Clinical caring: Shadow Hughes enjoys a quiet conversation with his oncologist, Giles Robinson, MD.
 

Medulloblastoma: DNA Points the Way

The largest study of its kind uncovers six inherited gene mutations that can lead to the brain cancer medulloblastoma.

By Maureen Salamon; Photos by Peter Barta

Shadow Hughes describes himself as a “lazy person,” a perception seemingly at odds with his pursuit of energetic hobbies such as swimming, snowboarding and hunting. But the straight-A student’s mellow manner came into sharper focus when he was diagnosed with the brain cancer medulloblastoma last year.

During six months of treatment at St. Jude Children’s Research Hospital, Shadow soaked in the serene approach he noticed in everyone involved in his care, appreciating the peaceful port in a storm.

“They were all at ease and calm. No one was rushing around,” recalls the 14-year-old, whom his mother calls “everybody’s favorite” in his family of six. “I knew if they were calm, I should be calm.”

The Oklahoma boy’s family has also drawn comfort from the hospital’s aggressive approach to treating Shadow, whose initial MRI scans showed nine tumors in his brain. Within hours of his arrival in Memphis in April 2017, surgeons performed complex brain surgery and told Shadow’s parents that a cure is possible.

“That was the first instance of hope we had in all of this,” says his mom, Jennifer. “It was a game-changer for us.”

That was the first instance of hope we had in all of this. It was a game-changer for us.

Jennifer Hughes

New awareness

New international research co-led by St. Jude, the European Molecular Biology Laboratory and German Cancer Research Center in Heidelberg, and the Hospital for Sick Children in Toronto, has transformed our understanding of the inherited risks for medulloblastoma.

Until now, scientists believed most cases of medulloblastoma occurred by chance and did not run in families. But in the largest study yet of genetic susceptibility in a pediatric brain tumor, scientists learned that inherited gene mutations often play a role in its development.

The discovery has prompted new recommendations for genetic testing and counseling of medulloblastoma patients and their families.

The massive effort, co-authored by St. Jude cancer biologist and medulloblastoma expert Paul Northcott, PhD, revealed germline variations—typically inherited and carried in cells throughout the body—in six genes that together explain 5% of cases of the malignancy.

These genes include APC, PTCH1, SUFU, TP53, BRCA2 and PALB2. Mutations in these genes can disrupt their normal function, causing cancer.

Although it is the most common malignant childhood brain tumor, medulloblastoma is diagnosed in only about 400 children in the United States each year. The cancer’s odds of appearing due to inherited genes vary depending on the disease’s four subgroups. These subgroups are named WNT, sonic hedgehog, Group 3 and Group 4.

Paul Northcott, MD

Monitoring risk

“When we break this down, we clearly see different risks according to the subgroups,” says Paul Northcott, PhD. The findings have important implications for the treatment and surveillance of patients and their families.

Assigning risk

Each subgroup carries different characteristics and treatment outcomes. Northcott’s paper, published in the journal Lancet Oncology, looked for gene variations related to each subgroup.

Among the most striking findings: As many as 20% of sonic hedgehog medulloblastoma cases stem from germline mutations in five out of six of the implicated genes. This places these patients and possibly their siblings not only at a higher risk of developing medulloblastoma, but also of having other cancers later in life.

Additionally, inherited genes account for about 10% of patients with WNT medulloblastoma.

“When we break this down, we clearly see different risks according to the subgroups,” says Northcott, who works in St. Jude Developmental Neurobiology. “Somewhere between one in five and one in six sonic hedgehog medulloblastoma patients have a clear hereditary predisposition based on mutations in these six genes. That’s an important finding that has significant implications for patients’ treatment and their surveillance—as well as for their siblings and parents.”

Northcott and his team have created detailed recommendations guiding clinicians on which types of other cancers to watch for in medulloblastoma patients—and in some cases, their relatives—based on genes and family history.

 

Discoveries and more discoveries

Northcott and his colleagues published another major international study in 2017 that revealed the genomic changes responsible for more than 75% of medulloblastoma tumors. The scientists also discovered two new suspected cancer genes found only in the least-understood disease subgroups.

Combined, the pair of efforts help round out scientists’ understanding of the genes at play in this brain tumor.

“In the earlier study, we focused on the medulloblastoma tumors themselves, describing their molecular landscape and the genes that were mutated,” Northcott says. “In this case, we’ve studied the germline—the normal DNA that’s present in every cell of these patients.”

Compelling comparisons

In their most recent study, the researchers analyzed germline DNA samples from 1,022 patients with medulloblastoma and 58,000 individuals with no history of cancer.

The scientists used next-generation gene sequencing— a process that deciphers the exact makeup of a DNA molecule—to compare how often variants in 110 cancer predisposition genes occurred in the medulloblastoma patients and the healthy subjects. These methods teased out six high-risk gene variants that show up in medulloblastoma patients far more often than in healthy people.

The findings, Northcott notes, are especially compelling because the healthy comparison group was so large.

“We focused only on the strong predisposition genes, where there’s plenty of evidence that mutations in these genes are known to cause cancer,” he says.

“When we see a variant that is much more prevalent in medulloblastoma patients compared to 58,000 normal, healthy controls,” Northcott adds, “we can be pretty confident that this gene is playing a role.”

Risky business

The study spotlights an urgent need to make genetic counseling and testing the standard of care for many medulloblastoma patients, particularly those in the sonic hedgehog and WNT subgroups.

Northcott and his team have created detailed recommendations guiding clinicians on which types of other cancers to watch for in medulloblastoma patients—and in some cases, their relatives—based on genes and family history.

For example, between 5 and 10% of WNT subgroup patients have an inherited mutation in the APC gene, which is tightly linked to the development of colorectal cancer and other malignancies.

“These medulloblastoma patients might assume they’re free and clear because they’ve survived their brain cancer, but they’re certainly at risk for other cancers,” Northcott says. “They also have to be careful with the treatments they’re given for their medulloblastoma, because that could make them more prone to develop second cancers.”

Collaborators around the globe are encouraging the World Health Organization to incorporate the St. Jude guidelines into its recommendations for managing and diagnosing brain tumors.

 
Infographic: proposed guidelines for genetic counseling and testing for patients with WNT, SHH, GRP3, and GRP4 subgroups of medulloblastoma

Every bit of research is building toward finding a better way to treat this.

Jennifer Hughes

 

Sharing guidelines worldwide

Among patients in the sonic hedgehog subgroup, Northcott points to five genes—including p53, a prominent cancer driver—commonly mutated in their inherited DNA.

“We’re proposing that these patients are all screened for these five genes up front,” he explains.

Just a few years ago, almost no medulloblastoma patients would be referred for genetic testing or counseling unless a rare companion syndrome was suspected, according to study co-author Giles Robinson, MD, of St. Jude Oncology.

“The screenings can help patients and families understand and manage their lifetime cancer risk,” Robinson says. But the new research greatly broadens the scope of patients who will be pegged for such consideration.

Already in use at St. Jude, the new guidelines fill a great need and should be adopted widely, Northcott says. His collaborators around the globe are encouraging the World Health Organization to incorporate the advice into its own guidelines for managing and diagnosing brain tumors.

“That’s the advantage of working together as a community,” Robinson says. “This isn’t something that will just stay at St. Jude. It will spread quickly.”

Total care

Shadow’s mom, Jennifer, says she’s grateful for the St. Jude efforts to uncover the genetic underpinnings of medulloblastoma. While the family’s Native American heritage isn’t specifically linked, the mother of four has certainly wondered if her son’s case stemmed from something in their DNA.

Ever the bookworm and smitten with adventure novels and outdoor-themed magazines, Shadow has “bounced back completely” from his treatment, which he finished in October 2017 with no evidence of disease. He undergoes imaging scans every three months at St. Jude to confirm he’s still cancer free.

“It’s a complete and total mystery to us, and we would love to find out more about where this has come from,” Jennifer says. “Every bit of research is building toward finding a better way to treat this. It’s just total care, all the way around.”

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