Molecular basis of subtotal complement C6 deficiency. A carboxy-terminally truncated but functionally active C6

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Subtotal complement C6 deficiency is characterized by a shortened C6 molecule (14% shorter) present at low levels (1-2% of normal).
• This altered C6 molecule retains bactericidal activity but lacks a specific epitope mapped to its carboxy-terminal end.

Purpose of the Study:

• To identify the molecular basis of subtotal complement C6 deficiency.
• To investigate the potential carboxy-terminal truncation of the abnormal C6 molecule.

Main Methods:

• Analysis of C6 cDNA fragments expressed as fusion proteins.
• Sequencing of PCR-amplified products from the carboxy-terminal region of the C6 coding sequence.
• Investigation of splice donor sites and downstream codons.

Main Results:

• A mutation in the 5' splice donor site of intron 15 was identified in three individuals from two families.
• This splice site mutation likely leads to intron retention and an in-frame stop codon, resulting in a C6 polypeptide 13.5% smaller than normal.
• The identified mutation affects the carboxy-terminal region, previously thought to be crucial for C6 function (C5b binding site/enzymatic region).
• All affected individuals were heterozygous for both subtotal and complete C6 deficiency.

Conclusions:

• The study identifies a specific splice site mutation responsible for subtotal complement C6 deficiency.
• The findings reveal that a truncated C6 molecule, lacking its carboxy-terminal portion, can still exhibit bactericidal activity.
• This challenges previous assumptions about the essentiality of the carboxy-terminal region for C6's functional integrity.