Wikipedia

Factor H

Factor H(FH) is a member of the regulators of complement activation family and is acomplement control protein.It is a large (155 kilodaltons), solubleglycoproteinthat circulates in humanplasma(at typical concentrations of 200–300microgramspermilliliter[5][6][7]). Its principal function is to regulate thealternative pathwayof thecomplement system,ensuring that the complement system is directed towardspathogensor other dangerous material and does not damage host tissue. Factor H regulates complement activation on self cells and surfaces by possessing both cofactor activity forFactor I–mediatedC3bcleavage, and decay accelerating activity against the alternative pathwayC3-convertase,C3bBb. Factor H exerts its protective action on self cells and self surfaces but not on the surfaces ofbacteriaorviruses.There are however, important exceptions, such as for example the bacterial pathogen,Neisseria meningitidis(also called the meningococcus).This human pathogen has evolved mechanisms to recruit human FH and down-regulate the alternative pathway.[8]Binding of FH permits the bacteria to proliferate in the bloodstream and cause disease.[9]

CFH
Available structures
PDBOrtholog search:PDBeRCSB
Identifiers
AliasesCFH,AHUS1, AMBP1, ARMD4, ARMS1, CFHL3, FH, FHL1, HF, HF1, HF2, HUS, complement factor H
External IDsOMIM:134370;MGI:88385;HomoloGene:20086;GeneCards:CFH;OMA:CFH - orthologs
Orthologs
SpeciesHumanMouse
Entrez

3075

12628

Ensembl

ENSG00000000971

ENSMUSG00000026365

UniProt

P08603

P06909

RefSeq (mRNA)

NM_001014975
NM_000186

NM_009888

RefSeq (protein)

NP_000177
NP_001014975

NP_034018

Location (UCSC)Chr 1: 196.65 – 196.75 MbChr 1: 140.01 – 140.11 Mb
PubMedsearch[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

The ability of Factor H to exert its protective action on self cells and self surfaces is thought to be the result of Factor H having the ability to adopt conformations with lower or higher activities as a cofactor for C3 cleavage or decay accelerating activity.[10]The lower activity conformation is the predominant form in solution and is sufficient to control fluid phase amplification. The more active conformation is thought to be induced when Factor H binds toglycosaminoglycans(GAGs) and orsialic acidsthat are generally present on host cells but not, normally, on pathogen surfaces ensuring that self surfaces are protected whilst complement fixation proceeds unabated on foreign surfaces.[11][12]

Structure

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The molecule is made up of 20complement control protein(CCP) modules (also referred to as Short Consensus Repeats or sushi domains) connected to one another by short linkers (of between three and eightamino acidresidues) and arranged in an extended head to tail fashion. Each of the CCP modules consists of around 60 amino acids with fourcysteineresiduesdisulfide bondedin a 1–3 2–4 arrangement, and a hydrophobic core built around an almost invarianttryptophanresidue. The CCP modules are numbered from 1–20 (from the N-terminus of the protein); CCPs 1–4 and CCPs 19–20 engage withC3bwhile CCPs 7 and CCPs 19–20 bind to GAGs andsialic acid.[13]To date atomic structures have been determined for CCPs 1–3,[14]CCP 5,[15]CCP 7,[16]CCPs 10–11 and CCPs 11–12,[17]CCPs 12–13,[18]CCP 15, CCP 16,[19]CCPs 15–16,[20]CCPs 18–20,[21]and CCPs 19–20.[22][23]The atomic structure for CCPs 6–8 bound to the GAG mimic sucrose octasulfate,[24]CCPs 1–4 in complex with C3b[25]and CCPs 19–20 in complex withC3d(that corresponds to the thioester domain of C3b)[26][27]have also been determined. Although an atomic resolution structure for intact factor H has not yet been determined, low resolution techniques indicate that it may be bent back in solution.[28]Information available to date indicates that CCP modules 1–4 is responsible for the cofactor and decay acceleration activities of factor H, whereas self/non-self discrimination occurs predominantly through GAG binding to CCP modules 7 and/or GAG or sialic acid binding to 19–20.[28][29]

Clinical significance

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Due to the central role that factor H plays in the regulation of complement, there are a number of clinical implications arising from aberrant factor H activity. Overactive factor H may result in reduced complement activity on pathogenic cells – increasing susceptibility to microbial infections. Underactive factor H may result in increased complement activity on healthy host cells – resulting in autoimmune diseases. It is not surprising, therefore, that raremutationsor commonsingle nucleotide polymorphisms(SNPs) in the complement factor H gene (CFH) often result in pathologies. Moreover, the complement inhibitory activities of factor H, and other complement regulators, are often used by pathogens to increasevirulence.

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In 2005, several independent research groups identified an SNP in CFH, which results in the protein change p.Y402H, as a risk factor forAge Releated Macular Degeneration(AMD) present in around a third of Europeans.[30]Although its allele frequency varies considerably between different populations, Y402H has been consistently associated with AMD onset and progression.[30]Homozygousindividuals have an approximately seven-fold greater odds of association with AMD whileheterozygoteshave a two-to-three-fold greater odds of association with the disease.[30]This SNP, located in CCP module 7 of factor H, has been shown to affect the ability of factor H protein to localise to sites of inflammation in retinal tissues (e.g. bypolyanionsand pentraxins) and to regulate the activation of complement and immune cells.[30]The SNP has also been shown to affect the function of factor H-like protein 1, an alternatively spliced version of factor H consisting of CCPs 1 to 7 only, which is thought to have a greater role in intraocular complement regulation.[30]However, the genetic variants in CFH with the greatest effect on an individual's risk of AMD have been shown to affect CCPs 1 to 4, which are involved in dampening the effects of the alternative pathway of complement.[30]A rare functional coding change, p.R1210C, in CFH results in a functional deficiency in factor H and leads to a substantially higher risk of macular degeneration as well as complement-mediated renal conditions.[30][31]

Variation in other genes of the regulators of complement activation locus, such as complement factor H-related genes, as well as in other complement proteins (e.g. factor I, C2/factor B, and C3) have also been associated with greater AMD risk.[30]The current theory is that complement dysregulation is a key driver of chronic inflammation in AMD.[30]

Atypical hemolytic uremic syndrome

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Hemolytic uremic syndrome(HUS) is a disease associated with microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. It can be either acquired (e.g. following infection with shigatoxigenicEscherichia coli), or inherited (also known as atypical hemolytic uremic syndrome, aHUS). aHUS has been strongly linked to mutations in genes of the complement system, especially factor H.[30]In contrast to AMD and C3 glomerulopathy (another complement-mediated renal disorder) which are mainly associated with variation in the N-terminus (CCPs 1 to 4), predisposing mutations in factor H mainly affect the C-terminus of the protein (CCP modules 19 and 20),[30]which has been shown to be responsible for adherence to renal tissues and the regulation of complement components and their downstream effectors.[30][32][33]

Schizophrenia

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Alterations in the immune response are involved in pathogenesis of many neuropsychiatric disorders includingschizophrenia.Recent studies indicated alterations in thecomplement system,including those which may result in the overactivation of thealternative complement pathway,may predispose to schizophrenia. For example, the CFH SNP rs424535 (2783-526T>A) was positively associated with schizophrenia.[34]

Ischemic stroke

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It was found that rs800292(184G >A) SNP was positively associated with stroke and rs800912 minor allele of the CFH gene might be considered as a risk factor for ischemic stroke.[34]

Recruitment by pathogens

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Given the central role of factor H in protecting cells from complement, it is not surprising that several important humanpathogenshave evolved mechanisms for recruiting factor H. This recruitment of factor H by pathogens provides significant resistance to complement attack, and therefore increased virulence. Pathogens that have been shown to recruit factor H include:Aspergillusspp.;Borrelia burgdorferi;B. duttonii;B. recurrentis;Candida albicans;[35]Francisella tularensis;Haemophilus influenzae;Neisseria gonorrhoeae;[36]N. meningitidis;Streptococcus pneumoniae;[10]andStreptococcus pyogenes.[37]

The Gram-negative bacteriumB. burgdorferihas five factor H–binding proteins: CRASP-1, CRASP-2, CRASP-3, CRASP-4 and CRASP-5.[38]Each CRASP protein also bindsplasminogen.[38]It is possible that the allele frequency of CFH variants across the globe reflects selective pressure from infectious diseases.[30]

Interactions

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Factor H has been shown tointeractwithcomplement component 3,amongst other complement proteins and factors, leading to regulation of the alternative pathway of complement in particular.[30][39][40]

Recombinant production

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Biologically active Factor H has been produced byRalf Reskiand coworkers in themoss bioreactor,[41]in a process calledmolecular farming.Large quantities of biologically active human Factor H, potentially suitable for therapeutic purposes, were produced using a syntheticcodon-optimised gene expressed in theyeastexpression host,Pichia pastoris.[42]

Potential use as a therapeutic drug

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Gemini Therapeutics Inc. is a Massachusetts based precision medicine company focused on the development of new therapies through a deeper understanding of disease. Based on the biological activity of human factor H, Gemini is developing a recombinant human factor H protein, GEM103, for the treatment of dry AMD. Gemini recently announced the completion of enrollment in a Phase 2a Trial of GEM103 in Dry Age-Related Macular Degeneration (AMD) in Patients with High-Risk Genetic Variants. Top line data are expected in 1H 2021.

References

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Further reading

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