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Updated: Sep 29, 2026
An Allele-specific Gene Expression Assay to Test the Functional Basis of Genetic Associations
Published on: November 3, 2010
Gene frequency and partial protein characterization of an allelic variant of mannan binding protein associated with
P Garred1, S Thiel, H O Madsen
1Department of Clinical Immunology, Rigshospitalet, Copenhagen, Denmark.
Abstract:
Low plasma concentration of mannan binding protein (MBP) has been shown to be the basis for a common opsonic deficiency and suggested to be caused by a single nucleotide substitution at base 230 of exon 1 in the MBP gene. This substitution causes a replacement of glycine (codon GGC) with aspartic acid (codon GAC). Of 123 healthy Danish individuals investigated by polymerase chain reaction performed on exon 1, followed by restriction fragment length polymorphism or allospecific probing, 93 were homozygous (75.6%) for GGC, 28 heterozygous (22.8%), and two homozygous for GAC (1.6%). The gene frequency of the GAC allele was found to be 0.13. DNA sequencing of the cloned exon 1 from one GAC homozygous individual revealed no other substitution. The median MBP concentration in the group containing the GAC allele was 6.4 times lower than in the GGC homozygous group (195 and 1234 micrograms/l respectively). However, the range in plasma concentrations of MBP was wide and overlapping between the groups. MBP protein was detected in both the GAC homozygotes (9 and 387 micrograms/l). Furthermore, no difference in relative mass and biological activity (mannan binding) was found when sera containing the two forms of MBP were investigated. Accordingly, it can be concluded that the GAC allele is able to produce a functional MBP protein which may be detected in serum at low concentrations.
Insights
A common genetic variant in the mannan binding protein (MBP) gene, replacing glycine with aspartic acid, leads to significantly lower MBP levels. However, this functional MBP protein is still detected in serum.
Area of Science:
- Immunology
- Genetics
- Biochemistry
Background:
- Low plasma concentrations of mannan binding protein (MBP) are linked to opsonic deficiency.
- A specific nucleotide substitution in the MBP gene (GGC to GAC at base 230, exon 1) is implicated.
- This genetic variation may explain reduced MBP levels in some individuals.
Purpose of the Study:
- To investigate the impact of the GAC allele in the mannan binding protein (MBP) gene on MBP plasma concentration.
- To determine if the MBP protein produced by the GAC allele is functional.
- To assess the prevalence of the GAC allele in a healthy Danish population.
Main Methods:
- Polymerase chain reaction (PCR) analysis of exon 1 of the MBP gene.
- Restriction fragment length polymorphism (RFLP) and allospecific probing for genotyping.
- DNA sequencing of cloned exon 1.
- Measurement of plasma MBP concentrations.
- Assessment of MBP relative mass and biological (mannan binding) activity.
Main Results:
- The GAC allele frequency was 0.13 in 123 healthy Danish individuals.
- Individuals with the GAC allele had a median MBP concentration 6.4 times lower than GGC homozygotes.
- MBP protein was detectable in GAC homozygotes, with no significant differences in relative mass or mannan binding activity compared to GGC homozygotes.
Conclusions:
- The GAC allele of the mannan binding protein (MBP) gene results in lower serum MBP concentrations.
- Despite lower concentrations, the MBP produced by the GAC allele is functional and retains biological activity.
- This genetic variation explains a common opsonic deficiency due to reduced, but functional, MBP levels.
