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Activated PI3K delta syndrome (APDS)

Activated PI3K delta syndrome (APDS)



Overview

Activated PI3K-delta syndrome (APDS) is a rare primary immunodeficiency caused by genetic mutations that lead to a hyperactive PI3K\(\delta \) pathway. This immune system impairment primarily causes recurrent respiratory infections, chronic viral infections, enlarged lymph nodes, and autoimmune issues, significantly increasing the risk of lymphoma.

Symptoms

1. Frequent and Severe Infections

Because the immune system’s white blood cells (B cells and T cells) fail to function properly, patients are highly susceptible to:

Respiratory Tract: Recurrent pneumonia, bronchitis, and chronic cough.

Ear and Sinus: Frequent ear infections (otitis media) and persistent sinusitis, which can sometimes lead to hearing loss

Viral Infections: Chronic or severe viral infections, particularly from the herpesvirus family (e.g., Epstein-Barr virus/EBV, cytomegalovirus/CMV, and herpes simplex).

2. Lymphoproliferation

Abnormal clumping and overproduction of white blood cells can cause organs and tissues to swell. Common signs include:

Swollen Lymph Nodes: Especially in the neck, armpits, and groin.

Enlarged Spleen and Liver: Can lead to a feeling of fullness or abdominal discomfort.

Nodules: Lumps or tissue growth in the airways or digestive tract.

3. Autoimmune and Inflammatory Disorders

In APDS, abnormal cells mistakenly attack normal, healthy tissues. Features include:

Cytopenias: The immune system destroys the body's own blood cells, leading to conditions like autoimmune hemolytic anemia (destruction of red blood cells) or immune thrombocytopenia (low platelets).

Gastrointestinal Issues: Inflammation resembling Crohn's disease or colitis, resulting in persistent diarrhea, abdominal pain, and poor nutrient absorption.

Kidney Inflammation: Conditions such as glomerulonephritis, which can cause blood in the urine.

4. Long-Term Complications

If left unmanaged, the chronic nature of the disease can lead to lasting damage:

Bronchiectasis: Scarring and widening of the airways in the lungs due to repeated respiratory infections.

Developmental Delay: Growth impairment or failure to thrive, often secondary to chronic illness and nutritional issues.

Lymphoma: An increased risk of developing a form of blood cancer.

Causes

The disorder is inherited in an autosomal dominant manner, meaning only one copy of the altered gene is needed to cause the condition. It is divided into two types based on the affected gene:

APDS Type 1: Caused by gain-of-function variants in the PIK3CD gene, which alters the p110\(\delta \) protein subunit.

APDS Type 2: Caused by loss-of-function variants in the PIK3R1 gene, which alters the p85\(\alpha \) protein subunit.

Diagnosis

1. Clinical Evaluation and Patient History

Doctors look for specific red flags that usually appear in early childhood:

Frequent Infections: Persistent or severe ear, sinus, and lung infections (like pneumonia)

Lymphoproliferation: Noticeably swollen lymph nodes, an enlarged spleen, or an enlarged liver.

Autoimmune Manifestations: Low blood cell counts (cytopenias), arthritis, or gut inflammation.

Developmental Delays: Short stature or neurodevelopmental delays, especially in APDS2.

2. Immunological and Laboratory Testing

Before ordering genetic tests, a clinical immunologist will typically run blood tests to assess the immune system. 

These tests often reveal: Low IgG and IgA antibody levels. Normal or elevated IgM antibody levels.

Abnormal lymphocyte counts: Low naive T and B cell counts, alongside elevated transitional B cells and CD8+ T cells.

Poor vaccine responses: Particularly to T-cell independent vaccines.

3. Definitive Genetic Testing

Because APDS symptoms overlap heavily with other conditions like Common Variable Immunodeficiency (CVID), genetic testing is required for a conclusive diagnosis.

Testing Methods: A doctor will order targeted gene sequencing, a gene panel, or whole-exome sequencing to search for pathogenic variants in the PIK3CD (APDS1) or PIK3R1 (APDS2) genes.

Inheritance: The syndrome can be inherited in an autosomal dominant pattern from an affected parent (50% chance of passing it on), or it can occur as a de novo (spontaneous) mutation with no family history.

Treatment

1. Targeted Therapeutics

Leniolisib: An oral PI3K\(\delta \) inhibitor approved for adult and pediatric patients 12 years and older. It acts directly on the root signaling mechanism of the disease to improve immune cell function.

mTOR Inhibitors: Drugs like sirolimus are frequently utilized off-label to control lymphoproliferation (swollen lymph nodes, spleen, or liver) and autoimmune issues.

2. Supportive Immune Care

Immunoglobulin Replacement Therapy (IRT): Because many APDS patients have poor antibody production, regular intravenous (IVIG) or subcutaneous (SCIG) infusions are used to help the body fight off infections.

Antimicrobial Prophylaxis: Chronic or recurring bacterial and viral infections require proactive use of antibiotics, antifungals, or antivirals.

3. Curative and Specialized Treatments

Hematopoietic Stem Cell Transplantation (HSCT): Often referred to as a bone marrow transplant, HSCT is currently the only potentially curative option for severe forms of APDS. It is generally reserved for patients with profound, life-threatening immunodeficiencies or lymphoma.

Airway Clearance: For individuals who develop chronic lung disease or bronchiectasis, therapies such as nebulized hypertonic saline, airway clearance techniques, and targeted respiratory physiotherapy are critical.

Type of Doctor Department : A Clinical Immunologist or an Allergist/Immunologist

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