Identification of a Novel Homozygous <i>C1QB</i> Mutation in an Iranian Girl: Expanding the Clinical Spectrum of C1q Deficiency

C1q deficiency is a rare autosomal recessive disease associated with recurrent skin lesions, chronic infections and an increased risk of autoimmune disorders, particularly systemic lupus erythematosus (SLE) or SLE-like disorders. Additionally, it has been linked to chronic glomerulonephritis and renal failure. C1q is the initial subcomponent of the classical pathway of the complement system and serves as a crucial linking factor between innate and acquired immunity. The C1 complex comprises three proteins: C1q, C1r and C1s. C1q comprises three chains: the A, B and C. The C1QB gene encodes the B-chain polypeptide of the serum complement subcomponent C1q. We report the case of an Iranian girl from a consanguineous family who suffers from C1q deficiency, presenting with some SLE symptoms that have not previously been described in the literature. She presented with progressive weakness in walking, tissue injury, skin lesions and subjective cognitive complaints regarding concentration and memory. The proband exhibited mild asymmetric uptake in the pelvic region, resulting in a waddling gait. Additionally, she showed abnormal circulating homocysteine levels, skin abnormalities and early inflammatory arthritis symptoms associated with SLE. Whole-exome sequencing (WES) was performed on the proband. A novel homozygous likely pathogenic missense variant in the C1QB gene, NM_001378156.1:c.263G>A, was identified. The variant was confirmed by Sanger sequencing in the proband, her parents and her healthy sister. This case highlights the significance of identifying a novel mutation in the C1QB gene, which expands the clinical spectrum of C1q deficiency. This finding contributes to the broader understanding of the disease's phenotype, helping to refine diagnostic criteria, particularly in cases with atypical manifestations. Furthermore, identifying such mutations in consanguineous families aids in genetic counselling and early diagnosis, allowing for better clinical management and prevention strategies.