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.