A hereditary homozygous deficiency of a component of the classical complement pathway may be associated with a number of clinical findings, including systemic lupus erythematosus (SLE).
Components of the classical complement pathway:
(1) C1q: gene located from 1p34.1-36.3
(2) C1r: gene located at 12p13
(3) C1s: gene located at 12p13
(4) C4: gene located at 6p
(5) C2: gene located at 6p
A hereditary deficiency of C1r usually occurs together with a deficiency of C1s.
Clinical associations of a hereditary homozygous deficiency of a component of the classical complement pathway:
(1) risk for developing clinical SLE (ranges from 93% for C1q to 10% for C2)
(2) glomerulonephritis (typically associated with the SLE)
(3) increased risk for infections
(4) premature cardiovascular disease
The SLE in these patients often starts in childhood and may be ANA negative.
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