Systemic Biomarkers of Brain Injury From Hyperammonemia

Ammonia is a waste product of protein and amino acid catabolism and is also a potent neurotoxin. The onslaught of high blood ammonia levels on the brain can manifest as cytotoxic brain edema and vascular compromise leading to intellectual and developmental disabilities. In addition, clinical hyperammonemia recurs at varying intervals, which can increase the cumulative damage to the brain and the chance of irreversible coma and death during a hyperammonemia episode due to vascular compromise or brain herniation. The threshold of tolerance for elevated blood ammonia is very low and concentrations above 100 µM can cause brain dysfunction manifested as nausea, vomiting, lethargy, and abnormal behavior; higher concentrations can cause coma and even death. Failure to remove ammonia can be due to inherited defects of the urea cycle, some defects in amino acid catabolism, and degradation of fatty acids.

Aim 1 - To determine the chronology of biomarkers of brain injury - S100B, NSE, and UCHL1 - in response to a hyperammonemic (HA) brain insult in patients with an inherited hyperammonemic disorder. We hypothesized that elevations of S100B, NSE, and UCHL1 will parallel the rise in blood ammonia. These biomarkers will be measured concurrently to ammonia levels throughout hospitalizations for HA until normalization of patient's blood ammonia and mental status.

Aim 2 - To determine if S100B, NSE, and UCHL1 are altered in patients with two other inborn errors of metabolism in which the primary pathology is neurological injury, Maple Syrup Urine Disease (MSUD) and Glutaric Acidemia (GA1). We hypothesize that neuronal and astroglial injury in these disorders may also result in increased levels of S100B, NSE, and UCHL1.

Metabolic patients will be enrolled either during a hospitalization or in outpatient clinic, but outpatient enrollment is preferred. Metabolic patients typically have multiple laboratory tests performed at their outpatient visits. We will obtain the discarded blood samples from such laboratory tests in order to measure S100B, NSE, and UCHL1 levels at baseline (normal blood ammonia), which will provide data on biomarker levels following recovery from a hyperammonemic episode.

During hospitalization for metabolic decompensation or for hypoxic-ischemic encephalopathy, sequential measurements of S100B, NSE and UCHL1 levels will be obtained from discarded blood samples. We will obtain S100B, NSE, and UCHL1 levels from collected discarded blood samples at all subjects' next outpatient visit following their hospitalization, to determine if levels return to baseline.