C1S Gene

The C1S gene (Complement C1s) is a protein coding gene located on chromosome 12p13.31. The C1S gene encodes a serine protease that is a major component of the human complement subcomponent C1. C1s associates with the other two complement components C1r and C1q to form the first component of the serum complement system. Defects in this gene are the cause of selective C1s deficiency.

The complement system is crucial for the defense against pathogens and the elimination of dying cells or immune complexes. Therefore, clinical indications of possible total complement deficiency include recurrent mild or severe bacterial infections and autoimmune diseases. In a larger sequencing analysis of 212 patients with total or subtotal complement deficiency (El Sissy C et al. 2019), 107 different hemizygous, homozygous, or compound heterozygous pathogenic variants were detected in 14 complement genes. The following genes were affected: C1Qβ (n = 1), C1r (n = 3), C1s (n = 2), C2 (n = 12), C3 (n = 5), C5 (n = 12), C6 (n = 9), C7 (n = 17), C8 β (n = 7), C9 (n = 3), CFH (n = 7), CFI (n = 18), CFP (n = 10), CFD (n = 2). This study confirms the strong association of meningococcal infections by terminal pathway deficiency and highlights the risk of pneumococcal and autoimmune diseases in the classical and alternative pathways.

Diseases associated with C1S include:

• Ehlers-Danlos syndrome, periodontal type, 2
• Complement C1s deficiency

1. El Sissy C et al (2019) Clinical and Genetic Spectrum of a Large Cohort With Total and Sub-total Complement Deficiencies. Front Immunol 10:1936.
2. Higuchi Yet al. (2013) The identification of a novel splicing mutation in C1qB in a Japanese family with C1q deficiency: a case report. Pediat Rheum Online J 11: 41.
3. Ling GS et al (2018) C1q restrains autoimmunity and viral infection by regulating CD8+ T cell metabolism. Science 360: 558-563.