STAT1

At least 35 STAT1 gene mutations have been identified in people with familial candidiasis, an inherited tendency to develop infections caused by the Candida fungus (commonly called yeast infections). Most people with familial candidiasis have chronic infections of the skin, nails, and mucous membranes such as the lining of the mouth, collectively called chronic mucocutaneous candidiasis, beginning in early childhood. 

Some people with STAT1 gene mutations have additional features such as increased susceptibility to other infections and an increased risk of autoimmune disorders, in which the immune system attacks the body's own tissues or organs. This combination of signs and symptoms caused by STAT1 gene mutations is sometimes called immunodeficiency 31C.

The STAT1 gene mutations that have been identified in people with familial candidiasis are described as "gain of function" because they increase the amount of activated STAT1 protein in cells. By increasing STAT1's inhibitory effect on the IL-17 pathway, the mutations impair the body's ability to fight Candida and result in the chronic infection that occurs in familial candidiasis. The effects of the increase in activated STAT1 protein on other signaling pathways are thought to underlie the variety of features that can occur in immunodeficiency 31C, but the specific mechanisms are not well understood.

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STAT1 gene mutations can also cause other immune system problems called immunodeficiency 31A and immunodeficiency 31B. In contrast with the gain-of-function mutations associated with familial candidiasis (described above), these STAT1 gene mutations are described as "loss-of-function" mutations because they impair or eliminate the protein's normal action. 

The STAT1 gene mutations that cause immunodeficiency 31A impair the STAT1 protein's function in the IFNG pathway, resulting in an increased susceptibility to mycobacteria. In the more severe disorder immunodeficiency 31B, the STAT1 gene mutations impair both the IFNG pathway and the IFNA/B pathway, resulting in susceptibility to both viral infections such as herpes simplex and viral encephalitis, and mycobacterial infections such as tuberculosis.