C6

Complement component C6 is a terminal complement protein that assembles into the membrane attack complex (MAC) to mediate cell lysis and shape innate immune responses. The C6 gene spans 18 exons on chromosome 5p12-14 with a mosaic domain architecture whose intron/exon boundaries do not conform to the conventional module-class 1 intron paradigm, suggesting a complex evolutionary origin for terminal complement components (PMID:8512929). A naturally occurring carboxy-terminal truncation (loss of ~14% of the protein) retains bactericidal activity, demonstrating that the C-terminal region is dispensable for MAC-dependent killing (PMID:7535801). Genetic ablation of C6 in mice reveals that MAC-dependent C6 activity is required for normal PMN and macrophage innate immune functions—including adhesion molecule expression, superoxide generation, phagocytosis, and NLRP3 inflammasome activation—and drives organ injury in sepsis and acute lung injury models (PMID:32444389).

1. 1977 Medium

   Establishing that C6 is a genetically polymorphic serum protein whose locus is unlinked to the MHC region addressed the fundamental question of where C6 maps and whether it co-segregates with other immune loci.

   Evidence High-voltage agarose electrophoresis, isoelectric focusing with hemolytic overlay, and LOD score linkage analysis in Norwegian families

   PMID:885533

   Open questions at the time

   • Chromosomal assignment was not determined in this study
   • Functional consequences of C6 allelic variants (C6A vs C6B) not characterized
2. 1993 Medium

   Determination that the C6 gene spans 18 exons whose boundaries do not follow the expected class 1 intron/module pattern resolved the structural genomic organization and challenged assumptions about how terminal complement component genes evolved.

   Evidence Genomic DNA sequencing and exon boundary classification across 17 of 18 exon junctions

   PMID:8512929

   Open questions at the time

   • One exon boundary was not characterized
   • Promoter and regulatory elements upstream of the gene were not defined
3. 1995 High

   Discovery that a naturally occurring C-terminally truncated C6 retains bactericidal function demonstrated that the distal ~14% of the protein is dispensable for MAC assembly and lytic activity, redefining the minimal functional unit of C6.

   Evidence PCR/sequencing of a splice-site mutation in intron 15 creating premature stop codon; hemolytic and bactericidal assays of patient serum; replicated across three individuals from two families

   PMID:7535801

   Open questions at the time

   • Whether the truncated C6 has altered kinetics of MAC insertion or assembly was not assessed
   • Structural basis for dispensability of the C-terminal region is unknown
4. 1995 Medium

   Multi-marker haplotyping across the C6/C7 locus revealed that complete C6 deficiency arises from multiple independent mutations, with subtotal C6 deficiency sharing a haplotype with combined C6/C7 deficiency, clarifying the genetic heterogeneity underlying C6 deficiency states.

   Evidence DNA marker haplotyping using 7 markers spanning ~300 kbp of chromosome 5p12-14 in South African and other patient cohorts

   PMID:7625764 PMID:7625765

   Open questions at the time

   • Causative mutations for each deficiency haplotype were not identified
   • Mechanism linking the C6 defect haplotype to associated C7 deficiency was not resolved
5. 1997 Medium

   Finding that C6 deficiency in the PVG/c rat arises from ~100-fold reduced mRNA rather than gene deletion established that regulatory or mRNA stability defects can abolish C6 protein without structural gene loss, and identified liver as the primary expression site.

   Evidence Northern blot, RT-PCR, and Southern blot in PVG/c rat liver and kidney; comparison with wild-type

   PMID:9276537

   Open questions at the time

   • The causative point mutation or regulatory lesion was not identified
   • Contribution of extrahepatic C6 expression (e.g. kidney) to circulating levels was not quantified
6. 2001 Medium

   Demonstrating that C6 mRNA is transcriptionally downregulated in esophageal, colon, and kidney carcinomas without genomic deletion raised the possibility that tumors evade complement-mediated surveillance by silencing terminal complement components.

   Evidence Differential display, in situ hybridization, semi-quantitative RT-PCR in 10 paired human tumor/normal specimens, and Southern blot in cancer cell lines

   PMID:11378347

   Open questions at the time

   • Mechanism of transcriptional downregulation (promoter methylation, transcription factor loss) was not identified
   • Functional consequence of C6 loss for tumor immune evasion was not tested experimentally
7. 2020 High

   Genetic ablation of C6 in mice revealed that MAC-dependent C6 activity is required for multiple innate immune effector functions and is a major driver of organ damage in sepsis and acute lung injury, establishing C6/MAC as a double-edged sword in innate immunity.

   Evidence C6 knockout mice assessed by flow cytometry, ROS/superoxide assays, phagocytosis assays, NLRP3 inflammasome activation in PMNs and macrophages, cecal ligation puncture sepsis model, and LPS-induced acute lung injury model

   PMID:32444389

   Open questions at the time

   • Whether C6/MAC effects on innate cells are direct (sublytic MAC signaling) or indirect (via downstream mediators) was not resolved
   • Findings from a single laboratory; independent replication in other KO or inhibitor models pending
   • Relevance of mouse C6 KO phenotypes to human C6 deficiency states not formally demonstrated

• Key unresolved questions include the precise mechanism by which MAC/sublytic MAC activates NLRP3 inflammasome in innate immune cells, the structural basis for the dispensability of the C6 C-terminal domain, and the epigenetic mechanism responsible for C6 silencing in carcinomas.
• No structural model of C6 within the assembled MAC pore at atomic resolution
• Sublytic vs lytic MAC signaling pathways downstream of C6 incorporation not delineated
• Therapeutic potential of C6 inhibition in sepsis not tested in clinical settings