Investigation of alpha-1-antitrypsin polymorphism

Abstract

Genetic variations are the basis for evolutionary change. There are different kinds of polymorphisms in living organisms: chromosomal polymorphism immunological polymorphism, protein polymorphism, DNA sequence polymorphism. The change in a structural gene can result in different manner, for example amino acid substitution in the polypeptide chain. In different variants of one protein the net charge on the polypeptide can be altered and variants can be separated by gel electrophoresis. Investigaton of protein polymorphism allows the study of a structural gene, a DNA sequence coding for protein synthesis. Polymorphism of some proteins can be used as a marker in population genetics and in diagnostics of inherited disease. Alpha-1-antitrypsin (AAT) is a highly polymorphic glycoprotein, serin protease inhibitor. Protease inhibitor locus (Pi locus) located on chromosome 14q32.1. AAT variants are inherited in autosomal codominant manner. Pi is highly polymorphic with >90 variants reported and used as a marker in population genetics. MM homozygote is the most common phenotype with normal plasma concentration to provide suitable antiprotese defence. There are many studies that suggested connection between alpha-1-antitrypsin phenotypes and different diseases. Most important are deficient phenotypes of alpha-1- antitrypsin. Only two variants, Z and S are common. Uncommon deficient alleles include: I, MMalton, MPittsburg, Mduarte, null and other rare alleles. Isoelectric focusing assay is the best method to type AAT alleles. Polymorphism of AAT can be use as a marker for genetic distance between different populations. The alleles that lead to a deficiency of AAT are clinically important. In neonates, inclusion of AAT can be found in the endoplasmic reticulum of the hepatocyte and cause liver damage. In adult patients with severe AAT deficiency, the lung no protect against breakdown by neutrophil elastase, increasing the risk for the development of emphysema. Phenotyping of AAT in serum samples has few advantages. Liver biopsy is an invasive method and Pi typing is recommended in children with liver disease. Also Pi typing is useful in family planning if one or both parents have deficient allele. Diagnostics of emphysema in adults with etiology of AAT deficient, can be done by AAT phenotyping.

Genske varijacije su osnova evolutivnih promena. U živim organizmima prisutni su različiti polimorfizmi: polimorfizam hromozoma, imunološki polimorfizam polimorfizam proteina, polimorfizam sekvenci DNK. Polimorfizam proteina je posledica polimorfizma gena koji dovodi do supstitucije jedne ili više aminokiselina polipeptidnog lanca. Tako nastale varijante poseduju različita neto naelektrisanja i razdvajaju se gel elektroforezom. Na ovaj način se posredno ispituje, strukturni gen odnosno sekvenca DNK koja kodira ovaj protein. Polimorfizam nekih proteina može se koristi kao: marker u populacionoj genetici i dijagnostici nekih naslednih oboljenja. Alfa-1-antitripsin (AAT) je visoko polimorfan glikoprotein, inhibitor proteaza. Gen za AAT (Pi lokus) je smešten na hromozomu 14q32.1, a nasleđuje se autosomno kodominantno. Pi lokus je veoma polimorfan (preko 90 različitih varijanti) i koristi se kao marker u populacionoj genetici. Najčešći fenotip jeste MM i ima normalnu koncentraciju u plazmi koja omogućava adekvatnu antiproteaznu zaštitu. Polimorfizam ovog proteina je doveden u vezu sa pojavom različitih oboljenja, a najznačajnije su deficijentne varijante. Najčešće deficijentne varijante su: Z i S, a retke: I, MMalton, MPittsburg Mduarte, nulta i druge. Metoda izbora za analizu varijanti AAT jeste izoelektrofokusiranje na tankom sloju poliakrilamida Ispitivanjem polimorfizma AAT u različitim populacijama utvrđuje se genska distanca između populacija. Klinički značaj ispitivanja polimorfizma AAT jeste dijagnostikovanje homozigota ili heterozigota, nosioca deficijentnog alela. Kod novorođenčadi se deficijentni protein nakuplja u endoplazmatičnom retikulumu hepatocita i uzrok je oštećenja jetre. Odrasli nosioci deficijentnog alela, usled deficita ove antiproteaze imaju neadekvantnu zaštitu pluća od neutrofilne elastaze, što povećava rizik za nastajanje emfizema. Određivanje fenotipa AAT iz uzorka krvi, ima više prednosti u odnosu na druge dijagnostičke metode. Kod novorođenčadi, izbegava se rizična i invazivna metoda uzimanja isečka jetre. Metoda je korisna za planiranje potomstva kod roditelja nosioca deficijentnog alela kod kojih je jedno od dece homozigot ili heterozigot. Kod odraslih se fenotipizacijom AAT utvrđuje etiologija emfizema.