José Luis Millán has spent decades studying a single enzyme — tissue-nonspecific alkaline phosphatase, or TNAP — and now his work is pointing toward a cure that patients with hypophosphatasia have waited years to hear about: a single injection, one lifetime of healing.
Hypophosphatasia, also known as soft bone disease, is a rare inherited disorder that distorts bone development and causes premature tooth loss. The most severe forms strike approximately one in every 100,000 live births and can be life-threatening. For the past decade, the only effective treatment has been asfotase alfa, an enzyme replacement therapy requiring injections three to six times each week — a grueling schedule that has nevertheless saved lives. Children who would have died shortly after birth now grow into adulthood. But the treatment's burden is real. Some patients experience reactions from frequent injections and stop treatment altogether, which motivated Millán and his team at Sanford Burnham Prebys to pursue the next frontier.
That frontier is gene therapy. A new study published January 12, 2025, in the Journal of Bone and Mineral Research demonstrates the safety and effectiveness of AAV8-TNAP-D10, a specially engineered virus designed not to cause disease but to deliver a gene capable of producing the missing TNAP enzyme and reversing bone and tooth malformation. "We believe the next evolution in treating HPP will be a gene therapy in which a single injected dose will provide a lifelong treatment for patients," Millán said.
The latest research tested different dosages in both male and female mice, examining both early-onset and late-onset forms of the disease. The team carefully calibrated the viral vector to determine which dose achieved efficacy without triggering harmful side effects like ectopic calcifications — unwanted bone crystal deposits in soft organs. The results provided what Millán called "a clear starting point for clinical trials."
An intriguing surprise emerged from the data: female mice with late-onset HPP responded more effectively to the gene therapy than males, and females achieved improvements in bone and teeth with lower doses. The team also observed that in females, the greatest enzymatic activity occurred at the injection site in limb muscle, while in males it was concentrated in the liver. When Millán presented these findings at the American Society for Bone and Mineral Research in Toronto, colleagues shared that this sexual dimorphism is known in mice but does not occur in non-human primates or humans — so future clinical trials may not see this effect, though now researchers will watch for it.
With preclinical evidence now solidly established, the next step is partnership with a pharmaceutical company to move AAV8-TNAP-D10 into human trials. Millán and his longtime collaborators, Drs. Takashi Shimada and Koichi Miyake of Nippon Medical School in Japan, are actively pursuing that path.
Yet Millán's vision extends beyond skeletal repair. The missing TNAP enzyme affects not only bones and teeth but also the brain, liver, kidney, and immune system. With patients now living longer — thanks to enzyme replacement therapy and the promise of gene therapy — long-term complications may emerge that previous generations never witnessed. "We need to anticipate long-term problems before they happen so we can be prepared to help patients with HPP throughout their lives," he said. That work of anticipation is already underway.