Mutations in the ADA2 gene cause reduced or absent ADA activity. The identified biallelic mutations of ADA2 accompanying the clinical phenotype contribute to the diagnosis; the plasma or serum ADA2 activity test is helpful[15]. Currently (up to 2020.04), 81 mutations have been registered at HGMD. However, genotype-phenotype correlations in DADA2 have been difficult to establish because of incomplete penetrance and variable clinical manifestations in patients with identical mutations[7]. The most frequent mutation G47R, which is thought to be more associated with the PAN-like phenotype, in Turkish and Georgian ancestry, and R169Q, which is thought to be associated with susceptibility to stroke, in the European population[4]. We identified 14 mutations, and 7 of which have not been reported among non-Chinese patients. These mutations are distributed throughout the gene, without a preferential location in specific domains. None of our patients carried the G47R mutation. Only one patient with the R169Q mutation was identified but without hematologic or neurological manifestations. Chinese DADA2 patients may have different mutation frequencies and genotypes. Furthermore, it remains to be elaborated whether DADA2 patients of different ethnicities share similar genotype-phenotype correlations.
There is increasing evidence that there are diverse presentations of ADA2 deficiency, even among patients with identical mutations[16]. Two subjects in our series had growth retardation compared to the 1-year-old standard, which has not been previously described in children with DADA2. The two cases were both early-onset, before 6 months, but these children were born with average weight and length and were cared for with adequate feeding. It remains to be established whether DADA2 affects the growth of children, or whether growth retardation is a new phenotype. Nonetheless, growth assessment should be taken into account when assessing pediatric patients. According to previous reports, the neurological manifestations of the central and peripheral nervous system represent permanent damage and dysfunction. Unilateral hearing loss can be observed during disease flares[17]. Three of our patients had neurologic involvement accompanied by ophthalmological dysfunction, such as eye external oblique and vision diminution, but all signs were reversed by suitable treatment. Early recognition of central nervous system dysfunction signs and appropriate therapy are essential for children with DADA2, because there might be a chance to avoid permanent sensory impairment.
None of our patients presented with hepatosplenomegaly or TBNK subset deficiency, which may have been influenced by the age of onset, race, and disease course. Other symptoms, such as recurrent fever, livedo reticularis, necrosis, and arthritis, are consistent with cases reported previously[3, 15]. According to the current comparison results, Chinese DADA2 patients have higher rates of fever and abdominal pain than non-Chinese DADA2 patients. Due to the small number of DADA2 in China, the number of cases at home and abroad is very different; thus, the reliability of the comparison results might be insufficient, and it is necessary to expand the sample size further.
Steroids are the mainstay of treatment, however, flares of inflammation and vasculitis upon tapering often occur. Immunosuppressive therapies, including azathioprine, cyclophosphamide, and methotrexate, have all been used, though little success in severe cases has been reported[3]. Except for P1, the patients received steroids together with immunosuppressants as initial treatment, and two achieved complete responses. This may suggest that immunosuppressive therapies can still provide good results in some cases. Anti-TNF agents are the currently preferred treatment based on the positive effect, in particular, in preventing stroke[6, 18]. Regardless, there is little mechanistic evidence for particular treatment choices. Recently, it was found that in the absence of ADA2, the formation of neutrophil extracellular traps could be triggered, resulting in the production of TNF-α[19]. This mechanism may explain the basis of anti-TNF therapy. Three of our patients received anti-TNF therapy; only one of them had a significantly elevated level of TNF-α before, and all showed a good response, with no stroke occurring after initiation. The only patient missing anti-TNF initiation, who had the lowest level of TNF-α, unfortunately had a stroke six months after onset. It is unclear whether the level of TNF-α matches the prognosis of DADA2.
In contrast to anti-TNF therapy, the anti-IL-6 agent tocilizumab seems to fail. Several studies have suggested that high serum IL-6 levels can be observed in DADA2 patients, and the use of tocilizumab has been reported to control recurrent fever[18, 20, 21]. Furthermore, it was reported that tocilizumab induced full remission in a DADA2 patient with an idiopathic multicentric Castleman disease-like phenotype[22]. Two of our patients with markedly increased serum IL-6 levels received tocilizumab. Unfortunately, the remission of both lasted for no more than half a year, and both cases relapsed. There are reports of three other patients with recurrent stroke while on tocilizumab[18, 23, 24]. The findings for our patients support that DADA2 patients without a Castleman disease-like phenotype are less responsive to anti-IL-6 therapy.
In some DADA2 patients, the hematological phenotype may be an accompanying feature., including bone marrow failure and pure red cell aplasia. The commonly used immunosuppressive drugs and TNF-α inhibitors may not reverse the hematological manifestation[7, 25], though HSCT has been considered a treatment, specifically for patients with this phenotype[26]. Indeed, one of our patients underwent HSCT. Before that, treatments with steroids, tocilizumab, and anticoagulant therapy were attempted (P7), but all proved unsuccessful. Although her HSCT was successful, this patient did not present with severe hematological defects such as pure red cell aplasia or bone marrow failure. In addition, anti-TNF therapy, which is preferential for DADA2 patients with vasculitis (as for P7), was missed. In general, the negative side effects of the HSCT should not be neglected, and the indication for the particular treatment choice should be considered twice.