Molecular basis of immunoglobulin heavy constant G4 gene (IGHG4)-related low serum IgG4 subclasses in Down syndrome

Objectives: To investigate the molecular mechanism of low serum IgG4 at the level of the immunoglobulin heavy constant G4 gene (IGHG4). Patients with Down syndrome (DS) are more likely to exhibit immunological abnormalities that predispose them to infection. Among other anomalies, individuals with DS have altered serum concentrations of some subclasses of immunoglobulin G (IgG), particularly the IgG4 subclasses. Methods: In this prospective study, quantitative real-time polymerase chain reaction (qPCR) was carried out from December 2017 to June 2019 in the University Hospital of Saint-Etienne, Saint-Etienne, France to measure the number of IGHG4 copies and to compare those outcomes with a reference gene (36B4). An IGHG4/36B4 ratio was considered normal when between 0.8 and 1.2. Forty-four DS patients, comprising 23 DS patients carrying severe low serum IgG4 and 21 DS patients with normal serum IgG4 (level >0.1 g/L). The patient group was compared with 38 healthy donors (controls) without DS. Results: The heavy chain gene IGHG4 heterozygous deletion was found in 16 (69.57%) DS patients with low serum IgG4 versus in 2 (9.52%) DS with normal serum IgG4 (p=0.0001). In the control group, deletion was found in 5.26% (2/38) of the sample. Conclusion: The heavy chain gene IGHG4 haploinsufficiency is highly correlated with low serum IgG4 in our population with DS, but other relevant factors must be assessed in future work.