Development of frataxin gene expression measures for the evaluation of experimental treatments in Friedreich's ataxia

PLoS One. 2013 May 17;8(5):e63958. doi: 10.1371/journal.pone.0063958. Print 2013.

Abstract

Background: Friedreich ataxia is a progressive neurodegenerative disorder caused by GAA triplet repeat expansions or point mutations in the FXN gene and, ultimately, a deficiency in the levels of functional frataxin protein. Heterozygous carriers of the expansion express approximately 50% of normal frataxin levels yet manifest no clinical symptoms, suggesting that therapeutic approaches that increase frataxin may be effective even if frataxin is raised only to carrier levels. Small molecule HDAC inhibitor compounds increase frataxin mRNA and protein levels, and have beneficial effects in animal models of FRDA.

Methodology/principal findings: To gather data supporting the use of frataxin as a therapeutic biomarker of drug response we characterized the intra-individual stability of frataxin over time, determined the contribution of frataxin from different components of blood, compared frataxin measures in different cell compartments, and demonstrated that frataxin increases are achieved in peripheral blood mononuclear cells. Frataxin mRNA and protein levels were stable with repeated sampling over four and 15 weeks. In the 15-week study, the average CV was 15.6% for protein and 18% for mRNA. Highest levels of frataxin in blood were in erythrocytes. As erythrocytes are not useful for frataxin assessment in many clinical trial situations, we confirmed that PBMCs and buccal swabs have frataxin levels equivalent to those of whole blood. In addition, a dose-dependent increase in frataxin was observed when PBMCs isolated from patient blood were treated with HDACi. Finally, higher frataxin levels predicted less severe neurological dysfunction and were associated with slower rates of neurological change.

Conclusions/significance: Our data support the use of frataxin as a biomarker of drug effect. Frataxin levels are stable over time and as such a 1.5 to 2-fold change would be detectable over normal biological fluctuations. Additionally, our data support buccal cells or PBMCs as sources for measuring frataxin protein in therapeutic trials.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Analysis of Variance
  • Biomarkers / metabolism*
  • Cohort Studies
  • Enzyme-Linked Immunosorbent Assay
  • Friedreich Ataxia / drug therapy*
  • Friedreich Ataxia / genetics
  • Gene Expression Regulation / drug effects*
  • Histone Deacetylase Inhibitors / pharmacology*
  • Humans
  • Iron-Binding Proteins / genetics
  • Iron-Binding Proteins / metabolism*
  • Leukocytes, Mononuclear / metabolism
  • RNA, Messenger / drug effects*
  • RNA, Messenger / metabolism
  • Real-Time Polymerase Chain Reaction

Substances

  • Biomarkers
  • Histone Deacetylase Inhibitors
  • Iron-Binding Proteins
  • RNA, Messenger
  • frataxin

Grant support

This work was supported by grants from Muscular Dystrophy Association (MDA), from Friedreich’s Ataxia Research Alliance (FARA), and from Penwest Pharmaceutical to DRL; from FARA and GoFAR to Repligen Corporation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. A representative of FARA and of GoFAR assisted in the collection of blood samples used in the study.