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Haemophagocytic lymphohistiocytosis type 4
HLH4 causes an overactive immune response, damaging organs and tissues. It primarily affects infants and young children, often leading to severe symptoms requiring prompt medical attention and specific treatments.
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Overview
Haemophagocytic lymphohistiocytosis type 4 (HLH4) is a severe, rare condition affecting the immune system. It belongs to a group of disorders called haemophagocytic lymphohistiocytoses (HLH), characterised by an uncontrolled inflammatory response [PMID:33651139]. In HLH4, certain immune cells, particularly T cells and natural killer (NK) cells, do not function correctly, leading to an overproduction of inflammatory molecules (cytokines). This can cause widespread tissue and organ damage.
HLH4 is one of several types of primary, or familial, HLH, which means it is caused by inherited genetic changes. Symptoms often appear in infancy or early childhood, though in some cases, they may present later. Without appropriate treatment, the condition can be life-threatening.
Symptoms & clinical features
The symptoms of HLH4 can vary but often include prolonged fever that does not respond to antibiotics, an enlarged spleen (splenomegaly), and sometimes an enlarged liver (hepatomegaly). Individuals may also experience low blood cell counts, affecting red blood cells (anaemia), white blood cells (leukopenia), and platelets (thrombocytopenia).
Other potential symptoms include skin rashes, neurological issues such as irritability, seizures, or developmental delays, and jaundice (yellowing of the skin and eyes). The uncontrolled inflammation can affect almost any organ system, leading to a wide range of clinical signs. Rapid diagnosis and treatment are crucial due to the severe nature of the condition [PMID:24040947].
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Affected organs
HLH4 can affect multiple organs and body systems due to the systemic inflammatory response. The bone marrow, where blood cells are produced, is often affected, leading to reduced blood cell counts. The spleen and liver frequently become enlarged as they try to process damaged blood cells and inflamed tissues.
Neurological involvement is also common, potentially affecting the brain and spinal cord, which can result in symptoms such as seizures, changes in consciousness, or difficulties with movement and coordination. Other organs that may be impacted include the skin, lymph nodes, and, less commonly, the lungs and kidneys.
Risks & severity
HLH4 is a severe condition with significant health risks. If left untreated, the uncontrolled inflammation can lead to multi-organ failure and can be life-threatening. The severity of the condition can vary; some individuals may experience a rapid onset of severe symptoms, while others may have a more gradual progression. However, most cases present with acute, severe manifestations.
Onset typically occurs in infancy or early childhood, though adult-onset cases have been reported in other forms of HLH [PMID:24040947]. The exact prevalence of HLH4 is not well established, but it is considered a rare disease. Early diagnosis and intensive treatment are critical to improve outcomes and manage the life-threatening complications associated with the condition.
Genetic causes
HLH4 is caused by pathogenic variants in the STX11 gene. The STX11 gene provides instructions for making a protein called syntaxin 11. This protein is important for the function of certain immune cells, particularly natural killer (NK) cells and cytotoxic T lymphocytes (CTLs).
Syntaxin 11 plays a role in the process by which these immune cells release cytotoxic granules, which are tiny packages containing proteins that kill infected or cancerous cells. When STX11 is altered, the cytotoxic granules cannot be released effectively. This impaired killing ability means that the immune system cannot properly clear pathogens or abnormal cells, leading to the sustained activation and proliferation of other immune cells, resulting in the uncontrolled inflammation characteristic of HLH [PMID:33651139].
- STX11 syntaxin 11
Inheritance pattern
HLH4 is inherited in an autosomal recessive pattern. This means that a person must inherit two altered copies of the STX11 gene - one from each parent - to develop the condition. Individuals who inherit only one altered copy of the STX11 gene are known as carriers. Carriers typically do not show symptoms of HLH4 because their unaffected copy of the gene can still produce enough functional syntaxin 11 protein.
If both parents are carriers of a pathogenic STX11 variant, there is a 25% chance with each pregnancy that their child will inherit two altered copies and develop HLH4. There is also a 50% chance the child will be a carrier like their parents, and a 25% chance the child will inherit two unaffected copies and not be a carrier.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
Diagnosing HLH4 involves a combination of clinical assessment, laboratory tests, and genetic testing. Doctors will look for characteristic symptoms such as prolonged fever, an enlarged spleen, and low blood cell counts. Blood tests may reveal high levels of inflammatory markers, such as ferritin, and specific immune markers indicating an overactive immune response. Reduced natural killer cell activity is a key diagnostic clue [PMID:24040947].
Confirmation of HLH4 relies on identifying pathogenic variants in the STX11 gene through genetic testing. This testing is typically requested by a clinical geneticist or a haematologist and is available through the NHS Genomic Medicine Service (GMS). Referral for genetic testing usually follows an initial clinical suspicion, and specific testing pathways (R-codes) exist within the GMS for individuals suspected of having HLH. Genetic counsellors can provide support and information throughout the diagnostic process.
Management & lifestyle
The management of HLH4 primarily focuses on suppressing the overactive immune response and addressing the underlying genetic cause. Treatment often involves immunotherapy, using medications to calm the immune system and reduce inflammation. In some cases, chemotherapy may also be used. The specific treatment regimen will depend on the individual's age, severity of symptoms, and response to therapy.
For some individuals, a haematopoietic stem cell transplant (HSCT) may be considered as a potentially curative treatment, especially if they do not respond to initial medical therapies. This procedure replaces the faulty immune cells with healthy ones. Ongoing care typically involves monitoring for complications, managing symptoms, and providing supportive care. Treatment decisions are made by a multidisciplinary team of specialists, often within specialist NHS centres, and are tailored to the individual patient.
UK care pathway
In the UK, individuals suspected of having HLH4 typically enter the NHS Genomic Medicine Service (GMS) pathway. This usually begins with a referral from a general practitioner or specialist (such as a paediatrician or haematologist) to a clinical geneticist or a specialist immunology centre. Genetic testing for HLH4 (and other forms of HLH) is available through specific GMS R-codes, which allow for comprehensive genomic analysis of relevant genes like STX11.
Following a diagnosis, patients and their families will have access to genetic counselling services. Genetic counsellors provide detailed information about the condition, its inheritance pattern, implications for other family members, and support in navigating the healthcare system. Ongoing management is coordinated by specialist teams within the NHS, often at regional or national centres of excellence for rare immune disorders.
Frequently asked questions
Is HLH4 contagious?
No, HLH4 is not contagious. It is an inherited genetic condition caused by specific changes in the STX11 gene. You cannot catch HLH4 from another person, nor can you pass it on through contact.
Can HLH4 be cured?
While there isn't a simple 'cure' in the traditional sense, some treatments, such as haematopoietic stem cell transplant (HSCT), aim to replace the affected immune system with healthy cells. This can potentially offer a long-term resolution for many individuals with HLH4.
How common is HLH4?
HLH4 is considered a rare disease. The exact number of people affected is not well established, but primary HLH conditions, including HLH4, occur in approximately 1 in 50,000 to 1 in 100,000 live births globally. It is more common in some populations due to founder effects.
What support is available for families affected by HLH4?
Families affected by HLH4 can access support through their clinical genetics team, genetic counsellors, and specialist immunology or haematology services within the NHS. Patient advocacy groups and charities also provide valuable resources, information, and peer support for rare diseases like HLH.
What is the typical age of onset for HLH4?
Symptoms of HLH4 most commonly appear during infancy or early childhood. However, the exact age of onset can vary, with some individuals showing symptoms later in childhood or, rarely, even into adulthood.