Frontal temporal dementia: dissecting the aetiology and pathogenesis

Frontotemporal dementia: Addressing the scattered harbingers of genetics and its relationship with glucose metabolism, bipolar disorder, and amyotrophic lateral sclerosis

Frontotemporal Dementia, also known by the name Pick's disease, is a rare form of dementia that can run for several generations. The two key characteristics are argyrophilic, spherical intraneuronal inclusions, which most frequently impact the frontal and temporal poles, and localized cortical atrophy (Pick bodies). Although personality decline and memory loss are frequently more severe than the visuospatial and apraxia disorders that are common in Alzheimer's disease, clinical overlap with other non-Alzheimer degenerative disorders is being increasingly recognized. The limbic system, which includes the hippocampus, entorhinal cortex, and amygdala, typically experiences the greatest levels of neuronal loss and degeneration. In the hippocampus's dentate fascia, several Pick bodies are frequently seen. Leukoencephalopathy and inflated cortical neurons are less specific symptoms (Pick cells). In this paper, we review the factors leading to Picks disease along with its pathophysiology, clinical manifestations, diagnosis, imaging, treatment, prognosis, and a comprehensive discussion on the same. We have also discussed the relationship of frontotemporal dementia with glucose metabolism, bipolar disorder, and amyotrophic lateral sclerosis, all of which are emerging fields of interest and need more studies.

Challenges and new opportunities in the investigation of new drug therapies to treat frontotemporal dementia

BACKGROUND

Frontotemporal dementia spectrum disorders are a set of neurodegenerative disorders affecting the frontal and anterior temporal lobes. They are often fatal, and currently no medications have been shown to slow their progression. Recent developments in understanding these disorders may aid in developing treatments.

OBJECTIVE

To discuss the development of drug therapies for frontotemporal dementia spectrum disorders, both those under current investigation and those that could be targets for future investigation.

METHODS

This review is divided into four sections: First, a brief review of frontotemporal dementia spectrum disorders; second, a discussion of the challenges in the development of drug therapies third, a review of the current clinical trials; and finally a discussion of some recent discoveries, which have sparked new areas of investigation.

CONCLUSIONS

Hopefully, advances in understanding of frontotemporal dementia spectrum disorders and clinical trial design will aid the development of new treatments.

Lrrk2 G2019S substitution in frontotemporal lobar degeneration with ubiquitin-immunoreactive neuronal inclusions

Leucine-rich repeat kinase 2 (LRRK2) mutation carriers can develop clinical symptoms other than typical parkinsonism such as dementia, amyotrophy or dystonia. To determine if LRRK2 mutations might be involved in frontotemporal dementia (FTD), 5 individuals with multiplex familial FTD kindreds and 41 pathologically confirmed cases of FTD, including 23 with a family history of dementia, were screened for genetic variations in the LRRK2 gene. We identified a LRRK2 mutation leading to the G2019S amino acid substitution in a 79-year-old woman with frontotemporal lobar degeneration with ubiquitinated neuronal intranuclear inclusions (FTLD-U/NII) and a possible family history of tremor. These findings may be coincidental; however, there is a small nucleus of LRRK2-positive patients displaying atypical features suggesting a role for this protein in other neurodegenerative disorders.

The complex aetiology of frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) is now a widely recognised form of dementia. This heterogeneous disease has been of particular interest to geneticists due to its high rate of heritability with up to 40% of patients reporting a family history of the disease in at least one extra family member. There have been several chromosome loci linked to this disorder and three genes have already been identified. Remarkably, it has been recently demonstrated that 2 of these are only 1.7 Mb from one another on chromosome 17q21, these being tau and progranulin. The identification of these genes has contributed greatly to our understanding of the differing neuropathologies associated with FTLD. Furthermore, the discovery that TDP-43 is a component of the neuronal inclusions seen in the most common neuropathological subtype has also helped expand the biochemical pathways that are the focus of much FTLD research. Nevertheless, other genes causing FTLD remain to be identified and their biology elucidated before we have a complete understanding of the complex aetiology of this disease.

Recent advances in the genetics of amyotrophic lateral sclerosis and frontotemporal dementia: common pathways in neurodegenerative disease

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease classically defined by the impairment of the voluntary motor system and ubiquitin-positive intraneuronal aggregates in anterior horn cells. Frontotemporal dementia (FTD) is a common form of neurodegenerative dementia and presents with personality change associated in a significant subgroup of patients with cortical ubiquitin-only neuropathology (FTD-U). Careful study of ALS as well as FTD patient cohorts suggests clinical as well as pathological overlap of ALS with FTD. The idea that this reflects a shared pathogenesis has received strong support from the identification of new genetic loci on chromosome 9p and of mutations in specific genes (CHMP2B and DCN1) in families with co-segregation of ALS and FTD. The identification of two further genetic causes of FTD-U with (rare) ALS (PGRN) or without ALS (VCP) also provides a starting point for exploring the pathways associated with ubiquitin-mediated protein mishandling in FTD-U and ALS. Pure ALS, through ALS with cognitive impairment and ALS-FTD to pure FTD-U, may represent a continuous spectrum of ubiquitin-associated neurodegenerative disease.

Characteristics of frontotemporal dementia patients with a Progranulin mutation

OBJECTIVE

Mutations in the Progranulin gene (PGRN) recently have been discovered to be associated with frontotemporal dementia (FTD) linked to 17q21 without identified MAPT mutations. The range of mutations of PGRN that can result in the FTD phenotype and the clinical presentation of patients with PGRN mutations have yet to be determined.

METHODS

In this study, we examined 84 FTD patients from families not known previously to have illness linked to chromosome 17 for identified PGRN and MAPT mutations and sequenced the coding exons and the flanking intronic regions of PGRN. We compared the prevalence, clinical characteristics, magnetic resonance imaging and 18-fluoro-deoxyglucose positron emission tomography results, and neuropsychological testing of patients with the PGRN R493X mutation with those patients without identified PGRN mutations.

RESULTS

We discovered a new PGRN mutation (R493X) resulting in a stop codon in two patients. This was the only PGRN mutation identified in our sample. The patients with the PGRN R493X mutation had a rapid illness course and had predominant right-sided atrophy and hypometabolism on magnetic resonance imaging and 18-fluoro-deoxyglucose positron emission tomography. The affected father of one of the patients with the PGRN R493X mutation showed frontal and temporal atrophy without neurofibrillary tangles on neuropathological examination.

INTERPRETATION

Known PGRN and MAPT mutations were rare and of similar prevalence in our sample (2 compared with 1/84). The patients with the PGRN R493X mutation had a clinical presentation comparable with other behavior-predominant FTD patients. The neuropathology of an affected family member of a patient with the PGRN R493X mutation appears not to be Alzheimer's disease.

The genetics of neurodegenerative diseases

In the last 50 years, an enormous amount of progress has been made in dissecting the etiology of hereditary neurodegenerative diseases, including the dementias, the parkinsonisms, the ataxias and the motor-neuron diseases. In addition, these genetic findings are beginning to provide insights into the pathogeneses of the sporadic forms of the diseases. Through animal and cellular modeling studies we are beginning to gain insights into the pathogenic pathways to disease. This mechanistic understanding is now leading to therapeutic strategies based on this new understanding. As yet, however, no mechanistic therapies are in use in the clinic.