Cryopyrin-associated periodic syndromes (CAPS) are a group of conditions that have overlapping signs and symptoms and the same genetic cause. The group includes three conditions known as familial cold autoinflammatory syndrome type 1 (FCAS1), Muckle-Wells syndrome (MWS), and neonatal-onset multisystem inflammatory disorder (NOMID). These conditions were once thought to be distinct disorders but are now considered to be part of the same condition spectrum. FCAS1 is the least severe form of CAPS, MWS is intermediate in severity, and NOMID is the most severe form.

The signs and symptoms of CAPS affect multiple body systems. Generally, CAPS are characterized by periodic episodes of skin rash, fever, and joint pain. These episodes can be triggered by exposure to cold temperatures, fatigue, other stressors, or they may arise spontaneously. Episodes can last from a few hours to several days. These episodes typically begin in infancy or early childhood and persist throughout life.

While the CAPS spectrum shares similar signs and symptoms, the individual conditions tend to have distinct patterns of features. People with FCAS1 are particularly sensitive to the cold, and exposure to cold temperatures can trigger a painful or burning rash. The rash usually affects the torso and limbs but may spread to the rest of the body. In addition to fever and joint pain, other possible symptoms include muscle aches, chills, drowsiness, eye redness, headache, and nausea.

Individuals with MWS develop the typical periodic episodes of skin rash, fever, and joint pain after cold exposure, although episodes may occur spontaneously or all the time. Additionally, they can develop progressive hearing loss in their teenage years. Other features of MWS include skin lesions or kidney damage from abnormal deposits of a protein called amyloid (amyloidosis).

In people with NOMID, the signs and symptoms of the condition are usually present from birth and persists throughout life. In addition to skin rash and fever, affected individuals may have joint inflammation, swelling, and joint deformities called contractures that may restrict movement. People with NOMID typically have headaches, seizures, and cognitive impairment resulting from chronic meningitis, which is inflammation of the tissue that covers and protects the brain and spinal cord (meninges). Other features of NOMID include eye problems, short stature, distinctive facial features, and kidney damage caused by amyloidosis.

CAPS are caused by variants (also known as mutations) in the NLRP3 gene. The NLRP3 gene provides instructions for making a protein called cryopyrin. Cryopyrin is a member of a family of proteins called intracellular "NOD-like" receptor (NLR) proteins. These proteins are involved in the immune system, helping to regulate the process of inflammation. Inflammation occurs when the immune system sends signaling molecules and white blood cells to a site of injury or disease to fight foreign invaders and help repair damaged tissues. After this has been accomplished, the body stops (inhibits) the inflammatory response to prevent damage to its own cells and tissues. Cryopyrin is involved in the assembly of a molecular complex called an inflammasome, which helps trigger the inflammatory process.

Researchers believe that NLRP3 gene variants that cause CAPS result in an overactive cryopyrin protein, which leads to inappropriate inflammatory responses. Impairment of the body's mechanisms for controlling inflammation results in the episodes of fever and damage to the body's cells and tissues seen in CAPS.

Normal Function

The NLRP3 gene provides instructions for making a protein called cryopyrin. Cryopyrin is a member of a family of proteins called intracellular "NOD-like" receptor (NLR) proteins. Cryopyrin is found mainly in white blood cells and in cartilage-forming cells (chondrocytes).

NLR proteins are involved in the immune system, helping to start and regulate the immune system's response to injury, toxins, or foreign invaders. NLR proteins recognize specific molecules and respond by helping to turn on (activate) certain parts of the immune system. Cryopyrin recognizes bacteria; chemicals such as asbestos, silica, and uric acid crystals; and compounds released by injured cells.

Cryopyrin molecules assemble themselves, along with other proteins, into structures called inflammasomes, which help trigger the process of inflammation. Inflammation occurs when the immune system sends signaling molecules as well as white blood cells to a site of injury or disease to fight foreign invaders and help repair damaged tissues. Once the threat is over, the body stops (inhibits) the inflammatory response, to prevent damage to its own cells and tissues.

Health Conditions Related to Genetic Changes

Cryopyrin-associated periodic syndromes

Several variants (also known as mutations) in the NLRP3 gene have been found to cause cryopyrin-associated periodic syndromes (CAPS). CAPS are a group of conditions that have overlapping signs and symptoms. The conditions are generally characterized by periodic episodes of skin rash, fever, and joint pain. CAPS include three conditions known as familial cold autoinflammatory syndrome type 1 (FCAS1), Muckle-Wells syndrome (MWS), and neonatal-onset multisystem inflammatory disorder (NOMID). These conditions were once thought to be distinct disorders but are now considered to be part of the same condition spectrum. FCAS1 is the least severe form of CAPS, MWS is intermediate in severity, and NOMID is the most severe form.

Many of the variants that cause CAPS are in a region of the NLRP3 gene known as exon 3. All of the variants likely result in the cryopyrin protein being overactive. Inflammasomes made with abnormal cryopyrin proteins trigger inflammatory responses even when there is no injury or disease. Impairment of the body's mechanisms for controlling inflammation results in episodes of fever and widespread damage to the body’s cells and tissues.

While the CAPS spectrum shares similar signs and symptoms, it is unclear why variants in different parts of the NLRP3 gene cause the patterns of features that distinguish FCAS1, MWS, and NOMID.

Other Names for This Gene

• AII/AVP
• AII/AVP receptor-like
• angiotensin/vasopressin receptor AII/AVP-like
• AVP
• C1orf7
• CIAS1
• CLR1.1
• cryopyrin
• NACHT domain-, leucine-rich repeat-, and PYD-containing protein 3
• NACHT, LRR and PYD containing protein 3
• NALP3_HUMAN
• NLR family, pyrin domain containing 3
• nucleotide-binding oligomerization domain, leucine rich repeat and pyrin domain containing 3
• PYPAF1
• PYRIN-containing APAF1-like protein 1

Genomic Location

CAPS are usually inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. People with the condition are born with one altered copy of the NLRP3 gene in each cell. In most cases, the altered gene is inherited from an affected parent. The remaining cases are a result of new variants in the NLRP3 gene and occur in people with no history of the disorder in their family.

Rarely, the genetic change that causes CAPS is not inherited but occurs at some point during embryonic development or later in life. As cells continue to grow and divide, some of these cells will have the genetic change, and others will not (a situation known as mosaicism).

CAPS are rare, with an estimated prevalence of 2 to 5 per million individuals, collectively. However, it is thought that the conditions are underdiagnosed since the features of CAPS are similar to other more common conditions.