Molecular diagnosis of severe combined immunodeficiency - Identification of IL2RG, JAK3, IL7R, DCLRE1C and RAG2 mutations in a cohort of Chinese and Southeast Asian children

Abstract

OBJECTIVES: Severe combined immunodeficiencies (SCID) is a group of rare inherited disorders with profound defects in T-cell and B-cell immunity. At present, over ten SCID genes were identified, accounting for T-B+SCID (IL2RG, JAK3, IL7R, CD45, CORO1A and CD3d/CD3e/CD3z) and T-B-SCID (ADA, PNP, RAG1, RAG2, LIG4, DCLRE1C, Cernunnos/XLF, DNA-PKcs and AK2). IL2RG mutations in X-linked SCID, the commonest form, were reported in mainland China and Taiwan, but other types were not characterized in Chinese and Southeast Asians. From 2005-2009, our unit developed molecular diagnostics for IL2RG, JAK3, IL7R, RAG1, RAG2, DCLRE1C, AK2 and ZAP70. Twenty-seven referrals for genetic diagnosis of SCID were received. This study aims to review the clinical features and genetic diagnoses of these patients. METHODS: Genetic studies were performed by candidate gene approach, based on patient’s gender, immune-phenotype and inheritance pattern. Mutations were identified by direct sequencing of the coding regions and splice sites of respective genes. MAIN RESULTS: Our cohort included Chinese (n=24), Malay (n=1), Korean- American (n=1) and Arabic (n=1) patients. The median age of initiating immunological investigation was 4 months. These infants suffered from major infections including regional (n=5) or disseminated BCG disease (n=3), disseminated CMV disease (n=1), candidemia (n=4), pulmonary aspergillosis (n=1), and bacteremia (n=4). Fourteen boys had T-B+NK-SCID, thirteen of whom had IL2RG mutations including 4 missense, 2 nonsense, 3 frameshift and 4 splice-junction mutations. Two girls had T-B+NK+ phenotype, and one of them had compound heterozygous IL7R mutations. Three patients had T-B+NKphenotype, and one of them had homozygous JAK3 splice-junction mutation. Six patients had Omenn syndrome or T-B-NK+ phenotype; 2 of whom had compound heterozygous mutations in RAG2 and DCLRE1C respectively. Reticular dysgenesis was suspected in a newborn baby presenting with severe sepsis, pancytopenia and T-B-NK- phenotype, but AK2 mutation was not found. A girl had severe CD8 lymphopenia, but ZAP70 mutation was not identified. Fifteen patients died while five were lost to follow-up. Four patients had successful match-unrelated hematopoietic stem cell transplantation (HSCT). A patient with T-B+NK-SCID received 1-antigen mismatch haploidentical transplant but unfortunately died of complications on D+59. CONCLUSION: Disseminated BCG and fungal infections should be recognized as clinical indicators of underlying immunodeficiency in infants, and detailed immunological studies are warranted. SCID is a heterogeneous genetic disorder. Identification of genotype facilitates diagnostic confirmation and genetic counseling. Moreover, HSCT protocols can be tailored to specific SCID-genotype for better outcomes. Early diagnosis, timely referral and securing resources for HSCT are urgently needed to improve prognosis of these patients.