Background/Purpose: Arterial calcification due to deficiency of CD73 (ACDC) is a hereditary autosomal recessive ectopic mineralization syndrome caused by loss-of-function mutations in the 5′-nucleotidase Ecto (NT5E) gene that encodes for CD73also known as ecto-5′-nucleotidase. In ACDC patients, arterial calcification predominantly involves the lower extremities causing early onset of intermittent claudication. Periarticular calcification has been reported in the first reports of ACDC, but the clinical characterization of arthritis has not been systematically investigated. Here, we describe the musculoskeletal manifestations in eight ACDC patients and characterize the microstructure and chemical composition of periarticular calcifications and synovial fluid crystals.

Methods: Eight ACDC patients with either homozygous or compound heterozygous mutations in the NT5E gene were included in this study and underwent extensive rheumatological and radiological evaluation over a period of 11 years. Periarticular and synovial biopsies were obtained from four ACDC patients with pathological evaluation of synovial tissue and synovial fluid, as well as characterization of crystal composition by compensated polarized light microscopy and Alizarin red staining for synovial fluid along with x-ray diffraction and x-ray micro tomosynthesis for periarticular calcification.

Results: Arthritis in ACDC patients has a distinctive presentation with episodic inflammatory manifestations of small joints along with mixed erosive-degenerative joint changes, as shown by x-ray, CT and MRI with a median age of onset of arthritis symptoms at 17. Over several decades, arthritis in these patients frequently leads to the development of fixed deformities and functional limitations. Large joints also become involved later in life with bulky periarticular calcifications and presence of enthesophytes. X-rays of cervical and thoracic spine vertebrae show osteoarthritis, osteophytes, intervertebral disk calcification, and arthritis of the facet joints. Further, we have identified calcium pyrophosphate and hydroxyapatite crystals in synovial fluid collected in four ACDC patients. With these samples, we identified by x-ray diffraction that periarticular calcifications are mostly composed of hydroxyapatite crystals. Some patients have multiple spherical calcifications with fibrosis showing histology similar to calcific tendinitis or tumoral calcinosis-like lesions.

Conclusion: This is the largest study to describe the musculoskeletal manifestations of ACDC patients over a period of 11 years and is the first study to report the crystal composition of periarticular calcifications and the presence of synovial fluid crystals in these patients. Based on the findings in this study, the joint involvement is best defined as an erosive arthropathy that includes peripheral and axial enthesopathic calcifications along with periarticular calcifications. Joint biopsy samples analysis reveals that calcium hydroxyapatite crystals are the main component of periarticular calcifications and synovial fluid, although pyrophosphate crystals were also found in the synovial fluid.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/musculoskeletal-manifestations-in-patients-with-cd73-deficiency/