Tuesday, 21 July 2015

Genetic variant of MS susceptibility gene influences inflammation

Gu BJ, Field J, Dutertre S, Ou A, Kilpatrick T, Lechner-Scott J, Scott R, Lea R, Taylor BV, Stankovich J, Butzkueven H, Gresle M, Laws SM, Petrou S, Hoffjan S, Akkad DA, Graham CA, Hawkins S, Glaser A, Bedri SK, Hillert J, Matute C, Antiguedad A; ANZgene Consortium, Wiley JS.A rare P2X7 variant Arg307Gln with absent pore formation function protects against neuroinflammation in multiple sclerosis. Hum Mol Genet. 2015  pii: ddv278. [Epub ahead of print]

Multiple sclerosis (MS) is a chronic relapsing-remitting inflammatory disease of the central nervous system characterized by oligodendrocyte damage, demyelination and neuronal death. Genetic association studies have shown a two-fold or greater prevalence of the HLA-DRB1*1501 allele in the MS population compared with normal Caucasians. In discovery cohorts of Australasian patients with multiple sclerosis (total 2941 patients, 3008 controls) we examined the associations of twelve functional polymorphisms of P2X7, a microglial/macrophage receptor with proinflammatory effects when activated by extracellular ATP. 


In discovery cohorts, rs28360457, coding for Arg307Gln was associated with MS and combined analysis showed a two-fold lower minor allele frequency compared with controls (1.11% for MS and 2.15% for controls, p=0.0000071). Replication analysis of four independent European MS case-control cohorts (total 2140 cases and 2634 controls) confirmed this association (OR 0.69, p=0.026). A meta-analysis of all Australasian and European cohorts indicated that confers a 1.8-fold protective effect on MS risk (OR 0.57, p=0.0000024). Fresh human monocytes heterozygous for Arg307Gln have >85% loss of 'pore' function of the P2X7 receptor. Analysis shows Arg307Gln always occurred with 270His suggesting a single 307Gln-270His haplotype which confers dominant negative effects on P2X7 function and protection against MS. Modelling based on the homologous zP2X4 receptor showed Arg307 is located in a region rich in basic residues located only 12Å from the ligand binding site. Our data show the protective effect against MS of a rare genetic variant of P2RX7 with heterozygotes showing near absent proinflammatory 'pore' function.
The P2X7 subunits can form homomeric receptors only with a typical P2X receptor structure. The P2X7 receptor is a ligand-gated cation (positively charged ion, like Na+) channel that opens in response to ATP binding and leads to cell depolarization . The P2X7 receptor requires higher levels of ATP (energy) than other P2X receptors; however, the response can be potentiated by reducing the concentration of divalent cations such as calcium (Ca2+) or magnesium (Mg2+).

Proteins are made up from the sequence of (the twenty) amino acids.

                                      The Sequence of P2X7
 10 20 30 40 50
MPACCSCSDV FQYETNKVTR IQSMNYGTIK WFFHVIIFSY VCFALVSDKL 
        60         70         80         90        100
YQRKEPVISS VHTKVKGIAE VKEEIVENGV KKLVHSVFDT ADYTFPLQGN 
       110        120        130        140        150
SFFVMTNFLK TEGQEQRLCP EYPTRRTLCS SDRGCKKGWM DPQSKGIQTG 
       160        170        180        190        200
RCVVYEGNQK TCEVSAWCPI EAVEEAPRPA LLNSAENFTV LIKNNIDFPG 
       210        220        230        240        250
HNYTTRNILP GLNITCTFHK TQNPQCPIFR LGDIFRETGD NFSDVAIQGG 
       260        270        280        290        300
IMGIEIYWDC NLDRWFHHCR PKYSFRRLDD KTTNVSLYPG YNFRYAKYYK 
       310        320        330        340        350
ENNVEKRTLI KVFGIRFDIL VFGTGGKFDI IQLVVYIGST LSYFGLAAVF 
       360        370        380        390        400
IDFLIDTYSS NCCRSHIYPW CKCCQPCVVN EYYYRKKCES IVEPKPTLKY 
       410        420        430        440        450
VSFVDESHIR MVNQQLLGRS LQDVKGQEVP RPAMDFTDLS RLPLALHDTP 
       460        470        480        490        500
PIPGQPEEIQ LLRKEATPRS RDSPVWCQCG SCLPSQLPES HRCLEELCCR 
       510        520        530        540        550
KKPGACITTS ELFRKLVLSR HVLQFLLLYQ EPLLALDVDS TNSRLRHCAY 
       560        570        580        590 
RCYATWRFGS QDMADFAILP SCCRWRIRKE FPKSEGQYSG FKSPY

P2X7 are made up of 595 amino acids and the variant of P2X7 that is not associated with MS and substitution at position 307. The Arg307Gln (Argenine =R a basic/positively charged amino acid at position 307 of the P2X7 protein, which would tend to interact with an acidic/negatively charged (glutamic acid,aspartic acid) partner has been changed to to glutamine which is a non charged amino acid). This combination looses the pore function. This is variant occurs with a histadine at postion 270. This is also a positively charged amino acid and replaces the Argine.

When this Gln307, Hist 270 gene combination occurs on one gene it blocks the pore irrespective of what the other gene variant on the other chromosome does. (Remember you get one chromeome from mum and one from dad each codes the p2X7 gene). This blocks the pore on the P2X7 receptor to inhibit its pro-inflammatory function.

2 comments:

  1. Curious: when they say 'Australasian' do they mean people with some Denisovan DNA (a percentage of Polynesian and Aboriginal Australians have this), or does Australasian mean just those of indigenous descent/ heritage (Aboriginal and Torres Strait islanders/people) or what? Just trying to unpick the terminology in these politically correct times!.

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  2. Common or garden white Aussie of European ascestry some times we can be too smart for our own good:-)

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