A new heterozygous TYK2 gene mutation: Case report and review of the literature.

Tyrosine kinase 2 (TYK2) deficiency is a rare primary immunodeficiency disease (PID). Patients carry TYK2 gene mutations and suffer from recurrent infections by intracellular pathogens, including mycobacteria. Delayed diagnosis often hinders timely and effective treatment, resulting in poor prognosis. In this study, we report a newly discovered TYK2 deficiency patient with recurrent pulmonary infections. The patient, a 27-year-old Chinese man with a history of tuberculosis, presented with recurrent cough, phlegm, and purulent sputum. Lung CT scan showed bronchiectasis with concomitant infection. Next-generation sequencing (NGS) identified Mycobacterium gordonae and Mycobacterium chelonae in lung, along with heterozygous c.997G>A&c.10C>T (p.V333M&p.R4C) mutation in TYK2. Further pathogenicity prediction analysis via dbNSFP (v5.1a) suggested the potential pathogenicity of this genetic variant. Additionally, TYK2 mRNA expression in peripheral blood mononuclear cells (PBMCs) also decreased significantly. Following anti-infective treatment, the patient improved and was discharged with regular human immunoglobulin infusion. However, the patient unfortunately succumbed to disease exacerbation in October 2021, 15 months after diagnosis. Furthermore, a literature review was conducted on cases of TYK2 deficiency. Previous studies have identified 24 mutation sites within TYK2 gene, which impair immune function and lead to early-onset recurrent infections. These mutations contribute to clinical heterogeneity, with the most common manifestation being recurrent infections by opportunistic pathogens, particularly mycobacteria. Our discovery of a novel TYK2 mutation expands the gene's mutation spectrum. Analyzing the characteristics of reported cases enhances understanding of TYK2 deficiency's clinical manifestations and facilitates early diagnosis of this rare condition.