Biomarkers in frontotemporal lobar degeneration

Toward diffusion tensor imaging as a biomarker in neurodegenerative diseases: technical considerations to optimize recordings and data processing

Neuroimaging biomarkers have shown high potential to map the disease processes in the application to neurodegenerative diseases (NDD), e.g., diffusion tensor imaging (DTI). For DTI, the implementation of a standardized scanning and analysis cascade in clinical trials has potential to be further optimized. Over the last few years, various approaches to improve DTI applications to NDD have been developed. The core issue of this review was to address considerations and limitations of DTI in NDD: we discuss suggestions for improvements of DTI applications to NDD. Based on this technical approach, a set of recommendations was proposed for a standardized DTI scan protocol and an analysis cascade of DTI data pre-and postprocessing and statistical analysis. In summary, considering advantages and limitations of the DTI in NDD we suggest improvements for a standardized framework for a DTI-based protocol to be applied to future imaging studies in NDD, towards the goal to proceed to establish DTI as a biomarker in clinical trials in neurodegeneration.

Diagnostic Value of Diffusion Tensor Imaging and Positron Emission Tomography in Early Stages of Frontotemporal Dementia

BACKGROUND

Due to suboptimal sensitivity and specificity of structural and molecular neuroimaging tools, the diagnosis of behavioral variant frontotemporal dementia (bvFTD) remains challenging.

OBJECTIVE

Investigation of the sensitivity of diffusion tensor imaging (DTI) and fluorodeoxyglucose positron emission tomography (FDG-PET) to detect cerebral alterations in early stages of bvFTD despite inconspicuous conventional MRI.

METHODS

Thirty patients with early stages of bvFTD underwent a detailed neuropsychological examination, cerebral 3T MRI with DTI analysis, and FDG-PET. After 12 months of follow-up, all patients finally fulfilled the diagnosis of bvFTD. Individual FDG-PET data analyses showed that 20 patients exhibited a "typical" pattern for bvFTD with bifrontal and/or temporal hypometabolism (bvFTD/PET+), and that 10 patients showed a "non-typical"/normal pattern (bvFTD/PET-). DTI data were compared with 42 healthy controls in an individual and voxel-based group analysis. To examine the clinical relevance of the findings, associations between pathologically altered voxels of DTI or FDG-PET results and behavioral symptoms were estimated by linear regression analyses.

RESULTS

DTI voxel-based group analyses revealed microstructural degeneration in bifrontal and bitemporal areas in bvFTD/PET+ and bvFTD/PET- groups. However, when comparing the sensitivity of individual DTI data analysis with FDG-PET, DTI appeared to be less sensitive. Neuropsychological symptoms were considerably related to neurodegeneration within frontotemporal areas identified by DTI and FDG-PET.

CONCLUSION

DTI seems to be an interesting tool for detection of functionally relevant neurodegenerative alterations in early stages of bvFTD, even in bvFTD/PET- patients. However, at a single subject level, it seems to be less sensitive than FDG-PET. Thus, improvement of individual DTI analysis is necessary.

Longitudinal Diffusion Tensor Imaging Resembles Patterns of Pathology Progression in Behavioral Variant Frontotemporal Dementia (bvFTD)

Objective: Recently, the characteristic longitudinal distribution pattern of the underlying phosphorylated TDP-43 (pTDP-43) pathology in the behavioral variant of frontotemporal dementia (bvFTD) excluding Pick's disease (PiD) across specific brain regions was described. The aim of the present study was to investigate whether in vivo investigations of bvFTD patients by use of diffusion tensor imaging (DTI) were consistent with these proposed patterns of progression.

Methods: Sixty-two bvFTD patients and 47 controls underwent DTI in a multicenter study design. Of these, 49 bvFTD patients and 34 controls had a follow-up scan after ~12 months. Cross-sectional and longitudinal alterations were assessed by a two-fold analysis, i.e., voxelwise comparison of fractional anisotropy (FA) maps and a tract of interest-based (TOI) approach, which identifies tract structures that could be assigned to brain regions associated with disease progression.

Results: Whole brain-based spatial statistics showed white matter alterations predominantly in the frontal lobes cross-sectionally and longitudinally. The TOIs of bvFTD neuroimaging stages 1 and 2 (uncinate fascicle—bvFTD pattern I; corticostriatal pathway—bvFTD pattern II) showed highly significant differences between bvFTD patients and controls. The corticospinal tract-associated TOI (bvFTD pattern III) did not differ between groups, whereas the differences in the optic radiation (bvFTD pattern IV) reached significance. The findings in the corticospinal tract were due to a “dichotomous” behavior of FA changes there.

Conclusion: Longitudinal TOI analysis demonstrated a pattern of white matter pathways alterations consistent with patterns of pTDP-43 pathology.

CSF sAPPβ, YKL-40, and neurofilament light in frontotemporal lobar degeneration

Objective:

To analyze the clinical utility of 3 CSF biomarkers and their structural imaging correlates in a large cohort of patients with different dementia and parkinsonian syndromes within the spectrum of frontotemporal lobar degeneration (FTLD).

Methods:

We analyzed 3 CSF biomarkers (YKL-40, soluble β fragment of amyloid precursor protein [sAPPβ], neurofilament light [NfL]) and core Alzheimer disease (AD) biomarkers (β-amyloid1-42, total tau, phosphorylated tau) in patients with FTLD-related clinical syndromes (n = 159): behavioral variant of frontotemporal dementia (n = 68), nonfluent (n = 23) and semantic (n = 19) variants of primary progressive aphasia, progressive supranuclear palsy (n = 28), and corticobasal syndrome (n = 21). We also included patients with AD (n = 72) and cognitively normal controls (CN; n = 76). We compared cross-sectional biomarker levels between groups, studied their correlation with cortical thickness, and evaluated their potential diagnostic utility.

Results:

Patients with FTLD-related syndromes had lower levels of sAPPβ than CN and patients with AD. The levels of sAPPβ showed a strong correlation with cortical structural changes in frontal and cingulate areas. NfL and YKL-40 levels were high in both the FTLD and AD groups compared to controls. In the receiver operating characteristic analysis, the sAPPβ/YKL-40 and NfL/sAPPβ ratios had areas under the curve of 0.91 and 0.96, respectively, distinguishing patients with FTLD from CN, and of 0.84 and 0.85, distinguishing patients with FTLD from patients with AD.

Conclusions:

The combination of sAPPβ with YKL-40 and with NfL in CSF could be useful to increase the certainty of the diagnosis of FTLD-related syndromes in clinical practice.

Classification of evidence:

This study provides Class III evidence that CSF levels of sAPPβ, YKL-40, and NfL are useful to identify patients with FTLD-related syndromes.

The role of diffusion tensor imaging in detecting microstructural changes in prodromal Alzheimer's disease

The MRI technique diffusion tensor imaging (DTI) is reviewed along with microstructural changes associated with prodromal Alzheimer's disease (AD) as a potential biomarker for clinical applications. The prodromal stage of AD is characterized by mild cognitive impairment (MCI), representing a transitional state between normal aging and AD. Microstructural abnormalities on DTI are promising in vivo biomarkers of gray and white matter changes associated with the progression of AD pathology. Elevated mean diffusivity and decreased fractional anisotropy are consistently found in prodromal AD, and even in cognitively normal elderly who progress to MCI. However, quality of parameter maps may be affected by artifacts of motion, susceptibility, and eddy current-induced distortions. The DTI maps are typically analyzed by region-of-interest or voxel-based analytic techniques such as tract-based spatial statistics. DTI-based index of diffusivity is complementary to macrostructural gray matter changes in the hippocampus in detecting prodromal AD. Breakdown of structural connectivity measured with DTI may impact cognitive performance during early AD. Furthermore, assessment of hippocampal connections may help in understanding the cerebral organization and remodeling associated with treatment response.

Patterns of brain atrophy in clinical variants of frontotemporal lobar degeneration

BACKGROUND/AIMS

The clinical syndromes of frontotemporal lobar degeneration include behavioral variant frontotemporal dementia (bvFTD) and semantic (SV-PPA) and nonfluent variants (NF-PPA) of primary progressive aphasia. Using magnetic resonance imaging (MRI), tensor-based morphometry (TBM) was used to determine distinct patterns of atrophy between these three clinical groups.

METHODS

Twenty-seven participants diagnosed with bvFTD, 16 with SV-PPA, and 19 with NF-PPA received baseline and follow-up MRI scans approximately 1 year apart. TBM was used to create three-dimensional Jacobian maps of local brain atrophy rates for individual subjects.

RESULTS

Regional analyses were performed on the three-dimensional maps and direct comparisons between groups (corrected for multiple comparisons using permutation tests) revealed significantly greater frontal lobe and frontal white matter atrophy in the bvFTD relative to the SV-PPA group (p < 0.005). The SV-PPA subjects exhibited significantly greater atrophy than the bvFTD in the fusiform gyrus (p = 0.007). The NF-PPA group showed significantly more atrophy in the parietal lobes relative to both bvFTD and SV-PPA groups (p < 0.05). Percent volume change in ventromedial prefrontal cortex was significantly associated with baseline behavioral symptomatology.

CONCLUSION

The bvFTD, SV-PPA, and NF-PPA groups displayed distinct patterns of progressive atrophy over a 1-year period that correspond well to the behavioral disturbances characteristic of the clinical syndromes. More specifically, the bvFTD group showed significant white matter contraction and presence of behavioral symptoms at baseline predicted significant volume loss of the ventromedial prefrontal cortex.

Targeting RNA binding proteins involved in neurodegeneration

Dysfunctions at the level of RNA processing have recently been shown to play a fundamental role in the pathogenesis of many neurodegenerative diseases. Several proteins responsible for these dysfunctions (TDP-43, FUS/TLS, and hnRNP A/Bs) belong to the nuclear class of heterogeneous ribonucleoproteins (hnRNPs) that predominantly function as general regulators of both coding and noncoding RNA metabolism. The discovery of the importance of these factors in mediating neuronal death has represented a major paradigmatic shift in our understanding of neurodegenerative processes. As a result, these discoveries have also opened the way toward novel biomolecular screening approaches in our search for therapeutic options. One of the major hurdles in this search is represented by the correct identification of the most promising targets to be prioritized. These may include aberrant aggregation processes, protein-protein interactions, RNA-protein interactions, or specific cellular pathways altered by disease. In this review, we discuss these four major options together with their various advantages and drawbacks.

Frontotemporal lobar degeneration: epidemiology, pathology, diagnosis and management

Frontotemporal lobar degeneration (FTLD) describes a spectrum of clinically, pathologically and genetically heterogeneous neurodegenerative disorders of unknown aetiology. FTLD spectrum disorders collectively represent a leading cause of early-onset dementia, with most cases presenting between 45 and 64 years of age. FTLD is characterized by progressive changes in behaviour, executive dysfunction and/or language impairment and can be differentiated clinically into three frontotemporal dementia (FTD) syndromes as follows: (i) behavioural variant (bvFTD); (ii) semantic dementia (SD); and (iii) progressive nonfluent aphasia (PNFA). Additionally, there is a significant clinical, pathological and genetic overlap between FTD and motor neuron disease/amyotrophic lateral sclerosis (FTD-ALS) and the atypical parkinsonian syndromes, progressive supranuclear palsy (PSP) and corticobasal syndrome (CBS). bvFTD is characterized by progressive behavioural impairment and a decline in executive function with frontal lobe-predominant atrophy, SD by a loss of object knowledge with prominent anomia and asymmetrical atrophy of the anterior temporal lobes and PNFA by expressive or motor speech deficits with predominantly left peri-sylvian atrophy. Recent advances in molecular biology and immunohistochemical staining techniques have further classified the FTLD spectrum disorders based upon the predominant neuropathological protein into three main categories: (i) microtubule-associated protein tau (FTLD-TAU); (ii) TAR DNA-binding protein-43 (FTLD-TDP); and (iii) fused in sarcoma protein (FTLD-FUS). Up to 40% of FTD patients report a family history of neurodegenerative illness, and one-third to one-half of familial cases of FTD follow an autosomal dominant inheritance pattern. Mutations in MAPT, PGRN, TARDBP, VCP and CHMP2B have been described, along with a recently identified C9ORF72 hexanucleotide repeat expansion. To date, there are no US FDA-approved treatments or disease-modifying therapies for FTD. Pharmacological strategies have focused on neurotransmitter replacement and modulation for the treatment of behavioural, motor and cognitive symptoms of FTD, and include selective serotonin reuptake inhibitors (SSRIs), atypical antipsychotics, acetylcholinesterase inhibitors and glutamate NMDA receptor antagonists. At present, adequate management of FTD symptoms involves a combination of pharmacological therapy with behavioural, physical and environmental modification techniques.

TDP-43 autoregulation: implications for disease

TDP-43 is a nuclear protein that has been shown to play a central role in RNA metabolism. In recent years, this protein has become very important in the study of neurodegenerative diseases such as amyotrophic lateral sclerosis and frontotemporal lobar degeneration (FTLD). These diseases share, as common feature, the presence of abnormally aggregated, posttranslationally modified, and mislocalized TDP-43 in the cell cytoplasm of both neurons and glial cells. A major question in TDP-43 research is represented by the investigation of the mechanism(s) that trigger this process and its potential consequences. Regarding the first issue, it is likely that relative protein expression levels might play an important role as has been demonstrated for many protein aggregation processes. In fact, the eventual misregulation of TDP-43 expression leading to enhanced protein production might well correlate with enhanced aggregation, and thus results in increasingly harmful gain- or loss-of-function effects on cellular metabolism. For this reason, it is important to determine the mechanisms that act to regulate TDP-43 levels within the cell. In normal conditions, it is now clear that TDP-43 can modulate its own protein levels through a negative feedback loop triggered by binding to its own RNA in the 3'UTR region leading to mRNA degradation. This work discusses how an eventual disruption of this mechanism might affect TDP-43 pathology, focusing in particular on its association with stress granules and intrinsic aggregation properties.

Neuroimaging findings in frontotemporal lobar degeneration spectrum of disorders

Frontotemporal lobar degeneration (FTLD) is a clinically and pathologically heterogeneous spectrum of disorders. In the last few years, neuroimaging has contributed to the phenotypic characterisation of these patients. Complementary to the clinical and neuropsychological evaluations, structural magnetic resonance imaging (MRI) and functional techniques provide important pieces of information for the diagnosis of FTLD. They also appear to be useful in distinguishing FTLD from patients with Alzheimer's disease (AD). Preliminary studies in pathologically proven cases suggested that distinct patterns of tissue loss could assist in predicting in vivo the pathological subtype. Recent years have also witnessed impressive advances in the development of novel imaging approaches. Diffusion tensor MRI and functional MRI have improved our understanding of the pathophysiology of the disease, and this should lead to the identification of additional useful markers of disease progression. This reviews discusses comprehensively the state-of-the-art of neuroimaging in the study of FTLD spectrum of disorders, and attempts to envisage which will be new neuroimaging biomarkers that could serve as surrogate measures of the underlying pathology. This will be central in the design of treatment trials of experimental drugs, which are likely to emerge in the near future, to target the pathological processes associated with this condition.