X-linked lymphoproliferative syndrome is a rare genetic disorder that primarily affects boys, causing their immune system to respond dangerously to a common virus that most people encounter without serious problems.
Epidemiology
X-linked lymphoproliferative syndrome, also known as Duncan syndrome, is an extremely rare condition that affects the immune system. The disorder occurs almost exclusively in males due to how the genes are inherited. According to available data, XLP1, the most common form of the condition, is estimated to occur in about one in every one million males worldwide. The second type, XLP2, is even more uncommon, affecting approximately one in every five million males.[1][4]
The condition has a specific demographic pattern related to its genetic nature. Because the disorder is X-linked, it manifests primarily in males who have only one X chromosome. Females have two X chromosomes, so in order for them to develop XLP, they would need to have changes on both X chromosomes, which is extremely rare. Most females with the genetic change are carriers who do not have the disease themselves but can pass it on to their male children.[4]
There are increasing numbers of reports of affected females with unfavorable X-chromosome inactivation favoring the X chromosome with the genetic change. These women may develop symptoms including hemophagocytic lymphohistiocytosis (a severe immune reaction), inflammatory bowel disease, and skin conditions.[3]
Causes
X-linked lymphoproliferative syndrome is caused by mutations in specific genes located on the X chromosome. There are two types of XLP, each caused by changes in different genes. XLP1 is caused by mutations in the SH2D1A gene, while XLP2, also called XIAP deficiency, is caused by mutations in the XIAP gene.[1][4]
The SH2D1A gene provides instructions for making a protein called signaling lymphocyte activation molecule associated protein, or SAP. This protein plays a vital role in the functioning of lymphocytes (white blood cells) that destroy other cells and is necessary for the development of specialized T cells called natural killer T cells. The SAP protein also helps control immune reactions by triggering self-destruction of certain lymphocytes when they are no longer needed.[1]
When the SH2D1A gene is mutated, some mutations impair SAP function, while others result in an abnormally short protein that is unstable or nonfunctional. In some cases, no SAP is produced at all. Without functional SAP, proper signaling in the immune system is disrupted, and the body may be unable to control its immune reaction to certain infections, particularly the Epstein-Barr virus.[1]
The XIAP gene provides instructions for making a protein that helps protect cells from undergoing self-destruction in response to certain signals. Mutations in the XIAP gene can lead to an absence of XIAP protein or decrease the amount produced. However, it remains unclear exactly how a lack of XIAP protein results in the signs and symptoms of XLP, or why features of this disorder differ between the two types.[1]
In many cases, XLP is an inherited condition, meaning it is passed down in families in the same way that physical characteristics like eye color are passed from parent to child. However, sometimes mutations can occur spontaneously by chance and are not inherited from parents.[7]
Risk Factors
The primary risk factor for X-linked lymphoproliferative syndrome is being male with a family history of the condition. Because XLP is caused by mutations on the X chromosome and follows an X-linked recessive inheritance pattern, males are at much higher risk than females. Males have only one X chromosome, so a mutation on that chromosome will cause the disease. A male who inherits an X chromosome carrying a mutation does not have a second X chromosome to compensate, making him vulnerable to this X-linked disorder.[7]
Having a mother who is a carrier of the genetic mutation significantly increases the risk. A female carrier has one normal X chromosome and one X chromosome with the mutation. While she typically does not show symptoms herself, each of her male children has a 50 percent chance of inheriting the mutated X chromosome and developing the disorder.[4]
Exposure to the Epstein-Barr virus represents a critical trigger for disease manifestation in individuals with XLP. Most people with the disorder do well until they are exposed to EBV. The virus is extremely common, eventually infecting most humans and often causing infectious mononucleosis, commonly known as mono. However, for individuals with XLP, EBV exposure can trigger life-threatening complications. This makes infection with EBV a major risk factor for developing severe symptoms in someone with the underlying genetic condition.[5][13]
Symptoms
Children with X-linked lymphoproliferative syndrome are usually asymptomatic until they develop an Epstein-Barr virus infection. The disorder is usually silent before this viral exposure, making it difficult to diagnose without family history or genetic testing. Symptoms typically begin appearing between 6 months and 10 years of age, though the timing and severity can vary significantly from person to person.[4][5]
More than half of individuals with XLP experience an exaggerated immune response to the Epstein-Barr virus. While EBV normally causes infectious mononucleosis with symptoms like sore throat, fever, and swollen lymph nodes in people with normal immune systems, children with XLP can develop potentially life-threatening complications. The most common presentation is hemophagocytic lymphohistiocytosis or severe mononucleosis, which represents an inappropriate and dangerous immune response.[1][3]
Hemophagocytic lymphohistiocytosis is characterized as an acute illness with prolonged and high fever, abnormally low levels of blood cells affecting multiple cell lines, and enlargement of the liver and spleen, which can be severe. During HLH, the immune system produces abnormally large numbers of T cells, B cells, and other immune cells called macrophages. This proliferation causes fever, destroys blood-producing cells in the bone marrow, and damages the liver. The spleen, heart, kidneys, and other organs and tissues may also be affected. Additional symptoms can include skin rashes, jaundice (yellowing of the eyes and skin), and anemia.[1][4]
About one-third of people with XLP experience dysgammaglobulinemia, which means they have abnormal levels of antibodies. Antibodies are proteins that attach to foreign particles and germs, marking them for destruction. Individuals with dysgammaglobulinemia are prone to recurrent infections, particularly respiratory infections. If untreated, this can lead to bronchiectasis and recurrent respiratory problems that may result in death.[1][3]
Cancers of immune system cells, specifically lymphomas, occur in about one-third of people with XLP1. These lymphoproliferative diseases often develop in childhood, usually following EBV exposure. The development of lymphomas may occur when defective lymphocytes are not properly destroyed by the body’s normal cell death mechanisms.[1][3]
Males with XLP2 are more likely to experience certain distinct symptoms compared to those with XLP1. They are more likely to have HLH without EBV infection and can experience recurrent episodes of HLH, which is not typically seen in those with XLP1. Splenomegaly, or an enlarged spleen, is common. Gastrointestinal disease is also more frequent in XLP2, including enterocolitis (inflammation of the digestive tract) and perirectal abscesses or fistulae (abnormal connections near the rectum). Rarely, individuals with XLP2 and inflammatory bowel disease have been reported to develop inflammatory liver disease, which can progress to fatal liver failure.[3]
Rarer symptoms that can occur in those with XLP1 include aplastic anemia (the bone marrow stops producing enough new blood cells), vasculitis (inflammation of blood vessels), and lymphoid granulomatosis. Some patients may also experience recurrent fevers and low blood counts.[3][4]
Without treatment, the prognosis is very poor. Death usually results from hemophagocytic lymphohistiocytosis, with most patients surviving only into childhood. Approximately 75 percent of patients die by age 10 years, and all die by age 40 years unless they receive hematopoietic stem cell transplantation.[5]
Prevention
Because X-linked lymphoproliferative syndrome is a genetic disorder caused by mutations in specific genes, there is no way to prevent the condition itself from occurring in someone who has inherited the genetic mutation. However, there are important steps that can be taken to prevent severe complications and improve outcomes for those with the condition or at risk for it.
Early genetic screening and diagnosis are the most important preventive measures. When a case or carrier is identified in a family, genetic testing should be done in relatives. Prenatal screening is strongly recommended for people if a mutation that causes XLP has been identified in their family. This allows families to know if a child has the condition before symptoms develop, enabling early intervention.[5]
For boys diagnosed with XLP before they are exposed to Epstein-Barr virus, preventing EBV infection becomes a priority. While there is no vaccine against EBV, avoiding exposure to the virus when possible can help delay the onset of severe symptoms. However, because EBV is so common and eventually infects most people, complete avoidance is extremely difficult in practice.
Early screening of infant boys in families that have had children with XLP is critically important so that they can receive curative treatment, specifically hematopoietic stem cell transplantation, before contracting an EBV infection. Transplantation is most effective when done before Epstein-Barr virus infection or other disorders become irreversible. This early intervention can prevent the life-threatening complications associated with EBV exposure in individuals with XLP.[5]
For patients who have been diagnosed with XLP but have not yet received a transplant, certain medications can help prevent severe EBV infection. Rituximab, a medication that targets certain immune cells, can help prevent severe EBV infection before transplantation. This provides a protective bridge until definitive treatment can be performed.[5]
In survivors of initial EBV infection who have XLP, ongoing monitoring is essential for preventing complications or detecting them early. Laboratory and imaging tests are typically done yearly to check for lymphoma and anemia. Regular follow-up allows healthcare providers to identify and address problems before they become life-threatening.[5]
Pathophysiology
The underlying mechanism of X-linked lymphoproliferative syndrome involves fundamental disruptions in how the immune system functions, particularly in the way certain immune cells respond to infections and regulate themselves. Understanding these changes helps explain why patients with XLP react so differently to the Epstein-Barr virus compared to individuals with normal immune systems.
In individuals with XLP1, the absence or dysfunction of the SAP protein creates multiple problems in immune system regulation. The SAP protein is normally involved in the functioning of cytotoxic lymphocytes, which are immune cells that destroy other cells, and is necessary for the development of natural killer T cells. Without functional SAP, lymphocytes proliferate unchecked in response to Epstein-Barr virus infection, and natural killer cells do not function properly.[5]
Normally, after initial EBV infection, the virus remains in certain immune system cells called B cells. However, the virus is generally kept inactive because it is controlled by other lymphocytes called T cells that specifically target EBV-infected B cells. In people with XLP, this normal control mechanism fails. The disrupted signaling in the immune system prevents the body from controlling the immune reaction to EBV infection properly.[1]
When someone with XLP encounters EBV, instead of mounting a controlled immune response, their body responds by producing abnormally large numbers of T cells, B cells, and other lymphocytes called macrophages. This uncontrolled proliferation of immune cells leads to hemophagocytic lymphohistiocytosis, where these overactive immune cells begin attacking the body’s own tissues and blood-forming cells.[1]
The mechanism behind HLH involves cytotoxic T cells that react to EBV-infected B cells or other tissue cells. These T cells, meant to eliminate infected cells, instead cause widespread tissue damage. They destroy blood-producing cells in the bone marrow, leading to low levels of multiple types of blood cells. They damage the liver, potentially causing liver failure. The excessive activity of these immune cells creates the fever, organ enlargement, and multi-system organ dysfunction characteristic of HLH.[5]
The SAP protein also plays a role in triggering apoptosis, or programmed cell death, of cytotoxic lymphocytes when they are no longer needed. Without this normal self-destruction mechanism, defective lymphocytes accumulate and may develop into lymphomas. This explains why about one-third of people with XLP1 develop cancers of immune system cells.[1]
In XLP2, the mechanism involves the XIAP protein, which normally helps protect cells from undergoing apoptosis in response to certain signals. The XIAP gene encodes the X-linked inhibitor of apoptosis protein. When this protein is absent or reduced due to genetic mutations, cells cannot properly regulate their survival and death pathways. Although the exact mechanism by which XIAP deficiency causes XLP symptoms remains unclear, it is known that XLP type 2 predisposes individuals to hemophagocytic lymphohistiocytosis, particularly without the trigger of EBV infection.[1][5]
The dysgammaglobulinemia seen in XLP results from disruptions in B cell function and antibody production. The altered immune signaling affects not only cellular immunity but also the humoral immune system responsible for producing antibodies. This leads to varying degrees of immune dysfunction, making patients susceptible to recurrent infections with bacteria and other pathogens beyond just EBV.[3]
In XLP2, the gastrointestinal manifestations appear related to improper regulation of inflammation in the digestive tract. The absence of XIAP affects how immune cells in the gut respond to normal intestinal bacteria and other stimuli, leading to chronic inflammation that manifests as enterocolitis, perirectal abscesses, fistulae, and sometimes inflammatory liver disease.[3]
