Molecular basis of hereditary C3 deficiency

Area of Science:

• Immunology
• Human Genetics
• Molecular Biology

Background:

• Hereditary complement deficiencies can lead to increased susceptibility to infections.
• Complement component C3 is central to all three pathways of complement activation.
• Deficiency of C3 is extremely rare and severely impairs immune function.

Purpose of the Study:

• To investigate the molecular basis of hereditary complement component C3 deficiency in a 10-year-old boy.
• To identify the specific genetic mutation responsible for the observed C3 deficiency.

Main Methods:

• Radioimmunoassay (RIA) to detect C3 levels in patient serum.
• Segregation analysis of C3 allotypes within the family.
• DNA sequencing of C3 gene exons and intervening sequences.
• Polymerase chain reaction (PCR) amplification of specific C3 gene regions from cDNA and genomic DNA.
• Analysis of mRNA splicing patterns.

Main Results:

• The patient exhibited undetectable serum C3 levels, consistent with a null gene.
• Sequence analysis revealed no coding sequence abnormalities in the C3 null gene.
• A GT-AT mutation at the 5' donor splice site of intervening sequence 18 was identified.
• A 61-bp deletion in exon 18, caused by aberrant splicing, was detected in the patient's C3 mRNA.
• This deletion resulted in a frameshift mutation and premature stop codon.
• Parents were heterozygous for the identified splice site mutation.

Conclusions:

• A novel splice site mutation in the C3 gene is the cause of hereditary C3 deficiency in this patient.
• The identified mutation leads to aberrant mRNA splicing, frameshift, and a premature stop codon, resulting in complete C3 deficiency.
• This finding expands the known spectrum of genetic defects causing complement deficiencies and highlights the importance of splice site integrity.