Recent advances in understanding frontotemporal degeneration

Frontotemporal degeneration (FTD) is a heterogeneous spectrum of neurodegenerative disorders characterized by diverse clinical presentations, neuropathological characteristics, and underlying genetic causes. In the last few years, several advances in the knowledge of clinical and biological aspects have been accomplished and three major scenarios have emerged that will represent the core issues in the FTD scene over the next few years. Foremost, the development of cerebrospinal fluid and blood biomarkers as well as neuroimaging techniques will aid the pursuit of new diagnostic and prognostic markers able to identify the ongoing proteinopathy and predict disease progression, which is key in identifying and stratifying patients for enrolment in clinical trials as well as evaluating response to treatment. On the other hand, current research has focused on the first attempts to slow down or revert disease progression, with the identification of disease modulators associated with disease onset and the ongoing development of the first pharmacological treatments for both sporadic and genetic FTD. Future research will certainly improve our knowledge of FTD and possibly open up a new era of disease-modifying therapies for this still-orphan disorder.


Introduction
Frontotemporal degeneration (FTD) is a highly heterogeneous spectrum of neurodegenerative disorders, not only clinically but also genetically and neuropathologically 1 . This significant heterogeneity has important implications for both diagnostic and therapeutic purposes. Considered in the past as a rare disease, FTD has been clearly shown by recent epidemiological studies 2 and the refinement of new clinical criteria 3,4 to be much more frequent than previously thought.
In the last few years, a giant step forward in the knowledge of clinical and biological aspects has been accomplished. Clinically, FTD is characterized by behavioral abnormalities, language impairment, and deficits of executive functions 5,6 . According to newly revised criteria, three different clinical variants have been defined: the behavioral variant of FTD (bvFTD) 4 , the agrammatic variant of primary progressive aphasia (avPPA), and the semantic variant of PPA (svPPA) 3 .
One of the most compelling chapters in the recent literature of FTD is represented by the identification of causative mutations responsible for monogenic autosomal dominant disease. Several genetic mutations have been associated with FTD, the most common being those in the microtubule-associated protein tau (MAPT) gene, the granulin (GRN) gene, or the expansion on chromosome 9 open reading frame 72 (C9orf72) gene 7,8 . However, other genes are associated with rare FTD cases, such as valosin-containing protein (VCP) mutations, which are linked to a specific condition called inclusion body myopathy with Paget disease of the bone and FTD (IBMPFD) 9,10 , charged multivesicular body protein 2B (CHMP2B), which is involved in the endosomal-lysosomal pathway 11,12 , fused in sarcoma (FUS) 13,14 , TAR DNA-binding protein (TARDBP) 15,16 , sequestosome 1 (SQSTM1) 17-20 , TANKbinding kinase 1 (TBK1) 21-24 , and ubiquilin 2 (UBQLN2) 25-27 .
Along with heterogeneity in both clinical presentations and different autosomal dominant inherited genetic traits, diversified neuropathological hallmarks may be responsible for distinct frontotemporal lobar degeneration (FTLD) diagnoses, which are constituted primarily by tau or TAR DNA-binding protein 43 (TDP-43) depositions 28,29 . Therefore, the critical issue in the current literature of FTD is the lack of a clear-cut relationship between clinical phenotype and neuropathological features. Up to now, the ongoing neuropathological process, and consequently the underlying pathogenetic mechanisms, may be predicted only in monogenic disease. In this case, it is well established that GRN mutations and C9orf72 expansions are associated with FTLD-TDP neuropathology while MAPT mutations are associated with FTLD-Tau 30 . Recent work has also explored the structural and functional neural correlates of behavioral symptoms in FTD. Among these studies, a few have focused on the gray matter correlates of apathy and disinhibition 31-34 , while others have attempted to connect the white matter associations with behavioral symptoms 33,35-37 .
According to these premises, three major scenarios have come out of the recent advances in the field and will represent the core issues in the FTD scene over the next few years. On one hand, the search for diagnostic and prognostic markers able to identify the ongoing proteinopathy in FTD and predict disease progression will be key in identifying and stratifying patients for enrolment in clinical trials. On the other hand, current research has focused on the first attempts to slow down or revert disease progression, with the identification of disease modulators associated with disease onset and the ongoing development of the first pharmacological treatments in a currently considered orphan disorder.

Looking for neuropathology and prognostic markers
In the past few years, many efforts have been devoted to finding reliable markers which reflect the ongoing neuropathological process in non-genetic FTD. These markers are critical for evaluating potential disease-modifying treatments targeting either tau or TDP-43 pathological mechanisms in homogeneous groups, independently of clinical phenotype, and to better understand the disease pathophysiology 38 .
Hu and colleagues proposed the cerebrospinal fluid (CSF) phospho-tau 181 to total tau (p/t-tau) ratio as a viable biomarker to identify FTLD with TDP-43 pathology as compared to FTLD-Tau, with a reduced ratio suggestive of TDP-43 pathology 39 . These findings have been confirmed in a few subsequent studies in FTD 40-44 , as well as in amyotrophic lateral sclerosis (ALS), which frequently is associated with TDP-43 pathology 45 . Conversely, a recent report investigated novel CSF tau fragments (N-123, N-mid-region, N-224, and X-368) in both FTLD-TDP and FTLD-Tau; however, none of the novel tau species showed a significant difference between pathological groups 41 . Similarly, CSF and plasma TDP-43 dosages have not yielded convincing results in differentiating FTLD-TDP from FTLD-Tau 46-51 .
If markers of the neuropathological process are far from being introduced in clinical practice, recent work has identified prognostic markers which will play an important role in forthcoming trials to assess treatment response. Above all, the assessment of neurofilament light chain (NfL), both in CSF and in blood, has recently gained great interest in the field of neurodegenerative disorders, with very high reliability of the assay obtained with advanced technologies 52 . Even though increased NfL levels, reflecting axonal damage, seem to be non-specific for FTD, this marker appears to be a measure of disease intensity and predicts progression and survival 44,53-57 . Longitudinal analysis of samples seems to suggest that levels change not long prior to symptom onset in genetic FTD, increasing by threefold to fourfold during conversion 58 . Thus, a decrease in NfL levels could be a measure of successful disease modification in clinical trials 59 .
Regarding C9orf72 expansion carriers, the putative pathophysiological mechanisms include loss of C9orf72 function as well as toxicity arising from the accumulation of sense and antisense transcripts of the expanded repeats. These RNA transcripts serve as templates for the synthesis of proteins of repeating dipeptides through repeat associated non-ATG (RAN) translation, as poly(GP) 60-62 . Increased CSF poly(GP) levels have been observed in C9orf72 expansion carriers in both the presymptomatic and the symptomatic phase, suggesting that this marker might be altered long before the onset of clinical symptoms and could be useful as a preclinical biomarker in clinical trials in genetic FTD 63-65 .
Concomitantly to biomarker research, positron emission tomography (PET) techniques have paved the way for breakthroughs to assess pathophysiology in vivo in FTD 66 . In particular, PET imaging of tau burden represents one of the most recent and promising applications in neurodegenerative dementias 67 , and to date a number of tau tracers have been developed, such as [ 18 F]flortaucipir (formerly known as [ 18 F]AV1451), [ 18 F]T807, and [ 11 C]PBB3 68,69 . However, studies of tau radioligands have so far not been proven to be particularly helpful in FTD, binding much more strongly to paired helical filament (PHF)-tau found mainly in Alzheimer's disease 70,71 than to other forms of tau found in the primary tauopathies such as progressive supranuclear palsy 72-75 or corticobasal degeneration 76-78 . Only two MAPT mutations (V337M and R406W) are associated with PHF-tau and have shown strong binding with [ 18 F]flortaucipir 79 . New-generation tracers are currently under evaluation to overcome the non-specific/off-target binding, as binding has been reported in non-tau diseases such as in C9orf72 mutation or svPPA, which are mainly associated with TDP-43 pathology 80-83 . Furthermore, the development of PET tracers able to specifically bind 3R or 4R tauopathies would have remarkable applications in clinical practice 66 .
Considering that most tau tracers show non-specific/off-target binding with considerable overlap with TDP-43 pathology, the ongoing development of PET tracers specific for TDP-43 aggregates could have significant implications not only for FTD but also for ALS research 84 . These ligands are expected to provide early and more accurate diagnosis of disease, help to monitor disease progression over time, and evaluate whether various therapeutic treatments are having a positive effect in individual patients.

Looking for disease modulators
Postponing disease onset in FTD is a mandatory issue in the field, especially in presymptomatic individuals carrying pathogenetic mutations. Disease onset is highly variable in each of the genetic forms of FTD, with even intrafamilial variability 85 .
A number of genetic modifiers have been identified in monogenic FTD, thanks to the development of collaborative international multicenter studies, such as GENFI in Europe and in Canada (http://genfi.org.uk) 86 and ARTFL/LEFFTDS in the US (https://ncrad.iu.edu/index.html) 87 .
In GRN and C9orf72 carriers, the TMEM106B genotype has been clearly associated with age at disease onset [88][89][90] . Another study of C9orf72 carriers identified two overlapping genes (LOC101929163 and C6orf10) in which a polymorphism was associated with disease onset 91 . The significance of the C9orf72 repeat expansion length remains unclear, with no definitive evidence of an association with age of onset 92-97 . Other genes, such as small G-protein signaling modulator 3 (SGSM3), which is involved in vesicular transport, have been identified as potential disease modifiers in C9orf72 carriers 98 . Little is known about factors that modify age at onset in MAPT mutation carriers 99 .
Beyond genetic modifiers that are inherited and immutable, the possibility to intervene with environmental and other modulating factors is attractive. Some evidence shows that cognitive stimulating environments lead to brain volumetrical advantages and better cognitive performances in healthy individuals and in neurodegenerative disorders 100 .
In the same view, in FTD patients, brain reserve, as measured by educational attainment, contributes to resilience against brain damage 101-103 . More interestingly, it has been recently demonstrated that highly educated at-risk subjects, carrying pathogenetic mutations associated with FTD, had better cognition and higher gray matter volume and showed slower loss of gray matter over time 104,105 . Thus, even in the presence of ongoing pathological processes, education may facilitate both brain reserve and brain maintenance in the presymptomatic phase of genetic FTD, virtually turning back the clock of the natural history of the disease. The demonstration that differences in early lifestyle may slow down later disease progression suggests that, even in monogenic disorders, outcomes are not completely determined from birth, and this opens up exciting perspectives for eventually delaying symptom onset 104 .

Disease-modifying therapeutic approaches
Clinical trial research in FTD spectrum disorders is in its infancy, and there are currently no approved disease-modifying therapies for sporadic or genetic FTD, but trials are now underway or planned for each of the three main FTD genetic mutations 99,106 .
GRN mutations, thought to act through loss-of-function mechanisms, are associated with decreased levels of the protein in serum and CSF [107][108][109] . Thus, restoring GRN function by targeting its receptors 110,111 or increasing protein synthesis is a promising strategy 112,113 . In this view, an emerging target is sortilin, which serves as a lysosomal trafficking receptor for progranulin, and sortilin-mediated progranulin endocytosis has been implicated in FTD pathophysiology 114 . Recombinant human anti-human sortilin monoclonal IgG1 antibodies (AL001) have been developed by Alector and are currently being investigated in a randomized controlled clinical trial (NCT03636204).
Antisense oligonucleotides (ASOs) are short oligonucleotide sequences that can disrupt RNA processing or transduction by activating RNA degradation or preventing ligation with particular RNA-binding proteins 115 . One study in ALS patients found that the ASO drug ISIS 333611 was well tolerated 116 , which suggests that ASOs could be beneficial in neurological disorder treatment 117 .
Small molecule therapies and tau monoclonal antibodies are also being developed for tauopathies (with the potential for use in MAPT mutations) 118 . Moreover, tau ASOs are also being planned for MAPT mutation carriers, with the objective of binding to tau messenger RNA and intercepting its translation, lowering the amount of tau protein in the brain. Currently, a clinical trial with a tau ASO (IONIS MAPTRx) is underway in mild Alzheimer's disease (NCT03186989).
The development of new pharmacological interventions and precision-medicine approaches will help to steer clinical trials in FTD spectrum disorders in a productive direction 119 .

Conclusions
Several advances have been accomplished in recent years in the scenario of FTD. Both CSF and blood biomarkers as well as novel neuroimaging techniques are emerging as promising markers of disease pathophysiology and disease progression. Further effort is needed to identify preclinical biomarkers in the view of innovative clinical trials, in which disease-modifying therapies might need to be administered early in the disease course, as it is becoming clear also for Alzheimer's disease. Indeed, up to now, only CSF and plasma progranulin levels for GRN carriers and CSF poly(GP) dipeptide repeat proteins for C9orf72 expansion carriers seem to be significantly altered long before symptom onset 99 , while neuroimaging abnormalities seem to become discernible about 10-15 years before symptom onset 30,120,121 , as evaluated in large cohorts of patients which may lack single-subject applicability. Emerging neurophysiological techniques such as transcranial magnetic stimulation may be useful in the identification of disease progression, even in the presymptomatic phase of monogenic FTD, long before the onset of clinical symptoms 107,[122][123][124] .
Disease modulators, both genetic and environmental, which play an important role in disease onset and progression, may also become targets of therapeutic clinical trials, inducing modifications in the underlying pathophysiological cascade which results eventually in FTD. In this view, several innovative therapies such as ASOs, small molecules, and monoclonal antibodies are in the pipeline for the treatment of both sporadic and genetic FTD. Future research will certainly improve our knowledge of FTD and possibly open up a new era of diseasemodifying therapies for this still-orphan disorder.