A novel variant associated with refractory lymphopenia in systemic lupus erythematosus: Dedicator of cytokinesis 2 gene c.2883_2886dup

Genetic and phenotypic heterogeneity present challenges for the diagnosis and personalized treatment of systemic lupus erythematosus (SLE).1 Leukopenia, a common clinical presentation of SLE, can vary in severity from mild to severe. Current thinking often attributes the pathogenesis of leukopenia to immune-mediated damage, infections, or iatrogenic factors (azathioprine and cyclophosphamide).2 However, the gene defects associated with leukopenia have only gained limited attention. Here, we report a novel dedicator of cytokinesis 2 (DOCK2) gene mutation that played a nonautoimmune pathogenic role in a woman with SLE who developed refractory lymphopenia. The 37-year-old female patient presented with prolonged thrombocytopenia, hypocomplementemia, and positive titers of antinuclear, antinucleosome, anti-double-stranded DNA (dsDNA), and anticardiolipin (IgG) antibodies. She was diagnosed with SLE 10 years ago according to the American College of Rheumatology 1997 criteria. Clinical evaluation revealed no damage to other systems. Over the course of her illness, she received steroid and hydroxychloroquine therapies and underwent regular reviews. The patient was hospitalized multiple times because of lupus activity, respiratory infection, or cryptococcal meningitis. Upon admission in September 2022, the patient was hospitalized due to lupus activity. Autoantibody testing revealed positive antinuclear antibody (1:320), anticardiolipin IgM, and anti-β2-glycoprotein IgG and IgM; antineutrophil cytoplasmic antibodies and other antibodies were negative. The C3 component levels decreased to 0.72 g/L (reference: 0.90–1.80 g/L). The systemic impairments of SLE predominantly affected the hematological system, with decreased white blood cell count (2.66 × 109/L, reference: [3.50–9.50] × 109/L), red blood cell count (2.08 × 1012/L, reference: [3.80–5.10] × 1012/L), and platelet count (115 × 109/L, reference: [125–350] × 109/L). The patient exhibited autoimmune hemolysis with a positive direct Coombs' test, a hemoglobin reduction (77 g/L, reference: 115–150 g/L), an increase in direct bilirubin (5.2 µmol/L, reference: 0–4.0 µmol/L) and indirect bilirubin (24.4 µmol/L, reference: 1.0–17.0 µmol/L), and splenomegaly. The relevant examination results revealed no involvement of other organs, including the skin, joints, kidneys, lungs, heart, or nervous system. The SLE disease activity score was 3 (hypocomplementemia and leukopenia), falling within the range of mild activity. The patient was treated with tacrolimus (2 mg/day), hydroxychloroquine (400 mg/day), methylprednisolone (40 mg/day), and other symptomatic medications. In the disease course of this case, the levels of the C3 component, red blood cells, platelets, and white blood cells (neutrophils and monocytes) reached a nadir in August 2022 and showed significant improvement by November 2022 (Supporting Information: Figure S1A–F). However, her lymphocytes showed limited response to hormone therapy combined with immunosuppressive therapy, a rarity in SLE. Throughout this period, lymphocyte levels consistently remained below the normal reference value ([1.1–3.2] × 109/L) and decreased to 0.61 × 109/L in November 2022 (Supporting Information: Figure S1G). Lymphopenia has been persistent since her SLE diagnosis 10 years ago (0.6 × 109/L, reference: [1.1–3.2] × 109/L). Further examination revealed that the absolute number of T cells (178.7/µL, reference: 603.0–2990.0/µL), B cells (0.4/µL, reference: 107.0–698.0/µL), and natural killer cells (40.7/µL, reference: 95.0–640.0/µL) were well below normal reference values. Her performance in the last 2 years showed that lymphocyte levels did not appear to be associated with SLE disease activity. Levels of serum immunoglobulins including IgG (10.3 g/L, reference: 7.0–16.0 g/L), IgA (1.8 g/L, reference: 0.7–4.0 g/L), IgM (1.6 g/L, reference: 0.4–2.3 g/L), and IgE (34.8 IU/mL, reference: 0–100.0 IU/mL) were normal, which did not support the possibility of autoantibody-mediated damage or lupus activity. Bone marrow examination revealed no significant abnormalities. Tacrolimus and hydroxychloroquine were the main immunosuppressive strategies used for treatment. However, both drugs rarely cause leukopenia. Consequently, we inferred the presence of other pathogenic factors, such as congenital immunodeficiency, which are distinguished from autoimmune damage, infections, and medication-related factors. Subsequently, whole-exome sequencing was performed. The heterozygous insertion in DOCK2 (NM_004946.2: c.2883_2886dup, p.Ser963Glnfs*3) was identified as a novel variant of SLE. The finding was validated by Sanger sequencing (Figure 1A) using the primers listed in Supporting Information: Table S1. The DOCK2 c.2883_2886dup resulted in the introduction of a premature termination codon at codon 965, leading to the loss of critical binding domains and regions in DOCK2 (Figure 1B). This alteration significantly impacted the structure and function of DOCK2. Notably, his variant is not documented in population databases and is considered likely pathogenic (PVS1 + PM2_Supporting) according to the American College of Medical Genetics and Genomics criteria. Lymphopenia and abnormalities in T, B, and natural killer cells are typical characteristics of pathogenic DOCK2 mutations, including c.3724_3725dup (p.Leu1244fs) and c.2229_2230dup (p.Phe744fs).3 DOCK2 is specifically expressed in hematopoietic cells and can explicitly activate the small G protein Rac1 through its Dock homology region-2 domain (residues 1196–1620).4 This activation can induce the exchange of GDP and GTP and regulates the proliferation, differentiation, and activation of T and B lymphocytes.5, 6 The DOCK2 c.2883_2886dup (p.Ser963Glnfs*3) mutation results in the destruction of the Dock homology region-2 domain, similar to the c.3724_3725dup (p.Leu1244fs) and c.2229_2230dup (p.Phe744fs) mutations. Combined immunodeficiencies have been observed in patients with homozygous DOCK2 mutations or heterozygous pathogenic variants.3, 7 Large-scale data analysis has shown that variants causing disease exclusively in a homozygous state (recessive inheritance) tend to exhibit modest heterozygous effects on the disease. This finding indicates a weaker but still significant loss-of-function effect in heterozygous individuals.8 This inborn error in immunity derived from the heterozygous DOCK2 c.2883_2886dup (p.Ser963Glnfs*3) manifested as a superposition of SLE-associated damage. The role of DOCK2 mutations in the development of lymphopenia warrants further investigation. Upon excluding infections, medication-related factors, and hematological disorders, the clinical characteristics of lymphopenia in this case were consistent with the biological effects of DOCK2 deficiency. This case of DOCK2 mutation showed persistent lymphopenia that was not correlated with lupus activity and had no positive response to hormone therapy combined with immunosuppressive therapy. Notably, congenital immunodeficiency increases the risk of infection. In light of these findings, we deemed it necessary to adopt a milder immunosuppressive regimen for this patient with SLE. Additionally, an even stronger emphasis should be placed on infection prevention and risk assessment, especially when combined with immunosuppressive therapy. Progression needs to be continuously monitored for benefit–risk assessment of hematopoietic stem cell transplantation. Minghua Huang, Yaqi Zhao, Wei Xu, and Zhenzhen Ma collected data and wrote the manuscript. Yanyan Bai and Qingrui Yang designed, supervised, and edited the manuscript. This work was supported by grants from the Natural Science Foundation of Shandong Province (No. ZR2022MH016) and the National Natural Science Foundation of China (Youth fund project, No. 82201994). The authors declare no conflict of interest. Written consent to publish was obtained from the patient. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.