Gene Therapy Worked. Four Years Later, a Tumor Grew in the Same Cells.

A 5-year-old child who received an experimental gene therapy for a devastating rare disease developed a brain tumor four years after treatment. The tumor's cells contained fragments of the therapeutic virus used to deliver it.

Researchers at Children's Hospital of Philadelphia reported the case in the New England Journal of Medicine: the child was treated with an adeno-associated virus (AAV) gene therapy for severe MPS I and later developed a surgically removable neuroepithelial brain tumor whose cells contained fragments of the therapeutic vector.

The child was born with Hurler syndrome, a condition in which certain sugars accumulate in the brain and cause damage, stemming from a mutation in the gene encoding an enzyme called IDUA. Without it, those sugars can't be broken down. Hurler syndrome historically killed patients in childhood, and while enzyme replacement therapy can extend life, it cannot stop cognitive decline. Gene therapy was the best shot at changing that.

One known risk of AAV-based therapy is that the viral vector can insert material near a cancer-causing gene. Researchers suspect a confluence of factors contributed to this patient's tumor: the AAV type used has an affinity for cells lining the brain's ventricles, which is exactly where the tumor developed, and the child was young enough that those cells may still have been dividing.

The tumor was found during a routine brain MRI. The child had no symptoms. No similar cases have appeared among the nine other patients treated with the same therapy or the 32 who received a related treatment. A causal link has not been established.

The therapy worked. The child’s cognitive development stayed on track. Four years later, a routine scan found a walnut-sized tumor in the same cells, a reminder that outcomes continue to unfold long after treatment is given.

Read the full story at STAT News, May 13, 2026

Hot Take: Medicine keeps calling on gene therapy for a fix, then acting surprised when it still has admin access four years later. We’ve been shipping miracles with no exit strategy and labeling the afterparty “long-term follow-up.”