Pyogenic sterile arthritis, pyoderma gangrenosum, and acne syndrome—known as PAPA syndrome—is a rare inherited condition that causes painful joint inflammation, severe skin ulcers, and troublesome acne, often beginning in early childhood and evolving throughout life.

Understanding PAPA Syndrome

PAPA syndrome is a rare genetic disorder that primarily affects the joints and skin. The name comes from the three main features of the condition: pyogenic arthritis (painful joint inflammation that appears to be infected but actually contains no bacteria), pyoderma gangrenosum (large, painful skin ulcers), and severe acne. This condition is considered an autoinflammatory disease, meaning the body’s immune system mistakenly attacks its own healthy tissues, causing inflammation without any infection being present.^[1]

The condition typically begins in early childhood with arthritis, while the skin problems usually become more noticeable during adolescence and may continue into adulthood. Not everyone with PAPA syndrome experiences all three symptoms at the same time, and the severity can vary significantly from person to person, even within the same family.^[2]

How Common Is PAPA Syndrome?

PAPA syndrome is extremely rare. According to available medical literature, only 34 patients from five families worldwide had been documented as of recent reports. These families were located in the United States, Italy, the Netherlands, and New Zealand.^[3] However, experts believe the actual number of cases may be higher, as the condition can be difficult to diagnose and may be underestimated in the general population.^[2]

The syndrome affects males and females equally, showing no preference for either gender. The disease usually appears during childhood, with joint problems typically being the first sign, followed by skin manifestations later in life.^[2]

Causes of PAPA Syndrome

PAPA syndrome is caused by mutations in a gene called PSTPIP1, which was previously known as the CD2 binding protein 1 gene. This gene provides instructions for making a protein that plays an important role in regulating the body’s inflammatory response.^[2] When mutations occur in this gene, the protein’s function changes, leading to abnormal inflammation throughout the body.

The PSTPIP1 gene is located on chromosome 15. Scientists have identified that only two specific mutations in this gene account for the known cases of PAPA syndrome. Recent research has shown that the PSTPIP1 protein binds to another protein called pyrin, which is encoded by the MEFV gene. Mutations in the MEFV gene cause a different condition called Familial Mediterranean Fever. In PAPA syndrome, the mutated PSTPIP1 protein shows increased binding to pyrin, which contributes to the excessive inflammation seen in the disease.^[3]

This genetic defect causes the protein to malfunction in ways that affect the regulation of inflammation. The PSTPIP1 protein is part of an inflammatory pathway that is also involved in other autoinflammatory conditions, which explains why PAPA syndrome shares some features with diseases like familial Mediterranean fever and other periodic fever syndromes.^[4]

How PAPA Syndrome Is Inherited

PAPA syndrome follows an autosomal dominant pattern of inheritance. This means that only one copy of the mutated gene—inherited from one parent—is sufficient to cause the condition. The inheritance pattern is not linked to gender, so both males and females can inherit and pass on the condition equally.^[2]

In families with PAPA syndrome, one parent typically shows at least some symptoms of the disease. Usually, more than one affected individual can be observed in a single family, with affected people appearing in each generation. When someone with PAPA syndrome plans to have children, there is a 50 percent chance with each pregnancy that the child will inherit the mutated gene and develop PAPA syndrome.^[2]

It’s important to understand that the parent who carries the mutation may not exhibit all symptoms of the disease. This is because PAPA syndrome shows what doctors call variable penetrance and variable expression. Variable penetrance means that not everyone who inherits the mutation will develop obvious symptoms. Variable expression means that the symptoms can vary widely in severity, even among family members who carry the same genetic mutation.^[2]

A child inherits the disease from one parent who carries a mutation in the PSTPIP1 gene. The disease cannot be prevented, but the symptoms can be managed and treated with appropriate medical care.^[2]

Main Symptoms of PAPA Syndrome

The three main symptoms that characterize PAPA syndrome are arthritis, pyoderma gangrenosum, and acne. However, it’s rare for all three to be present in the same patient at the same time. The symptoms typically appear at different stages of life and can change over time, usually improving as a person grows older.^[2]

Arthritis

Arthritis is usually the first symptom to appear and is considered the predominant feature of PAPA syndrome. The first episode typically occurs between 1 and 10 years of age, during early childhood. The arthritis usually involves one joint at a time, most commonly affecting large joints such as the elbow, knee, or ankle.^[2]

The affected joint becomes swollen, painful, and red. The clinical appearance closely resembles septic arthritis, which is joint inflammation caused by the presence of bacteria. In fact, the fluid inside the affected joint appears purulent (containing pus) with a significant accumulation of neutrophils (a type of white blood cell), but cultures are invariably negative, meaning no bacteria are actually present. This is why the condition is called “pyogenic sterile arthritis”—it looks infected but isn’t.^[3]

Episodes of arthritis often occur after minor trauma to the joint, though they can also occur spontaneously without any obvious trigger. The arthritis in PAPA syndrome can be quite destructive over time. Without proper treatment, repeated episodes can cause damage to the joint cartilage and surrounding bone. In severe cases, multiple joint replacements may be required. Fortunately, with improved treatment options available today, the hope is that this damage can be limited or prevented in newly diagnosed cases.^[4]

Pyoderma Gangrenosum

Pyoderma gangrenosum consists of large ulcerative skin lesions that usually have a later onset than arthritis. These lesions often appear on the legs, though they can occur elsewhere on the body. The ulcers present as poorly healing sores with distinct, raised, and undermined edges.^[2]

An important characteristic of pyoderma gangrenosum in PAPA syndrome is a phenomenon called pathergy. Pathergy refers to the tendency of ulcers to develop at sites of injury. There are documented reports of lesions developing at the site of a joint replacement wound, where a central venous line was inserted, or at intravenous drip insertion sites. This means that even minor injuries or medical procedures can trigger the formation of these painful ulcers.^[4]

Pyoderma gangrenosum is variably expressed in PAPA syndrome, meaning it is not always present in all individuals with the disease. Some patients may experience severe ulceration, while others may have minimal or no skin ulcers at all.^[4]

Acne

Acne affects most individuals with PAPA syndrome, though the severity varies considerably from person to person. The acne usually appears during adolescence and may persist into adulthood. It typically involves the face and trunk, and is characterized as severe cystic acne or nodulocystic acne. This type of acne forms deep, painful lumps beneath the skin rather than simple surface pimples.^[2]

If left untreated, severe cystic acne results in permanent scarring. Like the other symptoms of PAPA syndrome, acne episodes are often precipitated by minor injury to the skin.^[2]

Less Common Features

In addition to the three main symptoms, PAPA syndrome has been associated with other less common features. These include adult-onset insulin-dependent diabetes mellitus, protein in the urine (proteinuria), and abscess formation at the sites where injections are given. The abscess formation is another example of pathergy—the tendency for inflammation to occur at sites of injury.^[3]

Prevention Strategies

Because PAPA syndrome is a genetic condition caused by mutations present from birth, the disease itself cannot be prevented. However, understanding the condition and taking appropriate precautions can help prevent or minimize symptom flares and complications.^[2]

One key preventive strategy involves being cautious about activities that might cause joint or skin trauma, since minor injuries often trigger episodes of arthritis or skin ulcers due to pathergy. This doesn’t mean avoiding all physical activity, but rather being mindful and taking appropriate precautions during sports and daily activities.

For individuals with PAPA syndrome or those with a family history of the condition who are planning to have children, genetic counseling can provide valuable information. A genetic counselor can explain the inheritance pattern, discuss the 50 percent risk of passing the condition to children, and explore available options. However, it’s important to remember that even if a child inherits the gene mutation, the severity of symptoms can vary greatly due to variable expression and penetrance.^[2]

While the disease cannot be prevented, early diagnosis and appropriate treatment can help manage symptoms, prevent complications, and improve quality of life. Regular follow-up with healthcare providers familiar with PAPA syndrome is important for monitoring the condition and adjusting treatment as needed.

How the Disease Affects the Body

PAPA syndrome causes abnormal inflammatory responses in the body, primarily affecting the joints and skin. The underlying problem involves the immune system’s regulation of inflammation. In a healthy immune system, inflammation is a controlled response to infection or injury that helps protect the body. In PAPA syndrome, however, the genetic mutation causes the inflammatory response to become dysregulated.^[2]

The mutated PSTPIP1 protein fails to properly regulate inflammation, leading to excessive accumulation of neutrophils in affected tissues. Neutrophils are a type of white blood cell that normally help fight infections by releasing chemicals that kill bacteria. In PAPA syndrome, neutrophils accumulate in joints and skin even when no infection is present, causing inflammation and tissue damage.^[3]

In the joints, this neutrophil accumulation creates a purulent (pus-like) appearance in the synovial fluid (the fluid that lubricates joints). The affected joint tissue, called synovium, shows prominent inflammation with large numbers of neutrophils when examined under a microscope. Despite this appearance of infection, bacterial cultures remain negative because no actual bacteria are causing the problem. Over time, repeated inflammatory episodes can damage the cartilage that cushions joints and the bone tissue surrounding the joint.^[3]

In the skin, biopsies of pyoderma gangrenosum lesions show superficial ulceration with neutrophilic inflammation. The skin tissue displays prominent inflammation with a predominance of neutrophils. Biopsies of acne lesions show similar inflammatory patterns. The pathergy phenomenon—where injuries trigger new lesions—reflects the exaggerated inflammatory response to even minor trauma.^[4]

Recent research has revealed that the PSTPIP1 protein interacts with pyrin, another protein involved in inflammation regulation. In PAPA syndrome, the mutated PSTPIP1 protein shows increased binding to pyrin. This abnormal interaction is thought to contribute to the excessive inflammation characteristic of the disease. Understanding this mechanism has helped researchers develop targeted treatments that address specific aspects of the inflammatory pathway.^[3]

PAPA syndrome is considered a self-limiting disease, meaning that in some cases symptoms may improve over time without intervention. However, even though the disease may eventually stabilize or improve, it can lead to severe joint destruction and scarring before that happens, which is why appropriate treatment is important.^[3]