In the quest for a Huntington’s disease biomarker—one that reliably tracks disease progress and therapeutic response—researchers have so far come up short. Behavioral tests, neuroimages, and cerebrospinal fluid markers, which are panning out for Alzheimer’s and other neurodegenerative diseases, have shown some promise for HD, but none has yet emerged as a practical clinical measure. Could a simple blood test fit the bill? Researchers now report that in four separate case-control studies, mRNA encoding macroH2A1, a protein belonging to the histone H2A family, is elevated in the blood of patients with HD compared to controls. This mRNA rises before the onset of HD symptoms and drops in the blood of patients treated with a potential HD drug. If the findings, reported in the October 3 Proceedings of the National Academy of Sciences online, can be validated, this potential biomarker could help researchers track patients' disease progression and zero in on the most effective drugs for Phase 3 trials.
Scientists seeking HD treatments have their work cut out. "Because the phenotype is clinically so variable and the disease progresses so slowly, it's extremely difficult, if not impossible, to tell by clinical exam whether a new drug has had an effect on the disease progression," said senior investigator Clemens Scherzer of Harvard Medical School in Cambridge, Massachusetts. "What is really needed is a biomarker to get an informative readout about whether or not a drug looks promising." In AD research, the search for disease-modifying drugs was similarly stymied for some time because variability and slow progression made it hard to distinguish effects, especially for weak drugs, and that field, too, is focusing now on validating outcome measures for drug trials.
But can a blood marker reliably track a disorder where most of the damage occurs in the brain? Yes, according to Scherzer. Because huntingtin is expressed in almost all tissues, the mutant form may cause detectable biochemical or gene expression changes in blood cells that might track with disease.
Scherzer and colleagues pored over genomewide expression data from eight people with HD and 111 controls, many of whom had a different neurodegenerative disorder. From over 14,000 mRNAs, the team pulled out 99 that were up or down in patients with HD. Transcripts of H2AFY, which encodes the protein macroH2A1, were 1.6-fold higher in HD blood than in controls. MacroH2A1 is involved in transcriptional regulation, and profound transcriptional changes are a mark of mutant huntingtin toxicity (see ARF related news story on Cui et al., 2006 and Cornett et al., 2006). For this reason the scientists decided to scrutinize this transcript.
The researchers, who included co-first authors Yi Hu, Harvard Medical School, and Vanita Chopra, Massachusetts General Hospital in Boston, amplified RNA to measure it in blood taken from the same eight HD patients, eight healthy controls, and 29 additional neurodegenerative control patients. Six of those 29 had dystonia and were being treated with drugs commonly used for HD; they served as controls to weed out any potential drug effects. H2AFY mRNA was about two times higher in HD patients than in all the controls.
To validate their results, the authors conducted a cross-sectional and a longitudinal study. In the former, they tested blood of 36 patients with HD, nine asymptomatic HD mutation carriers, 50 healthy controls, and one subject with spinocerebellar ataxia-1, a disease also caused by a CAG trinucleotide repeat, but in a different gene. Patients with HD had 1.5-fold more H2AFY mRNA than any of the controls, while asymptomatic HD patients had 1.9-fold more. In the longitudinal study, raised serum H2AFY mRNA levels held steady over three years.
The researchers next checked for the macroH2A1 protein in the brains of HD patients and two lines of transgenic HD mouse model, R6/2 and 140-CAG. Both early-stage patients and mouse brains showed higher levels of macroH2A1 than did controls. Of 12 patients, only those with early HD had elevated macroH2A1 in the frontal cortex, while amounts in late-stage brains were more comparable to controls. The discrepancy might be due to neuronal loss as the disease progresses, the authors wrote. Though the disease is believed to begin in the striatum, research suggests it attacks the cortex as well (see Selemon et al., 2004). The R6/2 HD mouse models an aggressive disease course; macroH2A1 progressively rose in the striatum and cortex between four and 12 weeks of age and became abundant in the hippocampus and cerebellum by 12 weeks. The 140-CAG knock-in mouse more closely models slower human pathology; in its brains, macroH2A1 was high in the striatum.
Would this potential marker change in response to a drug? Sodium phenylbutyrate (SPB), a histone deacetylase inhibitor, helps normalize transcriptional dysregulation. In the R6/2 mice, the drug reduces brain atrophy, extends survival, and striatal macroH2A1 diminished after two weeks of treatment. The researchers tested frozen blood samples from people enrolled in the Phenylbutyrate Development for Huntington's Disease trial and found that the number of weeks of SPB treatment correlated with a drop in blood H2AFY.
"The data are tantalizing," wrote Michelle Ehrlich and Sam Gandy of the Mount Sinai School of Medicine, New York, in an accompanying PNAS commentary. They add, however, that it is unclear whether H2AFY will be useful in later stages of HD because H2AFY levels go back down as the disease progresses, even without treatment. Scherzer noted that the marker does seem to distinguish early symptomatic from pre-symptomatic HD. The authors acknowledge the need to validate results in other patient populations and confirm whether H2AFY tracks HD over time. Scherzer said prospective cohort studies will show if and how H2AFY levels change as asymptomatic HD mutation carriers develop the disease.
H2AFY "seems to have a connection to a plausible HD-related mechanism," said Christopher Ross of Johns Hopkins University in Baltimore, Maryland, who was not involved with the study." That suggests this would not only be interesting as a biomarker, but potentially as a clue to additional aspects of pathogenesis."—Gwyneth Zakaib
- Cui L, Jeong H, Borovecki F, Parkhurst CN, Tanese N, Krainc D. Transcriptional repression of PGC-1alpha by mutant huntingtin leads to mitochondrial dysfunction and neurodegeneration. Cell. 2006 Oct 6;127(1):59-69. PubMed.
- Cornett J, Smith L, Friedman M, Shin JY, Li XJ, Li SH. Context-dependent dysregulation of transcription by mutant huntingtin. J Biol Chem. 2006 Nov 24;281(47):36198-204. PubMed.
- Selemon LD, Rajkowska G, Goldman-Rakic PS. Evidence for progression in frontal cortical pathology in late-stage Huntington's disease. J Comp Neurol. 2004 Jan 6;468(2):190-204. PubMed.
- Hu Y, Chopra V, Chopra R, Locascio JJ, Liao Z, Ding H, Zheng B, Matson WR, Ferrante RJ, Rosas HD, Hersch SM, Scherzer CR. Transcriptional modulator H2A histone family, member Y (H2AFY) marks Huntington disease activity in man and mouse. Proc Natl Acad Sci U S A. 2011 Oct 11;108(41):17141-6. PubMed.