JAK3-deficient SCID

SCID stands for severe combined immunodeficiency. It is a group of rare genetic conditions in which a baby is born without a working immune system. JAK3 deficiency is one of the less common types. The US National Library of Medicine estimates it causes 7 to 14 percent of SCID cases, while the Immune Deficiency Foundation puts it at under 10 percent.

Babies with JAK3-deficient SCID have few or no T cells and natural killer (NK) cells, which attack cells infected by viruses. They usually have normal numbers of B cells, the cells that make antibodies, but those B cells do not work properly. Doctors call this pattern T-B+NK- SCID. It looks the same as X-linked SCID on blood tests, but the cause and the way it runs in families are different.

Some JAK3 changes leave the protein partly working. These can cause a milder or unusual form that may be found later, including a rare, overactive immune state called Omenn syndrome.

JAK3-deficient SCID is caused by changes in the JAK3 gene. This gene makes a protein that works inside immune cells and passes on the signals they need to grow and mature. It works hand in hand with the protein that is missing in X-linked SCID, which is why the two conditions look alike.

The condition is autosomal recessive. A child is affected only when both copies of the JAK3 gene have a change, one from each parent. Parents who carry one changed copy usually have no symptoms. When both parents are carriers, each pregnancy has a 1 in 4 chance of an affected child, and girls and boys are equally likely to be affected.

Once the family’s gene changes are known, relatives can have carrier testing and a pregnancy can be tested before birth. A genetic counselor can explain what the results mean for brothers, sisters and future children.

Babies look healthy at birth. Without treatment, they get infections that keep coming back or will not clear, often caused by germs that rarely harm other babies. Common problems include thrush (a yeast infection in the mouth), ongoing diarrhea, pneumonia, skin rashes and slow growth.

Where it is offered, newborn screening for SCID can find babies with JAK3-deficient SCID before they get sick.

An abnormal screen is not a diagnosis, but it needs urgent attention from an immunology team. Speed matters: in a large study in the United States and Canada, babies who had an active infection when they were transplanted were less likely to survive.

The weeks between diagnosis and transplant can be frightening. Families are usually asked to keep the baby at home or in hospital, away from crowds, daycare and anyone who is sick, even with a cold. Brothers and sisters who go to school or daycare can bring germs home, so daily routines often change for the whole family.

Treatment means a stay at a specialist children’s hospital, sometimes far from home. Afterward, checkups continue for years to see how well the donor cells have taken hold and how the child is growing. Preventive medicines and antibody replacement are stopped only when the new immune system is working well. Vaccines are given after that, on the team’s schedule.

Because the condition is recessive, parents may also face decisions about testing their other children and planning future pregnancies. Talking with other SCID families, a genetic counselor and the hospital’s social workers can help with the stress, the isolation and the practical costs.

A transplant for JAK3-deficient SCID uses blood-forming stem cells from a donor. A brother or sister who is a full HLA match is the first choice. But each full sibling has only about a 1 in 4 chance of being a match, and siblings are also checked to make sure they do not have the condition themselves. Many children do not have a matched sibling.

Without one, teams look for a fully matched unrelated donor through the registries. If that search would take too long for the baby’s safety, or no match exists, a half-matched family member, usually a parent (a haploidentical donor), or a cord blood unit is used instead. Parents who carry one changed copy usually have no symptoms, so a parent can be a half-matched donor.

This is why registry volunteers matter for SCID, and why speed matters. A matched volunteer who is ready to donate quickly can make an unrelated transplant possible while a baby waits in protective isolation. Joining a registry cannot promise a match for any one child, and many children are treated with a parent’s cells instead.