A novel HELLS mutation and its effect on B cell differentiation and immunoglobulin secretion

Immunodeficiency, centromeric instability and facial anomalies syndromes (ICF) are inborn errors of immunity (IEI) that arise due to B-cell abnormalities and DNA repair defects. ICF is a rare autosomal disorder characterised by reduced serum immunoglobulins leading to recurrent and potentially fatal infections. Patients with ICF4 harbour mutations in HELLS (lymphoid specific helicase), a regulator of pericentromeric methylation that also participates in nonhomologous end joining, which is required for generating diversity in antibody structure. The patients in this study are siblings exhibiting hypogammaglobulinemia, normal leukocyte panel and recurrent infections without presenting other classic ICF phenotypes. They also fail to initiate a immunological response to vaccinations. Whole exome sequencing identified two novel mutations in the HELLS coding region confirmed through Sanger sequencing. Patients were found to be compound heterozygous, with Arg224* inherited from the maternal allele and Ser674Asn from the paternal allele, confirming autosomal recessive inheritance. Genomic DNA was processed with isoschizomer restriction enzymes to analyse global methylation levels. No change in global methylation was observed, but multiradial chromosomes were present, consistent with a restricted effect on centromeric regions. To determine whether B-cells exhibited cell intrinsic defects in the production of antibody, patient B-cells were cultured in an in vitro plasma cell differentiation model. Patient B cells proliferated in a similar fashion to controls and generated phenotypically normal antibody-secreting cells. However, quantification of immunoglobulin production revealed reduced levels of switched isotypes. Additionally, B cell repertoire analysis demonstrated a normal distribution of IGHV gene usage but increased clonal expansion and reduced diversity in the patient memory B-cells. RNA sequencing was performed from samples obtained at four timepoints in the in vitro plasma cell differentiation model. Despite having some differences in gene expression, the differentially expressed genes were not vastly different. Thus, the novel mutations affect the molecular generation of a diverse antibody repertoire which results in immunodeficiency.