C1q is the molecule that initiates activation of the classical pathway of the complement system. It does this by interaction with the Fc portion of immunoglobulin (IgM, IgG1, IgG2, or IgG3) when bound to antigen or interaction with other nonimmune recognition biologically relevant initiators (C-reactive protein, cardiolipin, DNA, bacterial lipid A, multiple microorganisms). C1q deficiency is extremely rare, with only 41 cases reported worldwide. There are two forms of C1q deficiency. One form produces no C1q, and the other form produces a dysfunctional C1q. Genetic analysis of the C1q-deficient patients has revealed mutations affecting each of its three chains. C1q deficiency can clinically present with an systemic lupus erythematosus/SLE-like syndrome with fever, rash, arthritis, and glomerulonephritis. C1q deficiency is the most likely of all complement deficiencies to be associated with an SLE-like illness (93% of patients). Of the 41 reported cases of C1q deficiency, 38 had associated SLE-like illness. Cutaneous lupus is most frequently seen (in 98%), while glomerulonephritis (in 39%) and cerebral involvement have been described to a lesser extent in individuals with C1q deficiency. As is the same in all the early component deficiencies, it is hypothesized that the SLE-like association is a result of the inability to appropriately clear immune complexes and apoptotic cells and loss of complement-dependent B cell tolerance in C1q-deficient individuals.
Complement-mediated phagocytosis of apoptotic blebs may also be impaired, allowing formation of antinuclear antibodies. Antinuclear antibodies are usually positive (in 75% of patients), while anti-DNA antibodies are negative. Extractable nuclear antigen (ENA) is usually detected (70%). Because C1q is the initiating factor for opsonization, C1q deficiency can also present with increased susceptibility to infection, especially from encapsulated bacteria. Many individuals with C1q deficiency have died in early childhood from recurrent bacterial infections, viral infections, or renal failure.
1. Pettigrew H D, et al. (2009). Clinical significance of complement deficiencies. Annals of the New York Academy of Sciences, 1173(1), 108-123.
2. Morgan B P, et al. (1991). Complement deficiency and disease. Immunology today, 12(9), 301-306.
3. Botto M. (1999). C1q knock-out mice for the study of complement deficiency in autoimmune disease. Experimental and clinical immunogenetics, 15(4), 231-234.