Accumulation Of Subcortical Iron As A Modifier Of Volumetric And Cognitive Decline In Healthy Aging: Two Longitudinal Studies

Accumulation of non-heme iron in the brain has been theorized as a cellular mechanism underlying global neural and cognitive decline in normal aging and neurodegenerative disease. Relatively few studies of brain iron in normal aging exist and extant studies are almost exclusively cross-sectional. Here, I estimated iron content via T2* and measured volumes in several brain regions in two independent samples of healthy adults. The first sample (N = 89) was measured twice with a two-year delay; and the second sample (N = 32) was assessed four times over a span of 7 years. Latent models estimated change in iron and volume, and the effects of cardiovascular risk factors as modifiers of change trajectories. Iron significantly increased (T2* decreased) over time in the basal ganglia, but not in the hippocampus. Accumulation of iron accounted for shrinkage in the striatum. Elevated metabolic syndrome risk indicators were associated with greater iron at baseline, which accounted for individual differences in shrinkage. Increase in caudate iron content was associated directly with lesser improvement in virtual Morris water maze navigation, and indirectly via shrinkage with lesser improvement in verbal working memory. This study present the first longitudinal evidence in support of iron as a biomarker of age-related decline in regional volume and cognition

Determinants of age-related brain iron accumulation and links to neurocognitive functions

Iron is crucial for development and normal functioning of the brain. With increasing age, it accumulates in the cells and can cause irreparable damage, affecting both the structure and function of the brain. Despite these findings, the factors which influence iron accumulates and the longitudinal effects of iron are still poorly understood. This doctoral thesis aimed to explore what influences brain iron accumulation in normal aging, and how this accumulation impacts molecular, and functional properties of the brain, and working-memory. Study I investigated if iron accumulation in striatum and DLPFC affected working memory change in normal aging, and if this accumulation and relationship to performance varied based on availability of dopamine, specified by COMT genotype status. We found that iron accumulated in both striatum and DLPFC. Greater iron accumulation in DLPFC was related to more deleterious change in working-memory performance. In addition, iron accumulation was amplified in older adults with presumably lowest levels of dopamine. These individuals were also driving the link between changes in iron and working-memory performance. Study II investigated if iron was linked to dopamine receptor availability and whether this association affected working memory. The study revealed that more iron was related to lower receptor availability in DLPFC and that this, coupled together with older age, contributed to reduced brain activity during a working-memory task. Additionally, the reduction in brain activity was in turn related to poorer task performance. Study III assessed (1) if brain iron content and accumulation were related to longitudinal changes in in brain activity during working-memory performance in normal aging, (2) potential association with glutamate, and (3) whether glutamate mitigated iron-brain activity relationship. In this study, we found that younger adults with initial elevated iron down-regulated more brain activity over a 3-year period, while performing the task. The results also showed a potential age-dependent relationship between iron and glutamate, such that younger adults with elevated iron content had more glutamate in DLPFC. Study IV explored biological and lifestyle factors that might influence iron accumulation in normal aging. Here, blood iron markers, physical activity, diet, and cardiovascular health significantly influenced brain iron content and accumulation. Furthermore, the associations between these factors and brain iron were influenced by age, highlighting the complexity of these relationships. Collectively, our studies show that age-related brain iron accumulation can be influenced by a number of factors, both modifiable and non-modifiable, such as lifestyle choices and genetic predisposition respectively. The potential to attenuate the accumulation of brain iron is essential, as we have also shown that iron can have deleterious effects on brain function and cognition older age. Finally, the links between iron and the dopaminergic system could partially explain age-related alterations, such as diminished receptor availability. Understanding the role of neurotransmitters on attenuating iron accumulation can pave the way for tailoring interventions in neurodegenerative disorders

Association of copper levels in the hair with gray matter volume, mean diffusivity, and cognitive functions

Although copper plays a critical role in normal brain functions and development, it is known that excess copper causes toxicity. Here we investigated the associations of copper levels in the hair with regional gray matter volume (rGMV), mean diffusivity (MD), and cognitive differences in a study cohort of 924 healthy young adults. Our findings showed that high copper levels were associated mostly with low cognitive abilities (low scores on the intelligence test consisting of complex speed tasks, involving reasoning task, a complex arithmetic task, and a reading comprehension task) as well as lower reverse Stroop interference, high rGMV over widespread areas of the brain [mainly including the bilateral lateral and medial parietal cortices, medial temporal structures (amygdala, hippocampus, and parahippocampal gyrus), middle cingulate cortex, orbitofrontal cortex, insula, perisylvian areas, inferior temporal lobe, temporal pole, occipital lobes, and supplementary motor area], as well as high MD of the right substantia nigra and bilateral hippocampus, which are indicative of low density in brain tissues. These results suggest that copper levels are associated with mostly aberrant cognitive functions, greater rGMV in extensive areas, greater MD (which are indicative of low density in brain tissues) in subcortical structures in the healthy young adults, possibly reflecting copper’s complex roles in neural mechanisms

Anatomic & metabolic brain markers of the m.3243A>G mutation: A multi-parametric 7T MRI study

One of the most common mitochondrial DNA (mtDNA) mutations, the A to G transition at base pair 3243, has been linked to changes in the brain, in addition to commonly observed hearing problems, diabetes and myopathy. However, a detailed quantitative description of m.3243A>G patients' brains has not been provided so far. In this study, ultra-high field MRI at 7T and volume- and surface-based data analyses approaches were used to highlight morphology (i.e. atrophy)-, microstructure (i.e. myelin and iron concentration)- and metabolism (i.e. cerebral blood flow)-related differences between patients (N = 22) and healthy controls (N = 15). The use of quantitative MRI at 7T allowed us to detect subtle changes of biophysical processes in the brain with high accuracy and sensitivity, in addition to typically assessed lesions and atrophy. Furthermore, the effect of m.3243A>G mutation load in blood and urine epithelial cells on these MRI measures was assessed within the patient population and revealed that blood levels were most indicative of the brain's state and disease severity, based on MRI as well as on neuropsychological data. Morphometry MRI data showed a wide-spread reduction of cortical, subcortical and cerebellar gray matter volume, in addition to significantly enlarged ventricles. Moreover, surface-based analyses revealed brain area-specific changes in cortical thickness (e.g. of the auditory cortex), and in T1, T2* and cerebral blood flow as a function of mutation load, which can be linked to typically m.3243A>G-related clinical symptoms (e.g. hearing impairment). In addition, several regions linked to attentional control (e.g. middle frontal gyrus), the sensorimotor network (e.g. banks of central sulcus) and the default mode network (e.g. precuneus) were characterized by alterations in cortical thickness, T1, T2* and/or cerebral blood flow, which has not been described in previous MRI studies. Finally, several hypotheses, based either on vascular, metabolic or astroglial implications of the m.3243A>G mutation, are discussed that potentially explain the underlying pathobiology. To conclude, this is the first 7T and also the largest MRI study on this patient population that provides macroscopic brain correlates of the m.3243A>G mutation indicating potential MRI biomarkers of mitochondrial diseases and might guide future (longitudinal) studies to extensively track neuropathological and clinical changes

In vivo MRI mapping of brain iron deposition across the adult lifespan

Disruption of iron homeostasis as a consequence of aging is thought to cause iron levels to increase, potentially promoting oxidative cellular damage. Therefore, understanding how this process evolves through the lifespan could offer insights into both the aging process and the development of aging-related neurodegenerative brain diseases. This work aimed to map, in vivo for the first time with an unbiased whole-brain approach, age-related iron changes using quantitative susceptibility mapping (QSM)—a new postprocessed MRI contrast mechanism. To this end, a full QSM standardization routine was devised and a cohort of N = 116 healthy adults (20–79 years of age) was studied. The whole-brain and ROI analyses confirmed that the propensity of brain cells to accumulate excessive iron as a function of aging largely depends on their exact anatomical location. Whereas only patchy signs of iron scavenging were observed in white matter, strong, bilateral, and confluent QSM–age associations were identified in several deep-brain nuclei—chiefly the striatum and midbrain—and across motor, premotor, posterior insular, superior prefrontal, and cerebellar cortices. The validity of QSM as a suitable in vivo imaging technique with which to monitor iron dysregulation in the human brain was demonstrated by confirming age-related increases in several subcortical nuclei that are known to accumulate iron with age. The study indicated that, in addition to these structures, there is a predilection for iron accumulation in the frontal lobes, which when combined with the subcortical findings, suggests that iron accumulation with age predominantly affects brain regions concerned with motor/output functions

Genetic, magnetic resonance imaging and body fluid biomarker associations with severity of multiple sclerosis

Multiple sclerosis is a chronic and progressive neuroinflammatory disease that leads to demyelination and neurodegeneration in the central nervous system (CNS). Previous research has identified a wide range of environmental, lifestyle and genetic factors which increase MS susceptibility. However, the pathomechanisms that influence the severity of MS are largely unknown, and adequate biomarkers of disease severity are consequently lacking. Therefore, the aim of my thesis was to; 1) assess associations between the nerve injury biomarker neurofilament light (NfL) and brain atrophy and lesion volumes; 2) assess which brain/lesion volume measures show the strongest longitudinal association with clinical MS disability measures and to what degree these associations were affected by age; and to 3) identify genetic variants associated with brain atrophy, lesion volumes and plasma NfL (pNfL) levels in persons with MS. In Study I, we assessed how cerebrospinal fluid (CSF) and pNfL levels were associated with T1- and T2-lesion volumes as well as whole-brain, cortical and subcortical grey matter, white matter and thalamic volume fractions of total intracranial volume based on magnetic resonance imaging (MRI). High baseline CSF and pNfL levels were associated with lower whole-brain, subcortical grey matter, thalamic, white matter and corpus callosal volume fractions over time. A further analysis showed that there was an association between baseline pNfL and baseline cortical grey matter fractions also in absence of radiological signs of inflammatory disease activity. A topographic analysis of cortical thickness showed that loss of cortical volume preferentially involved frontotemporal cortical regions. These findings indicate that NfL levels contribute information about MS severity not provided by traditional MRI lesion metrics. In Study II, we showed that associations between baseline MRI variables, and baseline physical disability and self-reported impact of MS rapidly increased in strength in individuals beyond approximately 40-50 years of age. In separate longitudinal analyses using linear mixed-effects models, we showed that among the recorded brain volume measures, cortical and subcortical grey matter and thalamic volume fractions at baseline were the strongest predictors of future worsening in clinical disability over a median of approximately ten years’ follow-up time. They were also stronger predictors than T1- and T2-lesion volumes. In Study III, we assessed if a weighted risk score comprising 12 known MS risk human leukocyte antigen (HLA) alleles was associated with baseline and longitudinal MRI measures as described in Studies I and II. While this risk score was not significantly associated with baseline MRI measures, we found that a high score was associated with lower cortical grey matter fractions longitudinally. A further analysis showed that this effect was primarily driven by the HLA-DRB1*15:01 allele. These results suggest that MS HLA risk variants not only affect inflammatory, but also neurodegenerative aspects of the disease. In Studies IV and V, we performed genome-wide association studies of pNfL levels and whole-brain volume fractions, respectively, in persons with MS (and controls in Study IV). While no genome-wide significant associations were found in Study IV, gene set analyses highlighted a neural crest and odontogenesis development pathway in the regulation of pNfL levels, and a weighted MS susceptibility polygenic risk score was associated with higher pNfL levels in MS with statistical significance. These findings suggest that there is some degree of genetic regulation of pNfL levels, which partially overlap with MS risk. In Study V, we identified a genome-wide significant locus upstream of the glycerol kinase 2 (GK2) gene, previously implicated in the propensity for tobacco smoking, which is a known MS risk and severity factor. Gene set analyses in Study V also implicated Hypoxia Inducible Factor-1 (HIF1) in the regulation of whole-brain volume fractions, indicating that iron metabolism and response to hypoxia play a role in the neurodegenerative processes in MS

Neuroimaging Biomarkers in Paediatric Sickle Cell Disease

Sickle Cell Disease (SCD) is a collection of genetic haemoglobinopathies, the most common and severe being homozygous sickle cell anaemia. In the UK, it has been estimated that 1 in 2000 children are born with SCD. The disease is characterised by chronic anaemia, recurrent pain crises and vascular occlusion. Neurologically, there is a high incidence of stroke in childhood, as well as cognitive dysfunction. Newborn screening programmes and preventative treatments have allowed a much longer lifespan; however recently, neurological research has shifted to characterising subtler aspects of brain development and functioning that may be critically important to the individual’s quality of life. This thesis overviews the neurological and neurocognitive complications of SCD, and how magnetic resonance imaging (MRI) can provide biomarkers for severity of disease. During the PhD, retrospective and prospective cognitive and MRI data were collected and analysed. Diagnostic clinical MRI sequences and advanced MRI sequences were applied, as well as a neuropsychological test battery aimed at intelligence and executive function. First, this thesis reviews the intelligence literature in SCD and includes previously unreported data, finding patients, regardless of abnormality seen on conventional MRI, have lowered full-scale intelligence quotient than controls. Then, to determine imaging biomarkers, volumetric differences and diffusion characteristics were identified. Patients were found to have decreased volumes of subcortical structures compared to controls, in groups corresponding to disease severity. Results from a three-year longitudinal clinical trial suggest evidence of atrophy in paediatric patients, with no apparent protective effect of treatment. Diffusion tensor imaging revealed reduced white matter integrity across the brain, correlating with recognised markers of disease severity (i.e. oxygen saturation and haemoglobin from a full blood count). Overall, the four experiments bridge a gap in the cognitive and neuroimaging literature of the extent of neurological injury in children with SCD