CACP syndrome is a rare autosomal recessive disorder characterized by the triad of camptodactyly, non-inflammatory arthropathy with synovial hyperplasia, and coxa vara. Occasionally, non-inflammatory pericarditis and pleural effusion may also occur. This syndrome is likely underdiagnosed due to its rarity. Epidemiological information is limited to isolated case reports or small patient series, with the largest reported cohort including 35 patients.

The genetic cause of CACP syndrome is associated with mutations in the PRG4 gene, located on chromosome 1q31.1. While clinical signs (camptodactyly, non-inflammatory arthropathy, and coxa vara) and radiological findings suggest the diagnosis, genetic testing confirms it by identifying pathogenic biallelic mutations in PRG4.

To date, twenty-two mutations have been identified, all leading to premature stop codons and the absence of functional lubricin. However, the exact pathophysiology of CACP syndrome remains incompletely understood.

Clinical manifestations of CACP syndrome can vary, even within the same family. The progressive and slow onset can initially present as an incomplete clinical picture. However, camptodactyly (85- 100%) and arthropathy (100%) are constant features.

Although genetically homogeneous, CACP exhibits significant intra- and interfamilial phenotypic variability due to secondary genetic factors, environmental modifiers, and complex molecular mechanisms.

Camptodactyly is symmetrical, with variable distribution. It may affect fingers or toes and can be congenital or develop during childhood.

Arthropathy is symmetrical, primarily involving large joints (wrists, knees, ankles, elbows, and hips).

Coxa vara is present in 50-90% of cases, is progressive, and tends to worsen with age. Spinal abnormalities such as lordosis, scoliosis, and kyphosis are possible, though the cervical spine is generally spared.

The articular manifestations of CACP syndrome may mimic juvenile idiopathic arthritis (JIA), and patients are often initially misdiagnosed and treated inappropriately.

Joints appear swollen due to non-inflammatory synovial effusion and synovial thickening. They develop contractures, functional limitations, and sometimes musculoskeletal pain.

Non-inflammatory pericarditis is reported in 30% of published cases, with variable clinical courses that may require surgical intervention in cases of constrictive pericarditis.

The routine pathway of assessments and follow-up for patients with CACP syndrome includes an initial detailed evaluation and regular monitoring. Following the diagnosis, which is based on clinical history, imaging studies, and genetic confirmation of PRG4 mutations, patients undergo periodic clinical visits, generally scheduled every six months. During these visits, the progression of the disease, articular symptoms (e.g., camptodactyly, mobility limitations), and possible extraarticular complications, such as pericarditis, are assessed.

Radiological (e.g., X-rays, MRI) and laboratory assessments, however, can be spaced out over longer intervals compared to the schedule of clinical visits, typically every 1-2 years, unless specific indications arise. Nonetheless, these examinations may be requested based on contingent clinical needs, such as a sudden worsening of symptoms or suspicion of complications. This flexible approach helps to balance thorough disease monitoring with minimizing the burden on patients, while ensuring personalized and timely management of the condition.

At present, there is no specific pharmacological treatment for CACP. Management is primarily symptomatic and aimed at preventing joint deformities and extra-articular complications.

Currently, no experimental therapies are available for CACP syndrome, but future research could explore gene therapy, regenerative medicine, and biologics.

This study, involving pediatric and pediatric rheumatology centers across Italy and Europe, aims to collect epidemiological, clinical, and therapeutic data from a large cohort of patients. Its goals include better defining the disease's characteristics, understanding its natural history, and evaluating different therapeutic approaches and their efficacy. The study will also analyze potential genotypephenotype correlations.

Inclusion Criteria:

• Patients with clinical diagnosis and genetic confirmation of CACP syndrome.
• Patients diagnosed during pediatric age (<18 years).
• Time frame: Patients diagnosed with CACP between January 2005 and January 1, 2026.
• Informed consent obtained from parents or legal guardians.

Exclusion Criteria:

• Patients without genetic confirmation of the diagnosis.
• Lack of informed consent from parents or legal guardians.
• Patients diagnosed before January 1, 2005, or after January 1, 2026.

Study population will be selected at the participating centers according to inclusion criteria.