Brain urea increase is an early Huntington’s disease pathogenic event observed in a prodromal transgenic sheep model and HD cases
The neurodegenerative disorder Huntington’s disease (HD) is typically characterized by extensive loss of striatal neurons and the midlife onset of debilitating and progressive chorea, dementia, and psychological disturbance. HD is caused by a CAG repeat expansion in the Huntingtin (HTT) gene, translating to an elongated glutamine tract in the huntingtin protein. The pathogenic mechanism resulting in cell dysfunction and death beyond the causative mutation is not well defined. To further delineate the early molecular events in HD, we performed RNA-sequencing (RNA-seq) on striatal tissue from a cohort of 5-y-old OVT73-line sheep expressing a human CAG-expansion HTT cDNA transgene. Our HD OVT73 sheep are a prodromal model and exhibit minimal pathology and no detectable neuronal loss. We identified significantly increased levels of the urea transporter SLC14A1 in the OVT73 striatum, along with other important osmotic regulators. Further investigation revealed elevated levels of the metabolite urea in the OVT73 striatum and cerebellum, consistent with our recently published observation of increased urea in postmortem human brain from HD cases. Extending that finding, we demonstrate that postmortem human brain urea levels are elevated in a larger cohort of HD cases, including those with low-level neuropathology (Vonsattel grade 0/1). This elevation indicates increased protein catabolism, possibly as an alternate energy source given the generalized metabolic defect in HD. Increased urea and ammonia levels due to dysregulation of the urea cycle are known to cause neurologic impairment. Taken together, our findings indicate that aberrant urea metabolism could be the primary biochemical disruption initiating neuropathogenesis in HD.
My confusion arises over previous work, suggesting that gout (hyperuricemia) may actually PROTECT against dementia. Here's a press release from several years ago on the topic. Below see full-text of original paper:
Gout and the Risk of Alzheimer’s Disease: A Population-Based Cohort Study
While gout is associated with cardiovascular (CV)-metabolic comorbidities and their sequelae, the antioxidant effects of uric acid may have neuroprotective benefits. We evaluated the potential impact of incident gout on the risk of developing Alzheimer’s disease (AD) in a general population context.
We conducted an age-matched, sex-matched, entry-time-matched and body mass index (BMI)-matched cohort study using data from The Health Improvement Network, an electronic medical record database representative of the UK general population, from 1 January 1995 to 31 December 2013. Up to five non-gout individuals were matched to each case of incident gout by age, sex, year of enrolment and BMI. We compared incidence rates of AD between the gout and comparison cohorts, excluding individuals with prevalent gout or dementia at baseline. Multivariate hazard ratios (HRs) were calculated, while adjusting for smoking, alcohol use, physician visits, social deprivation index, comorbidities and medication use. We repeated the same analysis among patients with incident osteoarthritis (OA) as a negative control exposure.
We identified 309 new cases of AD among 59 224 patients with gout (29% female, mean age 65 years) and 1942 cases among 238 805 in the comparison cohort over a 5-year median follow up (1.0 vs 1.5 per 1000 person-years, respectively). Univariate (age-matched, sex-matched, entry-time-matched and BMI-matched) and multivariate HRs for AD among patients with gout were 0.71 (95% CI 0.62 to 0.80) and 0.76 (95% CI 0.66 to 0.87), respectively. The inverse association persisted among subgroups stratified by sex, age group (<75 and ≥75 years), social deprivation index and history of CV disease. The association between incident OA and the risk of incident AD was null.
These findings provide the first general population-based evidence that gout is inversely associated with the risk of developing AD, supporting the purported potential neuroprotective role of uric acid.
Anybody else confused? Does the answer lie in the difference between urea and uric acid outlined here. Is it possible that one (uric acid) is protective while the other (urea) is harmful? Or, just another red herring...