In 2006 a second form of XLP was identified through looking at specific gene mutations. This occurs in approximately 20% of XLP cases and is now called X-Linked Lymphoproliferative Syndrome type-2 (or XLP2 for short and now XIAP) with the initial XLP being reclassified as XLP1. XLP2 therefore is an even rarer genetic condition than XLP1, but the impact onto boys with the condition is no less severe.
XIAP is diagnosed when there is mutation in the gene XIAP (also known as BIRC4), but does not have a detectable defect in gene SH2DIA (the gene associated with XLP1). In addition XLP2 boys have a normal expression of SAP (SLAM Associated Protein) unlike XLP1 affected boys.
Both XLP2 (XIAP) and XLP1 (SH2DIA) genes are located in the same gene locus (Xq25) and the two genes are only separated by a physical distance of 2.5 Mb.
There are a number of similarities of XIAP and XLP1 and also a number of key differences.
As with XLP1, XIAP infected boys have a significantly increased susceptibility to EBV infection and subsequent consequences of that infection. Both XLP1 and XIAP boys are at risk of:
Most XIAP affected boys develop HLH related to EBV infection but in some cases they show HLH without having EBV as the obvious trigger. Typical symptoms of HLH, besides persistent fever, are pallor (paleness), jaundice, liver and spleen enlargement, and neurological symptoms, such as irritability or even seizures. The involvement of the bone marrow, the site of blood cell production, can lead to severe decline of the blood cell counts (red and white blood cells and platelets).
Before encountering EBV, XIAP boys are healthy but unlike XLP1 boys, some of them develop splenomegaly (an enlargement of the spleen) as the first sign of their disease which may represent minimal forms of HLH.
The key difference with XLP1, is that XIAP boys do not seems to be at increased risk to develop lymphoproliferative disorders such as lymphomas. Indeed it would actually appear that the absence of XIAP may actually protect XLP2 boys from cancer; more medical research is currently underway into this.
This is principally the same for both XIAP and XLP1 affected boys.
Initial treatment should include regular immunoglobulin treatment. This will give some level of protection against possible infections including EBV, but is not a cure. Please see the XLP Research Trust’s leaflet on immunoglobulin for more information.
The only current cure for XIAP is a bone marrow transplant which in effect replaces the faulty immune system with a healthy donor one. However this is a major undertaking and it is critical that you discuss the pros and cons with your medical team.
Medical research is underway looking at possible other alternative treatments such as gene therapy but this is still some years away from being a proven solution.
The discovery of XIAP and the ability now to accurately diagnose it in a patient will be helpful to correctly and more quickly prescribe the correct treatments for affected boys.
It also possible that there could be other forms of XLP as yet undiscovered which are caused by a different faulty gene but manifesting the same problems. However if this is the case then this will affect an even smaller number of boys.
First published July 2009.
Latest update November 2023.
This fact sheet is designed for educational purposes only and is not intended to serve as medical advice. The information provided here should not be used for diagnosing or treating a health problem or disease. It is not a substitute for professional care.
We would like to thank Sylvain Latour from Unité 768 INSERM Hôpital Necker-Enfants Malades, Paris, France for his help in putting this leaflet together.
