Falsely normal C4 in a case of acquired C1 esterase inhibitor deficiency.

A 59-year-old lady presented with recurrent angioedema without urticaria. The clinical history and examination were consistent with an acquired C1 esterase deficiency secondary to lymphoproliferative disease. Despite a low C1 esterase level, the C4 level assayed by nephelometry on our automated analyser was normal. Analysis using different nephelometric analysers revealed consistently low C4, despite consistent normal readings in our analyser. Further investigation revealed an IgM-kappa paraprotein that seemed to interfere with both this and haematology coagulation assays. Splenic marginal zone lymphoma was confirmed on bone marrow biopsy. Monoclonal paraproteins may interfere with nephelometric, turbidimetric and immunological assays in a non-antibody-specific manner and should be considered when there are unusual or unexpected results, particularly in a patient with lymphoproliferative disease.

Atypical haemolytic uraemic syndrome associated with a hybrid complement gene.

BACKGROUND: Sequence analysis of the regulators of complement activation (RCA) cluster of genes at chromosome position 1q32 shows evidence of several large genomic duplications. These duplications have resulted in a high degree of sequence identity between the gene for factor H (CFH) and the genes for the five factor H-related proteins (CFHL1-5; aliases CFHR1-5). CFH mutations have been described in association with atypical haemolytic uraemic syndrome (aHUS). The majority of the mutations are missense changes that cluster in the C-terminal region and impair the ability of factor H to regulate surface-bound C3b. Some have arisen as a result of gene conversion between CFH and CFHL1. In this study we tested the hypothesis that nonallelic homologous recombination between low-copy repeats in the RCA cluster could result in the formation of a hybrid CFH/CFHL1 gene that predisposes to the development of aHUS. METHODS AND FINDINGS: In a family with many cases of aHUS that segregate with the RCA cluster we used cDNA analysis, gene sequencing, and Southern blotting to show that affected individuals carry a heterozygous CFH/CFHL1 hybrid gene in which exons 1-21 are derived from CFH and exons 22/23 from CFHL1. This hybrid encodes a protein product identical to a functionally significant CFH mutant (c.3572C>T, S1191L and c.3590T>C, V1197A) that has been previously described in association with aHUS. CONCLUSIONS: CFH mutation screening is recommended in all aHUS patients prior to renal transplantation because of the high risk of disease recurrence post-transplant in those known to have a CFH mutation. Because of our finding it will be necessary to implement additional screening strategies that will detect a hybrid CFH/CFHL1 gene.