it, intestinal polyps were fewer, smaller and less likely to turn aggressive. The exact cellular mechanism is still being worked out, which is part of what makes the finding worth chasing further.

Could there be other protective genes we’ve simply failed to detect?

That is one of the more provocative implications. This variant evaded detection for years because its chemistry, a region rich in guanine and cytosine, made it hard for sequencing machines to read accurately. If one common variant could hide that long, others with real effects on disease may still be sitting in the genome’s harder-to-read corners, waiting for better tools or smarter analysis.