A Cohort Study on Deficiency of ADA2 from China

Deficiency of adenosine deaminase 2 (DADA2, OMIM 165,688) is an autosomal recessive autoinflammatory disease caused by biallelic loss-of-function variants in the ADA2 gene, formerly named CECR1 (cat eye syndrome chromosome region, candidate 1), located at chromosome 22q11.1 [1‐3]. DADA2 was first described in 2014 by two separate groups in individuals with polyarteritis nodosa (PAN) and recurrent strokes [1, 2, 4]. However, the clinical spectrum of the disease has expanded considerably since its initial description.

DADA2 typically presents in childhood with systemic inflammation, vasculitis, humoral immunodeficiency, and/or hematologic abnormalities [1, 2, 5‐13]. Inflammatory features of DADA2 can include recurrent fevers, mild to moderate anemia, and elevated inflammatory markers, such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) and musculoskeletal involvement (arthralgia/arthritis and/or myalgia/myositis). Vasculitis can involve in multiple organs, may manifest as cutaneous inflammation, early-onset ischemic (lacunar), and/or hemorrhagic strokes. Some patients may display humoral immunodeficiency of variable severity characterized by low immunoglobulin levels and increased risk of infection [1, 2, 5]. Hematologic symptoms are present in a subset of DADA2 patients and can include pure red cell aplasia (PRCA) that mimics Diamond-Blackfan anemia, lymphopenia, neutropenia, thrombocytopenia, or pancytopenia resulting in bone marrow failure (BMF) [6, 9, 14‐18]. More severe manifestations of disease include organ damage, such as ischemic injury to the intestine, kidney, and/or digits, hepatosplenomegaly, and strokes. Sequelae of strokes can include progressive central neurologic deficits, ataxia, dysarthria, cranial nerve palsies, and cognitive impairment, which contributes to significant morbidity of this disease [1, 2, 12, 13, 19‐23].

Adenosine deaminase 2 (ADA2) is an extracellular enzyme primarily secreted by myeloid cells [21, 24, 25]. ADA2 is a dimeric enzyme with four domains, including signal peptide, catalytic domain, putative receptor binding domain, and dimerization domain [11, 24, 26]. Biallelic variants in the ADA2 gene result in decreased levels of ADA2 enzyme, which can be used as a marker of pathogenicity [1, 2]. Missense variants are most common, but nonsense variants, insertions/deletions (indels), splice-site variants, copy number variations (CNV), and structural variations have been described [11, 21, 26‐29]

Early diagnosis and treatment initiation is essential for improving clinical outcomes and preventing more severe manifestation of disease. This study describes the clinical and genetic features in a Chinese cohort of thirty patients with DADA2 from seventeen centers in China. We discuss new clinical and genetic findings in this disease and summarize the current knowledge of DADA2 to help improve its recognition.

In this report, we describe the clinical features, laboratory findings, genotypes, and treatment responses in thirty Chinese patients with DADA2. This is the largest cohort study on DADA2 from China to date. The mortality of DADA2 in this cohort is 6.7%, which is in line with rates in other reported cohorts [35‐40]. Vasculitis with variable organ involvement and systemic inflammation was noted in majority of patients, and CRP appears to be a more sensitive index for inflammation compared to ESR.

DADA2 is increasingly being recognized as a monogenic etiology for PAN with systemic inflammation and vasculitis features and biallelic variants in ADA2 have been identified in ~ 25–31% of childhood PAN cases [1, 2, 8, 11, 21, 26, 28, 38‐42]. Genetic testing and/or ADA2 activity detection should be considered in all patients with recurrent fever accompanied unexplained elevated CRP and/or ESR, especially in those with livedo racemosa/reticularis and evidence of PAN-like vasculitis.

Though the prevalence of hypogammaglobinemia in DADA2 patients is high, immunodeficiency, with recurrent or severe infections, is relatively rare [1, 2, 6, 8, 10, 12, 35‐39]. There were seven patients (23.3%) with recurrent infections in this cohort, most of which had mild respiratory tract infections that resolved with minimal intervention. Interestingly, one patient (P28) developed recurrent fever and respiratory tract infection without and cutaneous or vascular symptoms. However, at 15 years old, he developed variable immunodeficiency with low levels of IgM, IgA, and IgG. Mild humoral immunodeficiency with low immunoglobulin levels appears to be a common clinical feature of DADA2, regardless of the initial phenotype [11, 28, 40]. This suggests that DADA2 screening should be considered in the differential diagnosis for patients with unexplained antibody deficiencies.

In some patients, the primary manifestation of DADA2 appears to be hematologic abnormalities, without the vasculitis or systemic inflammation that was first identified as a hallmark of this disease [6, 9]. BMF with variable cytopenia was observed in one patient who is without vasculitis and systemic inflammation in this cohorts. Mild to moderate anemia and leukopenia/neutropenia were noted in 73.0% and 20% of patients, respectively. The frequency of anemia in this cohort, which may be caused by chronic systemic inflammation, is consistent with that of recurrent fever (90.0%).

Most patients in this cohort were followed and characterized by rheumatologists. While the majority of features of this disease is rheumatologic in nature, it is possible that these findings were biased as symptoms including hematological abnormalities, humoral immunodeficiencies, and/or early stroke, which may be underrecognized by such providers [1, 2, 6, 8, 10, 12, 35‐39, 43]. Furthermore, there is likely ascertainment bias in the patients who are identified by rheumatology, as opposed to another specialty who may recognize other elements of this multifaceted disease. It is imperative that providers outside of rheumatology (i.e., hematologists, immunologists, and neurologists) also learn to recognize DADA2, as early diagnosis and treatment can change the narrative of this condition. To facilitate the evaluation of individuals with possible DADA2, we propose the following schema for evaluation (Fig. 2). Diagnostic testing for DADA2 is recommended for patients with at least one of following manifestations: unexplained reason of systemic inflammation, vasculitis, humoral immunodeficiency, or severe hematologic abnormalities, especially in children. In the future, we hope that consensus clinical guidelines will formalize further steps in the evaluation, monitoring and treatment of individuals with or suspected to have DADA2.

In total, thirty-nine unique, deleterious variants in ADA2 were detected in this cohort. Among these variants, six were novel (Table 2), expanding the spectrum of known pathogenic variants in ADA2. These novel variants may represent founder variants within the Chinese population. However, these patients with novel variant had classic features of DADA2 without any unique phenotype (Table S1). Variants detected in this cohort encompassed each coding exon of the ADA2 gene and involved all four domains of the ADA2 protein. We did not observe any hot spots for variants in this cohort. The most common variant is the frameshift variant p.T33Nfs*29, which was identified in four unrelated patients. Several known variants were detected in this cohort including: p.G47R which is frequently seen in Georgian Jewish and Turkish cohorts, p.R169Q which is mainly noted in individuals from the Netherlands, Belgium, and Finland, and the p.T360A which has previously been reported in those from Italy [2, 8, 36, 44]. Further research may be able to classify where these variants first emerged and how specific they truly are to specific ancestral populations.

In terms of treatment, patients in this cohort showed no response to glucocorticoids, NSAIDs, IVIG, and traditional DMARDs. TNFi was used in twenty-three patients with active disease and was found to effectively control fever episodes and vasculitis. In addition, no patients treated with TNFi develop strokes or disease relapse during the follow-up period which ranged from 5 to 36 months. Three patients did respond to thalidomide therapy. HSCT is an alternative choice for patients with severe hematologic manifestation of disease or disease that is refractory to TNFi. The two patients who received HSCT in this cohort successfully achieved remission and are doing well. For these patients, HSCT not only rescued the hematological and immunological phenotypes but also appears to have been curative for their vascular phenotype [9, 10, 39, 45‐48].