Multifocal calcific periarthritis with distinctive clinical and radiological features in patients with CD73 deficiency

Abstract

Objectives

Arterial calcification due to deficiency of CD73 (ACDC) is a hereditary autosomal recessive ectopic mineralization syndrome caused by loss-of-function mutations in the ecto-5′-nucleotidase gene. Periarticular calcification has been reported but the clinical characterization of arthritis as well as the microstructure and chemical composition of periarticular calcifications and SF crystals has not been systematically investigated.

Methods

Eight ACDC patients underwent extensive rheumatological and radiological evaluation over a period of 11 years. Periarticular and synovial biopsies were obtained from four patients. Characterization of crystal composition was evaluated by compensated polarized light microscopy, Alizarin Red staining for synovial fluid along with X-ray diffraction and X-ray micro tomosynthesis scanner for periarticular calcification.

Results

Arthritis in ACDC patients has a clinical presentation of mixed erosive-degenerative joint changes with a median onset of articular symptoms at 17 years of age and progresses over time to the development of fixed deformities and functional limitations of small peripheral joints with, eventually, larger joint and distinct axial involvement later in life. We have identified calcium pyrophosphate and calcium hydroxyapatite (CHA) crystals in SF specimens and determined that CHA crystals are the principal component of periarticular calcifications.

Conclusion

This is the largest study in ACDC patients to describe erosive peripheral arthropathy and axial enthesopathic calcifications over a period of 11 years and the first to identify the composition of periarticular calcifications and SF crystals. ACDC should be considered among the genetic causes of early-onset OA, as musculoskeletal disease signs may often precede vascular symptoms.

Rheumatology key messages

• ACDC is an autosomal recessive disorder caused by mutations in the NT5E gene encoding CD73.

• ACDC patients develop severe arterial calcifications as well as inflammatory arthritis.

• Hydroxyapatite crystals were present in synovial fluid and periarticular calcifications in ACDC patients.

Introduction

Arterial calcification due to deficiency of CD73 (ACDC) is a hereditary autosomal recessive ectopic mineralization syndrome caused by loss-of-function mutations in the ecto-5′-nucleotidase (NT5E) gene encoding CD73 [1]. CD73 is a glycosylphosphatidylinositol-anchored glycoprotein with 5′-ribonucleotide phosphohydrolase enzyme activity converting extracellular adenosine monophosphate (AMP) to adenosine and inorganic phosphate (P_i) [2, 3]. CD73 deficiency is thought to promote tissue calcification indirectly through a reduction in extracellular adenosine and an increase in tissue non-specific alkaline phosphatase activity [1] that subsequently leads to an increase inorganic pyrophosphate (PP_i) hydrolysis decreasing the PP_i/P_i ratio [4]. PP_i is a powerful anti-mineralization factor and reduced levels promote spontaneous chondrogenesis and arterial calcification [5, 6].

Clinical presentation of soft tissue and vascular calcifications may also be observed in chronic kidney disease, atherosclerosis, diabetes mellitus, chronic warfarin use and advanced age [7, 8]. In ACDC, early-onset arterial calcification predominantly involves lower extremities, resulting in symptomatic claudication [1]. However, periarticular calcification of the joint capsule of hands, wrists, ankles and feet has also been reported in the original description of ACDC [1] and two case reports [9, 10]. Clinical characterization of arthritis associated with ACDC has not been systematically investigated. Here we report the clinical, radiological and pathological findings of the joint involvement in eight ACDC patients with the genetically defined disease. Results show that ACDC has a distinctive presentation with episodic inflammatory manifestations in the small joints along with changes showing erosive joint space narrowing and multiple calcifications in both joint capsules and periarticular regions as assessed by radiographic imaging. We observed arthritis frequently leads to the development of fixed deformities with functional limitations. Articular involvement precedes vascular symptoms in most patients, who often first consult a rheumatology specialist. Clinical and radiographic features of ACDC are distinct from other forms of arthritis, and when encountered, should prompt investigation for potential subclinical vascular lesions and NT5E genetic testing.

Methods

Eight patients with genetically confirmed ACDC disease were evaluated for the rheumatic manifestations and joint involvement patterns in this disease. All patients were enrolled on several National Heart, Lung and Blood Institute (NHLBI) protocols: ClinicalTrials.gov Identifier: NCT01585402 and NCT03538639. Protocols were reviewed and approved by the NHLBI Institutional Review Board, and all subjects provided written informed consent before participation according to the ICH E6 Guidelines for Good Clinical Practice [11].

Based on genetic testing, all patients carry either a homozygous nonsense mutation or a compound heterozygous mutation in the NT5E gene. The cohort had a complete medical history and physical exam as well as yearly rheumatological assessments. Laboratory evaluation was performed during multiple visits and included a comprehensive metabolic panel, thyroid panel (thyroid stimulating hormone, T3, T4), parathyroid hormone and serum/urine electrolytes (calcium, magnesium, phosphorus), alkaline phosphatase, ESR, high-sensitivity CRP, 25-hydroxy-vitamin D, 1,25-dihydroxy-vitamin D, iron studies and immunological studies such as RF, anti-CCP and ANA. All results were within normal ranges. All patients underwent extensive radiological evaluation with standard radiographs. In select patients CT and MRI were also performed.

Additionally, four patients underwent open joint biopsies to obtain SF, synovial tissue and periarticular calcification samples of an affected MCP joint. Surgical procedures were performed under local anaesthesia by an orthopaedic hand surgeon. Specimens obtained were fixed in formaldehyde, embedded in paraffin or flash frozen immediately after the biopsy. Histopathological analysis was performed on paraffin-embedded sections that were stained with haematoxylin–eosin and Von Kossa reagents.

The collected SF (30–50 µl) was analysed under compensated polarized light using a Nikon Eclipse E400 inverted microscope (Nikon, Tokyo, Japan) and reviewed by two independent rheumatologists. Alizarin Red staining was performed in all SF samples according to previously described protocols [12].

Tissues containing bulk calcification were studied with X-ray diffraction and X-ray microtomosynthesis to characterize their morphology and chemical composition. In two patients, the periarticular joint calcification was also compared with a calcified blood vessel that had been previously collected during a femoral endarterectomy procedure.

Results

Population demographics and risk factors

Eight patients from four different families (five siblings in one family) with genetically confirmed NT5E deficiency were evaluated at the NIH Clinical Center and followed yearly for a period ranging from 3 to 11 years. Based on their demographic information (Table 1), the median age at onset of arthritis symptoms was 17, while the vascular phenotype had a median onset age of 28.5 years with arthritic symptoms pre-dating vascular symptoms by 8–25 years in seven patients.

Table 1.

Baseline demographic and clinical characteristics of ACDC patients

	Patient ID
Characteristic	1	2	3	4	5	6	7	8
Age at enrolment, years	51	56	46	55	5	45	52	44
Sex	Female	Female	Female	Male	Male	Female	Male	Male
Race	Caucasian	Caucasian	Caucasian	Caucasian	Caucasian	Caucasian	Asian	Caucasian
Risk factors
Smoking history	9 pack/year	No	No	No	45 pack/year	No	No	2 pack/year
Hypertension grade 1	No	No	No	No	No	No	Yes	Yes
Dyslipidaemia	Yes	Yes	Yes	Yes	Yes	No	Yes	No
Impaired glucose tolerance	No	No	No	No	No	No	Yes	No
History of illicit drugs	No	No	No	No	Yes	No	No	No
Onset age of arthritic symptoms	18	16	16	17	17	35	17	19
Onset age of vascular symptoms	35	25	30	25	33	16	42	27
Initial location of arthritis/arthralgia	Hands	Hands	Hands	Hands	Hands	Hands	Hands and Feet	Hands
Duration of arthritic flares/days	14–21	7–10	7–14	7–10	7–10	7–14	5–6	4–5
Frequency of the flare (initial)	6–8/year	3–4/year	2–3/years	2–3/years	2–3/years	1–2/years	1–2/month	1–2/month

ACDC: arterial calcification due to deficiency of CD73.

Various cardiovascular risk factors (Table 1) were evaluated in all patients: three patients had a history of smoking, while one patient had a remote history of illicit drug use. Mild mixed dyslipidaemia was present in six patients but was well controlled by statin administration. Neither kidney disease nor diabetes was present and only one patient was diagnosed with impaired glucose tolerance (HbA1C 6.3). Grade 1 hypertension was present in two patients but was controlled with either a low dose of calcium channel blockers or beta-blockers. Screening for endocrine abnormalities including hyperparathyroidism, collagen vascular disease, haemochromatosis or gout was uniformly unremarkable.

Genetics findings

Mutations in the NT5E gene causing ACDC was identified in 2011 by our group in nine persons, and six of these patients were included in our study [1]. For two patients, NT5E screening was performed based on the clinical presentation and in one patient, a novel homozygous splice-site variation of intron 3 was identified [10], while a previously reported mutation was found in the other patient [13]. Five patients were homozygous for mutation c.662C>A (p.S221X), one patient was homozygous for c.1387C>T (p.R463X), one patient was homozygous for c.751+2T>C (exon 3 skipping) and one patient was compound heterozygous for c.662C>A and c.1609dupA (p.S221X, p.V537fsX7) in the NT5E gene (NM_002526.3).

Rheumatological manifestations and radiological data

Prior to genetic confirmation of ACDC, all patients were historically assigned a diagnosis of seronegative RA, JIA or crystal-induced arthropathy based upon clinical assessments. In no patients were crystals historically identified in SF. Episodic arthritic flares of DIP, PIP, MCP or MTP joints lasting up to 2 weeks were consistently reported by all study subjects, and by and large, these flares they were successfully treated with either NSAIDs or short-term corticosteroid therapy. One patient was started on methotrexate for several months before study enrolment without an observed decrease in the frequency of the arthritis flares. Over several decades, arthritic symptoms in this patient group evolved into a pattern of oligoarticular joint flares, which diminished in frequency over time. All the patients continue to have three or four symptomatic episodes of arthritis per year.

Patients had yearly rheumatology assessments. While most of the patients did not present with an acute arthritic attack during these visits, some patients were evaluated several days after an acute arthritic peak. During these intercritical evaluations, we identified MCP, PIP and DIP as the most common joints to be clinically involved in these attacks (Table 2 and Fig. 1). Typically, patients manifested clinically with severe osteoarthritic-appearing changes of the DIP, PIP, MCP or MTP joints without erythema or tenderness. Most patients had Heberden’s nodules, Swan neck or Boutonniere deformities and ulnar deviation of the hands. In some patients, squaring of the base of the first carpometacarpal joint was present along with a decreased range of motion of the wrist. Historical reports of episodic pain, stiffness, swelling and erythema were consistent among all patients.

Table 2.

Main radiographic features in ACDC patients

	Subluxations	Periarticular calcification	Joint space loss	Erosions	Osteophytes	Enthesophytes
Wrist	0/8	6/8	8/8	4/8	0/8	0/8
Hands
MCP	3/8	8/8	8/8	7/8	4/8	0/8
PIP	5/8	8/8	8/8	8/8	8/8	0/8
DIP	5/8	8/8	8/8	8/8	7/8	0/8
Ankle	0/8	4/8	4/8	0/8	0/8	3/8
Feet
MTP	1/8	6/8	6/8	3/8	5/8	2/8
PIP	6/8	6/8	7/8	6/8	7/8	1/8
DIP	5/8	6/8	7/8	6/8	6/8	0/8
Elbows	0/8	7/8	5/8	1/8	3/8	6/8
Shoulders	0/8	8/8	6/8	1/8	1/8	5/8
Knees	0/8	3/8	7/8	0/8	2/8	2/8
Hips	0/8	4/8	1/8	0/8	2/8	0/8

ACDC: arterial calcification due to deficiency of CD73.

Fig. 1.

ACDC characteristics hand deformities and X-ray changes over a period of 35 years

(A, B) Example images of a patient’s hands (A) with deforming joint changes and radiological changes in the same patient (B). (C) Hand X-ray performed in an ACDC patient at the age of 19 years with periarticular calcifications. (D) Hand X-ray in the same patient performed 35 years later showing the absence of initially observed calcifications (blue arrow) and the development of new calcifications (arrowhead) with osteoarthritic changes of ITP joints, osteophyte formations and more periarticular calcifications. ACDC: arterial calcification due to deficiency of CD73.

We reviewed radiography images obtained at yearly visits from all eight patients as well as historical images when available. In one patient, serial hand radiographs were available over a period of 35 years (Fig. 1C and D), revealing the progression of periarticular calcifications that developed in a manner atypical for degenerative osteophytes. Plain radiographs and MRI confirmed joint space loss, capsular and pericapsular calcification, homogeneous areas of soft tissue calcification and degenerative changes that at times were erosive (Table 2 and Fig. 1B–D). Small joint erosions were frequently at, or close to, enthesis organs and the collateral ligaments were often calcified. At times, productive and ‘healed’ bone changes were evident. Frank carpal erosions were occasionally seen but chondrocalcinosis of the wrist’s triangular fibrocartilage complex was not present. In several instances, joint MRI assessment was available to confirm the periarticular erosive process that was evident without associated bone marrow oedema.

Remarkably, systematic cataloguing of calcification in the hands and wrists in our cohort demonstrated that some calcifications progressed while others regressed over time. For example, over a 35-year period, calcifications in one patient’s hands increased in three sites, while they also diminished in three other distinctly separate anatomical locations (Figs 1C and 2D). This was observed in all patients to a varying extent.

Fig. 2.

CT of a patient with ACDC

Coronal three-dimensional reconstruction (A), coronal (B) and sagittal (C) reformat CT images of thoracic spine of a 62-year-old patient demonstrates osteophytes (blue arrow), syndesmophytes (arrowhead), narrow disc spaces and intervertebral disc calcification. ACDC: arterial calcification due to deficiency of CD73.

Radiographic axial involvement was also present in all patients with asymmetric thoracic and cervical spine (right side predominantly affected) while relatively sparing the lumbar spine. The radiological abnormalities included multiple osteophytes, syndesmophytes, intervertebral disk narrowing, intervertebral disk calcification, facet joint OA, anterior and posterior longitudinal ligament calcification and loss of vertebral height (Fig. 2 and Supplementary Table S1, available at Rheumatology online). All our ACDC patients present with various degree limitations in rotation and lateral flexion on the cervical spine’s physical examination. In patients where larger joints such as the hip, elbow and shoulder were affected, plain radiographs revealed periarticular calcifications and the presence of enthesophytes, and joint space loss and a decreased range of motion were detected during the physical exam of these specific joints.

Vascular manifestations

ACDC patients present with severe lower extremity arterial large vessel calcification (Supplementary Fig. S1, available at Rheumatology online) causing obstruction of the iliac, femoral, popliteal and tibial arteries and leading to the development of extensive collateral circulation. The onset of the arterial calcification process is unclear, but patients typically develop symptomatic claudication in their thirties. Four of our patients required femoral surgical bypass or stenting for worsening of claudication or impending ischaemia. We observed less extensive, asymptomatic calcification in other vascular beds. We analysed bypass surgery samples with X-ray microtomosynthesis and X-ray diffraction and found hydroxyapatite crystals were the main component of vascular calcifications. Histology on these samples showed arterial wall thickening and severe calcification with the fragmentation of the elastin fibres in the lamina elastica interna.

Laboratory and pathological findings

All patients were negative for RF, anti-CCP antibodies and ANA, and all exhibited normal inflammatory markers during intercritical periods with mild elevation during arthritic attacks as documented by local treating healthcare providers at the time of the acute arthritic flare. As described in the original ACDC report, serum calcium levels, phosphate and alkaline phosphatase were within normal limits.

Synovial tissue and fluid specimens were obtained from four ACDC patients. MCP joints adjacent to periarticular calcifications also presented with osteoarthritic changes (Fig. 1B and D) and all calcifications had been present for several years as observed by hand X-rays.

Histological examination of the synovial biopsy material revealed dense fibrocartilaginous and collagenous tissue containing spherical microcalcifications in two out of four biopsies and pyrophosphate-like calcifications in one biopsy. The other two joint biopsies had negative Von Kossa stain for calcifications. No inflammatory cells were noted but a histiocytic reaction associated with calcifications resembling tumoural calcinosis was observed on one biopsy (Fig. 3D and E) [14].

Fig. 3.

Polarized light microscopy of SF, Alizarin Red staining and synovial biopsy histological sections

(A, B) Birefringent crystals obtained from SF under compensated polarized light showing that the CPPD crystal is blue (A) when oriented parallel and yellow (B) when perpendicular to axis of slow vibration of polarized light by the compensator filter. (C) Alizarin Red staining of SF showing red–brown deposits that appear to be hydroxyapatite accumulations. (D, E) Synovial pathology images (haematoxylin–eosin staining). (D) Multiple spherical calcifications were found in the tenosynovial tissue with dense fibrosis. (E) Calcified debris lined by a histiocytic reaction showing tumour calcinosis-like features.

When analysing surgically harvested SF under direct polarized light, several rhomboid or square-shaped crystals weakly positive birefringent were identified in all samples, consistent with CPPD (Fig. 3A and B), but no identifiable urate crystals. Further, Alizarin Red staining of SF was strongly positive in all four patients, suggesting the presence of basic calcium phosphate or calcium hydroxyapatite crystals (Fig. 3C).

In contrast, bulk mineral deposits in the synovial tissue and a previously collected vascular calcification sample of the same patient were identified by X-ray diffraction study as mostly of hydroxyapatite crystals. Calcium pyrophosphate (CPP) crystals were not identified [15]. This did not exclude a minimal presence of other compounds in the specimens at levels below the detection threshold. Microscopically, in one patient with a receding periarticular lesion, the calcification in the lesion appeared to be collections of fragmented particles (Fig. 4A), while tightly packed fat globules of 40 µm size were seen in soft tissue (Fig. 4B). In another patient with a larger periarticular lesion, calcification appeared to be a concentration of fragmented particles of larger sizes (Fig. 4C) and fat globules were not seen. In a high-resolution extremity CT scan of the same patient at a level below the patella, the bulk of the arterial calcifications appear to consist of heterogeneous fragments and particles (Fig. 4D). Microscopically, the calcification fragments in the vascular tissue sample of this patient showed a porous internal structure (Fig. 4E). Description of the X-ray microtomosynthesis technique and X-ray diffraction analysis of periarticular and vascular calcification is available in a separate publication [15].

Fig. 4.

X-ray microtomography of periarticular calcification and CT scan of vascular calcification from ACDC patient

(A) An X-ray micro tomosynthesis scanner cross-section of the synovial tissue biopsy sample showing fragmented particles of calcification. (B) Another cross-sectional image of the same sample showing tightly packed fat globules in soft tissue. (C) X-ray microtomography section of the synovial tissue sample of another patient showing fragmented particles of calcification. (D) A high-resolution CT scan of the same patient showing the calcification in the right popliteal artery (red circle), in the form of heterogeneous fragments and particles. (E) At a microscopic scale, calcification fragments in a vascular tissue sample from the same patient showing a porous internal structure. ACDC: arterial calcification due to deficiency of CD73.

Discussion

Our longitudinal evaluation of eight ACDC patients from four unrelated families better characterizes the rheumatological features of ACDC. Although this condition was initially described as a syndrome associated with lower extremity claudication due to vascular calcifications, we found that the disease first and most often presents with rheumatological symptoms. Moreover, many symptoms were initially misattributed to a diagnosis of inflammatory arthritis (e.g. RA or JIA). By virtue of a detailed assessment, including documentation of the disease trajectory over several decades, we identified a pattern of arthritis distinct from both RA and typical crystal-induced arthropathy. The main distinguishing features include: (i) younger age of onset, with an initial presentation of episodic arthritis and evolving chronic symptoms; (ii) multiple periarticular calcifications that wax and wane over time in large and small joints; (iii) a predilection for erosive joint changes affecting hands and feet; (iv) cervical and thoracic spine involvement with osteoarthritic-appearing changes; and (v) non-inflammatory histology at the site of periarticular calcifications.

Although calcium pyrophosphate dihydrate crystal deposition disease (CPPD) is a pro-calcifying systemic disorder, we did not detect radiological features of typical chondrocalcinosis in our patients despite extensive longitudinal radiological assessments. In spite of initial diagnoses of RA, none of the patients progressed to develop RF, anti-CCP or other classic features of RA over time, nor any other signs of autoimmunity. Wrist erosion was an uncommon feature of the disease and when present, it was generally localized and not pancompartmental. Although calcium deposits were mainly observed within the joint capsule and periarticularly, the observed mixed features of both degenerative and erosive joint disease observed suggest that either primary or secondary intrinsic joint damage occurs. These distinctive features should help guide earlier ACDC diagnosis, and rheumatologists should become familiar with this condition as it is likely that they will be the first specialists to see these patients.

Spinal radiographic abnormalities were identified in all our patients, including osteophytes and syndesmophytes of the cervical and thoracic spine, but also narrow intervertebral disk spaces with a relatively normal lumbar spine (Fig. 2A–C). Seven patients had evidence of arthritis of cervical facet joints and four patients had similar findings in the thoracic spine. Ossification of the anterior and posterior longitudinal ligaments was observed in two patients in both cervical and thoracic regions (Fig. 2A and Supplementary Table S1, available at Rheumatology online). Such axial skeleton calcification, with ossification of spinal ligaments and entheses, was reminiscent of DISH [16, 17]. For some of our patients, more than four contiguous vertebral bodies were affected, yet they did not meet the Resnick and Niwayama and Utsinger criteria [18, 19] to be classified as DISH due to decrease in disc space height in the affected areas and the presence of large intervertebral disk calcifications. In fact, there exist overlapping similarities in the spine imaging of ACDC patients both with DISH and with AS like asymmetrical osteophytes and bridging syndesmophyte (more prominent on the right side of the cervical and thoracic spine) but without the involvement of sacroiliac joints.

Histologically, we found that the periarticular calcifications consisted of multiple spherical microscopic and often bulky calcifications deposited within the periarticular soft tissue and even within the joint space. These were similar to the known pathological descriptions of calcific tendinitis, periarthritis and tumoural calcinosis, all of which may be formed from calcium hydroxyapatite, thereby implicating a similar pathological process of ectopic calcification in ACDC.

Although this is the first comprehensive description of ACDC arthritic manifestations, remarkably similar descriptions of arthropathy accompanied by vascular calcifications have been reported as far back as 1914 [20]. In 1954, Sharp reported two siblings from a consanguineous family with MCP periarticular calcification, new bone formation and extensive calcification of lower extremity arteries [21]. An autopsy of one of the siblings 24 years later revealed severe dystrophic calcification and periarticular calcifications along with severe vascular calcification retrospectively suggesting that CD73 deficiency may have been a diagnostic consideration for this family (J. Ball, personal communication). ACDC was also reported in the published literature as ‘calcification of joints and arteries’ ([21, 22]. More recently, with the advent of a molecular description of ACDC, additional case reports of new NT5E mutations in Asian populations have been described [13, 22–24]. Notably, in most reports, patients with CD73 deficiency similarly presented with articular symptoms prior to the presence of vascular symptoms [13, 22, 23].

One of the eight patients reported herein presented with claudication prior to an arthritis diagnosis. This may be due to differences in CD73 expression as the patient harbours compound heterozygous mutations in NT5E, or it is possible that phenotypic variation may also be influenced by environmental, epigenetic or undetermined polygenetic factors.

The ability of CD73 deficiency to cause multifocal erosive and degenerative polyarthritis with periarticular calcifications may shed light on soft tissue calcification’s pathogenesis. Other than the polyarticular nature of the lesions, the inflammatory episodes reported by these patients resemble spontaneous acute calcific periarthritis, both in terms of presentation and in the resolution of attacks using NSAIDs.

Mechanistically, CD73 has ecto-5′-nucleotidase enzyme activity that converts extracellular AMP to adenosine and P_i and is expressed in physiological conditions by various cells, including stromal cells, lymphocytes (B cells and several T cells subsets) and endothelial cells [25]. CD73 expression is increased under hypoxic and inflammatory conditions [2, 26]. Loss-of-function CD73 mutations result in a decrease of extracellular adenosine and P_i and a compensatory upregulation of tissue non-specific alkaline phosphatase activity, resulting in an increase in P_i and promotion of calcifications formation. Another potential mechanism leading to arthritis in ACDC patients may result from a local deficiency of adenosine levels [27]. Adenosine can be released by multiple cells (endothelial, vascular smooth muscle cells and platelets) and several studies have suggested that it may function as an endogenous inhibitor of arterial calcification [28–31]. Treatment with activators of adenosine 2B receptors (A_2BR) reduces calcifications in vitro via a mechanistic target of rapamycin pathway mechanism and rapamycin or etidronate treatment can reduce calcification in induced pluripotent stem cell-derived mesenchymal stromal cells [4]. Surprisingly, MTX, which increases intracellular adenosine concentration [32], did not appear to effectively control the arthritic episodes in our patients. It is possible that in these patients, extracellular adenosine resulting from MTX activation may be insufficient to compensate for the physiological loss of adenosine from CD73 deficiency. Periarticular calcifications have also been reported in patients with generalized arterial calcification in infancy caused by deficiency of ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), an enzyme catalysing conversion of extracellular ATP to AMP and PP_i [33]. ENPP1 plays an essential role in increasing PP_i levels under normal physiological conditions and loss-of-function mutations in the ENPP1 gene result in a PP_i/P_i imbalance leading to vascular calcification and ectopic mineralization of multiple tissues including periarticular calcification [34, 35].

Another genetic condition affecting pyrophosphate metabolism is a gain-of-function mutation of the progressive ankylosis protein homologue (ANKH) gene, which encodes the PP_i transport regulator and causes several variants of familial autosomal dominant CPPD deposits [36]. ANKH gene mutations cause familial chondrocalcinosis 2 leading to high extracellular PP_i resulting in CPPD [37, 38]. The periarticular nature of periarticular calcifications in ACDC vs the intraarticular CPPD deposition in ANKH mutant patients highlights how mutations of various proteins controlling adenosine and phosphate metabolism may result in different clinical phenotypes.

In conclusion, we provide the first full description of the clinical, radiological and histological features of arthritis associated with ACDC. We describe the clinical presentation and evolution of the arthritis over a period of years, its radiological characteristics and, most importantly, these data provide the first identification of the composition and the types of crystals in the calcifications present in joint biopsies and vessels obtained from ACDC patients using state-of-the art imaging techniques such as microtomography. Further studies linking these findings to the mechanism that drives these changes will be needed. Recognition of the clinical and radiological features of CD73 deficiency by rheumatologists will lead to earlier diagnosis and help avoid misattribution of symptoms and inappropriate treatment.