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Molecular analysis of C3 allotypes in patients with nephritic factor
1Department of Medicine, University of Cambridge, UK.
Insights
The study found a significant association between the C3F allele and nephritic factor (NeF), a key factor in certain autoimmune kidney diseases. This finding confirms a link between C3F and NeF development, suggesting a potential genetic predisposition.
Area of Science:
- Immunology
- Genetics
- Nephrology
Background:
- Autoantibody nephritic factor (NeF) causes complement consumption and is linked to mesangiocapillary glomerulonephritis (MCGN II) and partial lipodystrophy (PLD).
- The third component of complement (C3) has two allotypes, C3S and C3F, with C3F being rarer and previously suggested to associate with autoimmune conditions.
- Analyzing C3 allotypes at the protein level is challenging in NeF patients due to low circulating C3 levels.
Purpose of the Study:
- To investigate the association between C3 polymorphisms (C3S/F and a linked polymorphism defined by MoAb HAV 4-1) and nephritic factor (NeF) in a cohort of patients.
- To utilize DNA-level analysis, specifically the amplification refractory mutation system (ARMS) and polymerase chain reaction (PCR), to overcome challenges in protein-level allotyping.
Main Methods:
- Employed the amplification refractory mutation system (ARMS), a polymerase chain reaction (PCR) modification, for DNA-level analysis of C3 polymorphisms.
- Analyzed two C3 polymorphisms, C3S/F and the linked MoAb HAV 4-1 polymorphism, in DNA samples from 26 patients with nephritic factor (NeF).
Main Results:
- Observed allele frequencies for C3S and C3F in NeF patients were 0.673 and 0.327, respectively, differing significantly from predicted values (0.79 and 0.2).
- Calculated a relative risk of 2.1 for NeF development associated with the presence of a C3F allele (chi-squared = 4.813, P < 0.05).
- Allele frequencies for the linked MoAb HAV 4-1 polymorphism showed no significant deviation from predictions based on C3S/F frequencies.
Conclusions:
- This study, the largest series of NeF patients to date, confirms a statistically significant association between the C3F allele and the development of nephritic factor (NeF).
- The findings suggest a potential genetic predisposition to NeF conferred by the C3F allele, warranting further investigation into the underlying mechanisms.
- The study highlights the utility of DNA-level analysis for C3 polymorphism typing in conditions with low complement levels.
Abstract:
The autoantibody nephritic factor (NeF) leads to complement consumption in vivo and is associated with type II mesangiocapillary glomerulonephritis (MCGN II) and partial lipodystrophy (PLD). The third component of complement (C3) exists in two common allotypic forms, C3S and C3F, distinguished at the protein level by electrophoresis. An increased frequency of the rarer C3F allele has been reported in several autoimmune conditions, including one small series of patients with NeF. However, patients with NeF have low levels of circulating C3 so that allotyping at the protein level is difficult. The molecular basis of the S/F polymorphism has recently been established: a single base change at the DNA level encodes a single amino acid substitution at the protein level. A second polymorphism, closely linked to the first, is defined by the MoAb HAV 4-1, and is also due to a single base change. These polymorphisms can therefore be analysed at the DNA level. We have used the amplification refractory mutation system (ARMS), a modification of the polymerase chain reaction (PCR), to analyse these two C3 polymorphisms at the DNA level in 26 patients with NeF. The allele frequencies of C3S and C3F were 0.673 and 0.327 (predicted values 0.79 and 0.2, chi 2 = 4.813, P < 0.05), giving a relative risk of 2.1 for the development of NeF conferred by the presence of a C3F allele. The HAV 4-1 allele frequencies were (-) 0.71 and (+) 0.29, i.e. not significantly different than predicted from the linked C3S/F allele frequencies. This is the largest series of patients with NeF yet published, and our data confirm an association between C3F and NeF. Possible mechanisms for for this link are discussed.