STAT1 gain-of-function disease

STAT1 is a protein that passes signals inside immune cells. It helps the body fight viruses and bacteria, and it plays a part in the defense against Candida, a yeast-like fungus. In STAT1 GOF disease, a gene change makes the protein too active. That holds back the IL-17 pathway too much. This is the part of the immune system that fights Candida on the skin and in the mouth.

The overactive protein also makes cells respond too strongly to interferons, the body's natural alarm signals. This is thought to drive the autoimmune problems many people have. So the condition combines a weak defense against some germs (immune deficiency) with an overactive, misdirected immune system (immune dysregulation).

STAT1 gain-of-function changes were first identified in 2011 as a cause of chronic mucocutaneous candidiasis (CMC). CMC means Candida infections of the skin, nails and moist linings of the body that keep coming back. Since then, doctors have learned that the condition can affect much more than the skin.

The cause is a change in one of the two copies of the STAT1 gene. One changed copy is enough, so it follows an autosomal dominant pattern. A person with the condition has a 50% chance of passing the change to each child.

The change can also appear for the first time in a person whose parents do not have it. In an international study of 274 people, 61% belonged to families with more than one affected member, and 39% were the only known case in their family. Nearly everyone with the change develops candidiasis, usually in the first years of life. The condition is not caused by anything a parent did, and it cannot spread from person to person.

Candida infections are the hallmark. In the study of 274 people, 98% had CMC, starting at a median age of 1 year. It affects the mouth and other moist linings, the skin and the nails, and it can involve the food pipe (esophagus). Many people also had bacterial infections (74%), often of the lungs or skin, and viral infections (38%), mostly from the herpes family.

More than 1 in 3 people had autoimmune problems, when the immune system attacks the body. The most common was an underactive thyroid (hypothyroidism). Others included type 1 diabetes, low blood counts and lupus. Some people have severe autoimmune disease that looks like IPEX syndrome, a condition of serious gut inflammation and autoimmunity.

Three problems carry the most risk. The first is invasive infection, which spreads beyond the skin, such as deep fungal or mycobacterial infection. The second is an aneurysm, a weak, bulging spot in a blood vessel, often in the brain. The third is cancer, mostly squamous cell cancer of the skin, gut or voice box. In the large study, 25% had invasive infections, 6% had brain aneurysms and 6% had cancers.

Doctors suspect STAT1 GOF disease when a child has Candida infections of the mouth, skin or nails that keep coming back. The suspicion grows when there are also other infections, an underactive thyroid or other autoimmune problems. A family history of CMC is another clue, but many people are the first in their family.

Genetic testing looks for a change in STAT1. Many different changes are known: more than 100, found in over 400 people worldwide by 2020. The Immune Deficiency Foundation says a firm diagnosis rests on genetic testing confirmed by functional tests. These tests check whether STAT1 is overactive in the person's cells, and so far they are available only in research labs.

Other immune tests often show few T cells that make IL-17, the cells that help fight Candida. In the large international study, this count was low in 82% of the people tested. Some people also have low T-cell numbers, which can raise the risk of severe infections.

Antifungal medicines treat and prevent Candida infections, and many people take them for years. They do not fix the cause, and Candida often comes back. In the large study, CMC persisted in 39% of the 202 people on long-term antifungal treatment. Care also includes treating other infections and autoimmune problems, such as giving thyroid hormone for an underactive thyroid.

JAK inhibitors, such as ruxolitinib and baricitinib, are pills that damp down the overactive signal. None is licensed for STAT1 GOF disease, but doctors often prescribe them off-label, meaning for a use not on the drug's label. In a European study of 45 people with STAT1 GOF disease, symptoms improved partly or fully in 87% on a JAK inhibitor. Side effects such as infections and weight gain were common but mostly mild. International expert guidance on their use was published in 2026.

A donor stem cell transplant can replace the immune system. It is used for people with severe disease, such as combined immune deficiency, severe or hard-to-treat infections, or serious autoimmunity. Early results were poor; one earlier series of 15 people reported 40% survival. Results have since improved. The authors of the latest study recommend a JAK inhibitor before transplant, called bridging, to control the disease and lower the risk that the new cells fail.

Living with STAT1 GOF disease often means years of Candida infections that keep coming back. Many people take antifungal medicine for long stretches. Some also need treatment for problems such as an underactive thyroid or lung damage. Experts in the large international study advised that care take place at centers experienced with the condition.

Deciding on a transplant is hard. JAK inhibitors can control symptoms for many people, but long-term experience with them is still missing. A transplant brings conditioning treatment, weeks in hospital and months of follow-up for infection and graft-versus-host disease. In the latest study, 21 of 36 people were alive and well at last follow-up. Five more were alive with ongoing problems, such as graft failure, bronchiectasis or kidney failure.

A transplant needs blood-forming cells from a donor with a close HLA match. Because the gene change can run in families, related donors are checked for it. European transplant guidelines say family donors should be screened for the same genetic defect, especially in conditions whose signs can start late or vary.

Unrelated volunteers were the most common donors in the latest study. Of 36 first transplants, 23 used a matched or closely matched unrelated donor and 10 used a matched brother or sister. Two used a partly matched relative, and one used unrelated cord blood. Survival did not differ significantly between matched family donors and unrelated donors.

In that study, 20 people took ruxolitinib before transplant, starting a median of 6 months before. New registry members widen the choices for patients who need an unrelated donor, though joining cannot promise a match for any one person.

When a transplant from a donor is planned, the team usually tests brothers and sisters first. Each full sibling has about a one in four chance of being a full match.

Most patients do not have a matched relative. In the words of NMDP, the U.S. registry, “75% of patients don’t have a fully matched donor in their own family.” The team then searches registries of volunteer donors around the world and banks of donated cord blood. In some transplants, a half-matched parent, child or sibling can also be the donor.

Matching depends on inherited tissue markers called HLA, so a patient is most likely to match someone who shares their ancestry. Every person who joins makes the search a little more likely to succeed, especially for patients from groups that are underrepresented on registries.

Many people with STAT1 GOF disease live well into adulthood. The large international study included people up to age 71. The outlook depends mostly on whether invasive infections, aneurysms or cancer develop.

In that study, 34 of 274 people (12%) had died, at a median age of 30. The main causes were severe infections, cancer and bleeding from a brain aneurysm. JAK inhibitors and better transplant results have changed care since then, but long-term data on both are still limited.

These figures describe groups of people, many diagnosed before today's treatments. They cannot predict how any one person will do.