What Genetic Conditions Cause HLH?
“Primary” or “familial” HLH is caused by inherited problems in genes that control how the immune system kills virus-infected or other abnormal cells in a person’s body. These genes include PRF1, MUNC13-4, STXBP2 and STX11.
Primary HLH also occurs in some closely related inherited diseases. These include:
- X-linked lymphoproliferative disease (XLP), which is due to mutations in the SH2D1A gene (XLP1) or XIAP gene (XLP2)
- Griscelli syndrome type II, which is due to mutations in the Rab27a gene
- Chediak-Higashi syndrome, which is due to mutations in the LYST gene.
“Secondary” HLH can occur when the immune system is disturbed (e.g., infections), but not necessarily because of an inherited condition.
Why Do These Conditions Result in HLH?
The T and NK cells in patients with primary HLH can’t kill virus-infected or other abnormal cells like they normally would. T cells and NK cells usually do this by secreting death signals into targeted abnormal cells. When they can’t do this they get ‘overexcited’ and this leads to severe and unusual inflammation.
The proteins made by the MUNC13-4, STXBP2, STX11, Rab27a and LYST genes work like the machinery of a conveyor belt and are responsible for the secretion of the death signals produced by T and NK cells. The PRF1 gene makes a protein called perforin. It works like a key that allows the secreted death signals to enter inside a targeted abnormal cell, where the death signals can work.
SH2D1A is responsible for a more specialized killing mechanism and controls how the T cells themselves die. It is not yet entirely clear why XIAP mutations cause HLH.