Huntington’s disease (HD) is a fatal, inherited neurodegenerative disorder caused by an expanded CAG repeat in the gene encoding huntingtin (HTT). Therapeutic approaches to lower mutant HTT (mHTT) levels are expected to proceed to human trials, but noninvasive quantification of mHTT is not currently possible. The importance of the peripheral immune system in neurodegenerative disease is becoming increasingly recognized. Peripheral immune cells have been implicated in HD pathogenesis, but HTT levels in these cells have not been quantified before. A recently described time-resolved Förster resonance energy transfer (TR-FRET) immunoassay was used to quantify mutant and total HTT protein levels in leukocytes from patients with HD. Mean mHTT levels in monocytes, T cells, and B cells differed significantly between patients with HD and controls and between pre-manifest mutation carriers and those with clinical onset. Monocyte and T cell mHTT levels were significantly associated with disease burden scores and caudate atrophy rates in patients with HD. mHTT N-terminal fragments detected in HD PBMCs may explain the progressive increase in mHTT levels in these cells. These findings indicate that quantification of mHTT in peripheral immune cells by TR-FRET holds significant promise as a noninvasive disease biomarker.
Huntington’s disease (HD) is caused by an autosomal dominant pathogenic mutation, resulting in an expanded stretch of 36 or more glutamine residues in the N terminus of the huntingtin (HTT) protein (1). Mutant HTT (mHTT) expression in the CNS is the primary pathogenic factor for the development of HD, with increasing expression levels associated with disease severity and toxicity in various models (2–4). However, patients with HD also exhibit multiple systemic changes (5). We have previously demonstrated peripheral immune system dysfunction in HD, including increases in innate immune proteins in patient plasma, many of which track with disease progression (6). Inflammatory cytokines and chemokines are elevated in the plasma of patients with HD, the origin of which appears to be hyperreactive monocytes carrying the HD mutation (7, 8). Recent studies using peripherally administered kynurenine 3-monooxygenase inhibitors (9) and bone marrow transplantation (10) highlight further that the peripheral immune system is altered in HD. The hyperreactivity observed in monocytes is reflected by a similar pattern in HD microglia (7), and microglial activation is seen before symptom onset in HD mutation carriers (11). Peripheral immune dysfunction may be pathogenically important and could offer a window onto relevant CNS dysfunction.
Many putative therapeutic approaches for HD attempt to lower mHTT levels in the CNS or systemically. However, noninvasive means of quantifying mHTT in the CNS do not exist. Peripheral readouts may be of value in assessing the effects of systemically delivered HTT-lowering therapies, i.e., small molecules aimed at enhancing HTT clearance, and detecting peripheral effects of centrally delivered therapies. If mHTT levels track with disease progression, they may have value as markers of disease progression. Therefore, we aimed to use a recently described time-resolved Förster resonance energy transfer (TR-FRET) immunoassay (12) to quantify total HTT and mHTT levels in peripheral immune cells in patients with HD.