DEFINING MRI BIOMARKERS FOR UCD RESEARCH Pacheco-Colon I1, Seltzer RR1 , Shattuck K1, Prust M2, Breeden A1, Sprouse C2, Gertz B2, King J2, and Gropman AL1,2. Georgetown University, Washington, D.C., Children’s National Medical Center, Washington, D.C., USA Background/Objective: Although several theories exist, it is not well understood how hyperammonemia (HA) disrupts brain function. In addition, the pathogenesis of brain injury and recovery from neurological sequelae associated with UCDs remains largely unexplored. Clinicians who care for affected patients commonly encounter clinical settings characterized by significant elevations of plasma concentrations of ammonia and glutamine without neurological dysfunction, as well as situations in which patients manifest confusion, vomiting and ataxia in the presence of only mild elevations of blood ammonia and glutamine. Surrogate brain markers that can be used clinically to predict severity of insult and potential treatment response will impact management decisions and the development of neuroprotective strategies that can be used within a critical time window. However, prediction of outcome is not straightforward. There is currently no direct correlation between genotype, peak ammonia level, and structural changes in the brain and/or phenotype. The age of onset, duration and degree of HA are used to establish the prognosis and the extent to which the neurological changes may be reversible, but the predictive value is limited. As part of our NIH- funded Rare Diseases Clinical Research Center in Urea Cycle Disorders, we are investigating these questions by using advanced magnetic resonance imaging methods that allow assessment of brain metabolic perturbations and biomarkers in UCD. Our work focuses on a defined population of UCD patients, those with ornithine transcarbamylase deficiency (OTCD), the only X-linked urea cycle disorder. Neuroimaging with 1HMRS, DTI, and fMRI has uncovered abnormalities in the brains of subjects with partial OTCD, reflecting cellular injury in otherwise normal appearing brain by conventional MRI. Several neuroimaging platforms exist to study neural networks underlying cognitive processes, white matter/myelin microstructure, and cerebral metabolism in vivo. Patients and methods: We have now enrolled a total of 80 subjects with partial OTCD (males and females) and controls ages 7-60 years into two protocols and have applied structural MRI, DTI, fMRI and MRS to this cross sectional group at baseline. All imaging has been performed on a 3T GE Tim Trio Siemens scanner. Patients had a minimum IQ of 70 and were “stable” at the time of study. The protocol engendered 3 hours of imaging split into several sessions as well as neurocognitive testing using standard batteries to assess working memory, attention and reaction time. To study white matter microstructural features, all patients underwent DTI using a matrix of 36 directions and two b values. For biochemical assessment, 1H MRS with TE of 30ms and 250 averages were used and functional connectivity was assessed by fMRI using an N-back and STROOP task to assess working memory. Results: patients with OTCD differed in structural, biochemical and neural networks. Brain findings segregated into the three groups: controls, asymptomatic OTCD females and symtomatic males and females showing normal, intermediate, and severe findings. Routine T1 and T2 imaging failed to detect any significant changes. The yield of finding white matter lesions in asymptomatic and symptomatic OTCD, however, increased with the use of FLAIR imaging and should be part of clinical routine in metabolic disorders. With regard to biochermical findings, using MRS, we detected significant increases of Gln levels (p< 0.007) in posterior cingulate gray matter (PCGM), parietal white matter (PWM) in asymptomatic and symptomatic OTCD, compared to controls. The utility of MRS was demonstrated in one subject had suspected HA episode during the study as evidenced by large brain gln peak, despite denial of clinical symptoms by the parent who endorsed the participant was at baseline. mI concentrations were significantly decreased in parietal and frontal white matter (PWM, FWM), thalamus and PCGM and the degree of decrease correlated with disease severity. The degree of brain mI depletion was inversely correlated with brain Gln level. This inverse relationship between Gln and mI was not observed in controls. Interestingly, reduced level of mI in white matter was also observed in women with OTCD who were asymptomatic and suggests the possibility of unrecognized biochemical disturbances (such as edema and volume changes in the astrocyte) in these subjects. The reduction of mI also correlated with cognitive impairments in a pattern suggesting a white matter injury model. In terms of white matter injury, DTI disclosed decreased FA in the frontal white matter of subjects with OTCD, including “asymptomatic” carrier females. This is an area in the superior extent of the corpus callous and provides key hemispheric connections to brain areas involved in executive function. Lastly brain activation maps generated from fMRI showed decreased neuronal activation of working memory networks, most likely reflecting damage caused by HA. The scope and limitations of these methods will be discussed in the context of information they provided regarding pathology of hyperammonemia (HA) in subjects with OTCD who were enrolled as part of the UCRDC rare disorders network. Conclusions: multimodal imaging has the potential to investigate impact of HA on cognitive function by interrogating neural networks, connectivity and biochemistry. As neuroimaging methods become increasingly sophisticated, they will play a critical role in clinical monitoring and following treatment. Many of these techniques are available in the clinical setting and protocols that can be performed on clinical scanners should be explored.