Dissecting the Clinical Heterogeneity and Genotype-Phenotype Correlations of MAPT Mutations: A Systematic Review

Background : Microtubule-associated protein tau ( MAPT ) mutations are one of the main causes of genetic Frontotemporal dementia (FTD) and are characterised by high clinical heterogeneity. A behavioural variant of FTD is the principal phenotype, but other rarer phenotypes are described, mostly reported as single cases. In this review, we provide an overview of the clinical phenotypes associated with MAPT mutations in order to define their characteristics and explore genotype-phenotype correlations. Methods : We performed systematic bibliographic research on the Pubmed database, focusing on articles published between 1998 and 2022. We analysed the clinical phenotype of 177 patients carrying MAPT mutations, focusing on the rarest ones. We performed a narrative synthesis of the results. Results : Regarding language phenotypes, the most frequent were the non-fluent variant and the semantic variant of Primary Progressive Aphasia (nfvPPA, svPPA), approximately in the same proportion. Almost 20% of the whole group of patients present a clinical phenotype belonging to the corticobasal syndrome-progressive supranuclear palsy (CBS-PSP) spectrum. While no clear genotype-phenotype correlation could be identified, some mutations were associated with a specific phenotype, while others gave origin to multiple clinical pictures and mixed phenotypes. Conclusions : A high clinical heterogeneity exists in FTD associated with MAPT mutations without a clear phenotype-genotype correlation in most cases. However, some characteristics can be helpful to drive genetic testing. Deep phenotyping of patients, together with functional studies of single mutations, particularly those associated with atypical phenotypes, are necessary to better understand the biological mechanisms underlying this clinical variability.


Introduction
Frontotemporal dementia (FTD) refers to a group of disorders characterised by striking clinical and pathological heterogeneity.Up to 40% of FTD are inherited forms caused by mutations in the progranulin (GRN), in the chromosome 9 open reading frame 72 (C9orf72) or in the microtubule-associated protein tau (MAPT) gene in most cases.The latter is responsible for approximately 5-10% of all familial FTD cases [1].Almost 90 pathogenic MAPT mutations have been discovered so far (Human Gene Mutation Database, https://digitalinsights.qiagen.com/products-overview/clinical-insights-portfolio/human-gene-mutation-database/; [2]), causing different effects on tau protein, with alteration of: (i) tau affinity for microtubules, leading to microtubule destabilisation; (ii) tau aggregation properties; (iii) the physiological proportion of 4R and 3R tau isoforms [3].The clinical phenotypes associated with FTD are highly heterogeneous, with some differences between the three main genetic forms [4].In MAPT mutation, the age at onset is usually earlier than in GRN and C9orf72 mutations and wide phenotypic variability, in terms of age of onset, clinical presentation and features of pathological tau deposits in the brain [1], is evident.The most common phenotype is the behavioural variant (bvFTD), while primary progressive aphasia (PPA) is less frequently described.bvFTD is characterised by progressive atrophy in the frontal and temporal lobes, causing behavioural, cognitive and personality alterations.PPAs include a heterogeneous group of diseases mainly affecting language.Depending on the brain area involved, symptoms vary from progressive loss of meaning and single-word comprehension deficits (semantic variant of PPA) to slow, effortful, hesitant and distorted speech (non-fluent variant of PPA).In addition, other phenotypes such as Corticobasal Syndrome (CBS), Progressive Supranuclear Palsy (PSP) and amnestic syndrome consistent with Alzheimer's disease (AD-like) are associated with MAPT.CBS is characterised by asymmetric motor signs (rigidity, tremor, dystonia, and myoclonus) and is often associated with apraxia, cortical sensory deficits and alien limb phenomena.PSP affects body movements, causing early loss of balance, difficulty in walking or swallowing, slurred speech and eye movement impairment.AD-like phenotypes caused by MAPT mutations are usually characterised by predominantly episodic memory deficits at onset.In this review, we provide an overview of the clinical phenotypes associated with MAPT mutations, focusing on the rarest ones and on those belonging to the Frontotemporal Lobar Degeneration (FTLD) spectrum, trying to better define their characteristics and to explore genotype-phenotype correlations.

Materials and Methods
We conducted systematic bibliographic research on the Pubmed database.The search focused on articles published between 1998 (the year of the discovery of the MAPT gene) and 2022.We applied the following terms of search: (MAPT) AND (FTD), (MAPT) AND (frontotemporal dementia), (MAPT) AND (FTDP-17), (MAPT) AND Since we primarily focused on the FTLD spectrum, we did not include the terms "AD" or "Alzheimer Disease" in our research.The few articles that included these terms that we found were described here because they were somehow included in an FTLD cohort or accidentally found while studying FTLD patients.
We found a total of 1610 articles.All the articles not written in English, the reviews and the studies without a clear description of the clinical phenotypes and/or without a formal diagnosis were excluded.We also excluded articles that did not specify the MAPT mutation or that described just haplotypes or risk factors or variants with uncertain significance (Fig. 1).
We selected 86 articles, including 177 patients.Among them, 28 patients showed one of the PPAs diagnosis (including semantic variant and non-fluent variant), 23 had PSP phenotype, 12 CBS phenotype, 30 had AD-like pheno-type, and 84 were diagnosed with bvFTD.Some of these papers are listed in different groups, as the same article often comprises patients with different disease phenotypes.We analysed the clinical data of the patients, including age at onset, familial history, neuroimaging data and MAPT mutations.We compared the ages at onset between groups by using ANOVA with Bonferroni correction.A narrative synthesis of the results has been performed.All MAPT mutations were reported referring to the NCBI Reference Sequence Database (RefSeq) transcript NM_005910.6.

General Overview
We identified 21 articles describing 28 patients with different linguistic phenotypes (Table 1, Ref. ), including two patients recently reported by our group [19,27].Ten patients had the clinical diagnosis of nfvPPA (35.71%) and ten subjects of svPPA (35.71%).Six patients exhibited clinical symptoms consistent with the right temporal variant of FTD (rtvFTD) (21.43%).In three of them, we revised the diagnosis on the basis of the recent rtvFTD criteria [30].Two patients had a clinical diagnosis characterised by linguistic symptoms plus other non-verbal symptoms (7.14%): one patient with Primary Progressive Apraxia of Speech and one subject with a Primary Progressive Anarthria or Apraxia of Speech.In the whole PPA group, familial history was reported as positive in 23 cases (82.14%).Age at onset varied between 24 and 69, with a mean age of onset of 51.68 (±11.62).

Non-Fluent Variant of Primary Progressive Aphasia
Ten patients of our series had the diagnosis of nfvPPA.The average age at onset was 51.7 years (±15.74).One patient had a very early age at onset, before 30 years [21].The main symptoms at onset were verbal production deficits with non-fluent, hesitant, and stuttering speech, anomia and difficulties in word finding.Two patients presented behavioural changes; one patient had concomitant Apraxia of Speech (AOS), suggesting a slow evolution to CBS, and one patient presented signs of CBS.V363I MAPT mutation was found in two distinct articles [17,18], while the other mutations were isolated case reports.nfvPPA associated with AOS was found in association with the V363I mutation [18]; this patient's phenotype was characterised by progressive difficulties in speech production (hesitant, stuttering, non-fluent), followed by apraxia of speech (dysprosodia) and bradykinesia, articulatory difficulties and mild buccofacial apraxia.All these characteristics suggested a progression to CBS.CBS features were reported in another patient with nfvPPA linked to the R5H MAPT mutation [15] who presented non-fluent speech and expression difficulties (word finding, naming deficits, stuttering and hesitant speech) at onset, followed by mask face, right-hand apraxia, bradykinesia, rigidity and stimulus-sensitive my-oclonus on his right-hand and forearm.Two studies reported behavioural changes associated with prominent nonfluent aphasia linked to G389R and S305I MAPT mutations [22,25].

Semantic Variant of Primary Progressive Aphasia
Ten patients had the diagnosis of svPPA; the main symptoms at onset were language deficits (anomia and word finding) and memory disturbances.Average age at onset was 48.3 (±9.33).Three MAPT mutations were associated with this phenotype: the P301L in three patients belonging to the same family, the G389R in one case and Q336H in another patient.Predominant anterior temporal lobe atrophy was described in all svPPA patients.

Right Temporal Variant of Frontotemporal Dementia
Interestingly, the diagnosis of rtvFTD was found in six patients.The mean age at onset was 53 years (±6.19), and the main symptoms at onset were memory disturbances, familiar faces identification deficits and behavioural disturbances.MAPT mutations related to the right temporal variant of FTD were: V363I, P332S, R406W, S352L and P301L.Two brothers shared the P332S mutation.Remarkably, their mother carried the same mutation and presented Primary Progressive Anarthria or a Progressive Apraxia of Speech [11].At Magnetic resonance imaging (MRI), predominantly right frontotemporal or temporal atrophy was found in three patients [10,29]; the other three patients, two of them belonging to the same family, showed bilateral and symmetric temporal and frontal atrophy, barely consistent with the recently published criteria [30].Their clinical description highlighted initial memory deficits and prosopagnosia signs followed by typical semantic deficits.

Primary Progressive Aphasia-Plus Phenotypes
Two patients showed a PPA-plus phenotype, in which language disturbances were present but not predominant.These two subjects presented Primary Progressive Apraxia of Speech or Anarthria [11,21] linked to P332S and K317N MAPT mutations.The first report [11] was the mother of the two subjects carrying the same mutation and who developed right temporal variant FTD.Her first symptom was a modification in her voice followed by orofacial apraxia, brisk face reflexes and hypernasal dysarthria; across the 25 years of illness, the patient had never manifested cognitive or behavioural modifications.The subject of the second paper [21] presented with difficulty in expression and production of sounds.Over time, the patient developed PSP-like symptoms, such as vertical gaze palsy, axial rigidity, backward falls and apraxia of eyelid opening.We also found one CBS case with nfvPPA signs at onset carrying the V363I MAPT mutation; even though the main diagnosis was actually CBD, the patient showed unspecified primary progressive aphasia, left-sided parkinsonism and CBS [31].MRI, Magnetic resonance imaging; y, yes; n, no; N.A., not available.>, atrophy is more pronounced in the specified hemisphere.svPPA, semantic variant primary progressive aphasia; nfvPPA, non-fluent variant primary progressive aphasia; rtvFTD, right temporal variant frontotemporal dementia; AOS, apraxia of speech; CBS, corticobasal syndrome; CBD, corticobasal degeneration.
Casseron and colleagues [33] reported two cases of P301S mutation found in a large family with a diagnosis of CBD.Both patients had an age of onset in their late 30s.The patient described by Bugiani and colleagues [32] had, in turn, a very early age of onset (27 years-old) and presented asymmetric symptoms from the beginning.Interestingly, his father was diagnosed with schizophrenia and neuropathologically showed frontotemporal atrophy with widespread neuronal tau inclusions.Erro and colleagues [34] reported three cases carrying another mutation at the same codon, the P301T, with an older age of onset and, in one case, with speech difficulties at the beginning.Most patients in this group had a positive family history of neurodegenerative/psychiatric diseases (83.33%).
Mesencephalic atrophy was described in only two patients.In seven articles (fourteen patients), an MRI description wasn't available.Family history was positive in most patients (82.61%).
The most frequent MAPT mutation associated with the PSP phenotype was N279K (39.13%), followed by the N296del and the S285R mutations (both 13.04%).The other eight cases were characterised by different MAPT mutations.MAPT N279K mutation was described in Japanese families [43,44].Comparing their phenotypic characteristics with sporadic PSP patients, Ogaki and colleagues [43] found that the visual grasping symptom could be a sign specifically related to the presence of a MAPT mutation.
Pastor and colleagues [46] described two patients with atypical PSP linked to the N296del MAPT mutation.This mutation could be a risk factor for Parkinson's disease [46] in the homozygous state and, interestingly, even in the heterozygous state [50].
Two MAPT mutations at the same residue, V363I and V363A, are associated respectively with CBS [31] and PSP phenotypes [18].In the latter study, the PSP patient had a mixed diagnosis of CBS, PPA and left-sided parkinsonism, with complete anarthria and severe dysphagia in the advanced stage.
The R406W MAPT was the most frequent mutation with AD-like phenotype (76.67%).Two patients carried the P301L MAPT mutation.All the remaining patients (23.33%) carried rarer mutations.The almost totality of the AD-like group was characterised by a positive family history of neurodegenerative diseases (96.67%).It's worth mentioning the only article describing a phenotype consistent with posterior cortical atrophy (PCA) [18]: a 54-yearold woman who developed difficulties in recognising faces and visuospatial impairment at the age of 51 and carried the V363I mutation.y, yes; n, no; N.A., not available.>, atrophy is more pronounced in the specified hemisphere.AD, Alzheimer's Disease; PCA, posterior cortical atrophy.AAO, age at onset; PPAOS, Primary Progressive Apraxia of Speech.*some articles did not specify a family history of disease.

Behavioural Variant of Frontotemporal Dementia (bvFTD)
We identified 84 patients with bvFTD phenotype at onset (Supplementary Table 1, Ref. [16,20,43,).More than half of patients had a family history of dementia (71.43%), considering that several papers did not specify this information (25%).The mean age at onset was 47.62 (±10.85), and 18 patients (21.43%) were younger than 40 years old, 15 of them with positive family history.Most of the patients (28.57%) carried the IVS10+16C>T MAPT mutation, followed by the P301L (14.28%) and the P397S mutation (7.14%).The most frequent phenotype was "typical" bvFTD (90.48%); eighteen patients (21.43%) presented parkinsonism, and fourteen patients had speech disturbances during the course of the disease (16.67%).Brain MRI showed, in most cases, atrophy of bilateral frontal and temporal regions.Notably, in 10 patients, the atrophy was mainly on one side; among them, the majority showed a left-predominant atrophy, consistent with the presence of speech impairment (70%).Four patients who showed a primary diagnosis of bvFTD had remarkable semantic features [68,83,88].Even though the authors did not make a PPA diagnosis, their main MRI patterns involved bilateral anterior temporal lobe atrophy, with less marked frontal lobe involvement.Moreover, they showed naming deficits, reduced verbal fluency, poor single-word comprehension and general semantic deficits.These phenotypes were linked to P397S, S356T and D252V MAPT mutations.

Clinical-Genetic Correlations in the Whole Cohort
A general overview of the whole cohort is reported in Table 5.Using ANOVA, we compared ages at onset between groups, and we only found a significant difference between AD and all other phenotypes (p < 0.05) (nfvPPA, svPPA, rtfFTD and PPAOS or Primary Progressive Anarthria are intended as part of the great PPA group).The age at onset in the AD group was higher than in the others.Conversely, the age at onset also differed between bvFTD and PSP/CBS subgroups and between PPAs and CBS (p < 0.05).We did not find any difference between bvFTD and PPAs (p > 0.05).In Fig. 2 and in Supplementary Table 2, all the MAPT mutations that we found associated with different clinical phenotypes were reported.
Notably, some of the mutations are associated with multiple phenotypes, in particular CBS, PPA and bvFTD.The P301L appears to be a cross-group mutation, as it was found at least once in every phenotype group except for the PSP group.IVS10+16C>T mutation was the most frequent in bvFTD patients.In general, the PSP group shares fewer mutations with the other phenotypes, with most patients carrying the N279K mutation.Similarly, almost all AD carry the R406W MAPT mutation, shared with only one rtvFTD patient and with one bvFTD.

Discussion
In this review, we analysed the clinical phenotypes of patients carrying the MAPT mutations reported in the literature between 1998 and 2022 to delineate possible clinicalgenetic correlations, paying particular attention to the rarest clinical presentations such as PPAs, atypical parkinsonism and AD-like phenotypes.
Among the entire group of patients included in this review, 52.54% carried a phenotype rarer than bvFTD: AD-like phenotype (16.95%), followed by the PPAs group (15.82%), which may reflect the fact that atypical phenotypes are more fully characterised in the literature than the classical ones.
Interestingly, considering all patients, seven had an onset before their 30s, most of them with a diagnosis of bvFTD, in agreement with previous findings on very early age at onset FTD, in which most of the subjects present the bvFTD phenotype [96].In this review, we extend the findings of very early onset FTD to other phenotypes, with 18 MAPT patients with PPAs and PSP/CBS having age at onset at or before their 40s.Early age at onset does not have a clear link with specific mutations.
Regarding the language phenotypes, we found almost the same number of patients with svPPA and nfvPPA, while logopenic variant was absent.An almost equal ratio of svPPA and nfvPPA was found in MAPT mutations in one of the largest worldwide cohorts of genetic FTD [4].This result differed from genetic TDP-43 proteinopathies, where the predominant language phenotype was the logopenic variant in a large cohort of GRN carriers [97] and the nfvFTD in C9orf72 expansion [4,98].Unlike MAPT patients, svPPA is exceptional in these other two genetic forms of FTD [97].This result suggests a relative homogeneity of pathology for the semantic phenotype with a major vulnerability of the temporal anterior lobe for tau accumulation [99].
Another difference with TDP43 genotypes is that the mean age at onset in PPA patients by our review is earlier than PPA linked to C9orf72 expansion and GRN mutations reported in the literature [97].
Interestingly, although the criteria for rtvFTD have only recently been defined, we identified at least six patients whose clinical presentation was consistent with this phenotype.RtvFTD is rarer than the left variant and represents a "unique", usually considered sporadic and associated with TDP-43 pathology [100].The recognition of this clinical entity has become even more important as the involvement of the right temporal lobe seems to be associated with a more widespread neurodegeneration than in patients with main involvement of the left temporal lobe and a shorter survival, possibly linked to a diagnostic delay [101].RtvFTD has also been associated, although rarely, with GRN mutations, suggesting that these two genes should be tested first.
Among the PPA group, we classified five patients as "PPA-plus" because of other neurological symptoms associated, mainly parkinsonism, myoclonus and dystonia.Conversely, we could not classify patients as mixed PPAs, a diagnostic category largely represented in PPA, because of the lack of a detailed neuropsychological assessment in several cases.
Considering the overall group of PPA, the most frequent MAPT mutation was the P301L.This is the most frequent mutation worldwide, and it is found to be linked to bvFTD phenotypes [4].In our review, P301L was also found associated with svPPA, with one case of nfvPPA, with one rtvFTD and one CBS.The rarer V363I mutation has been found in four PPA cases, half of them also characterised by CBS signs.This mutation has also been described in bvFTD [102], showing high heterogeneity in its clinical presentation.Some authors hypothesise that the rare presence of this mutation in the general population can be due to an incomplete penetrance and late disease onset [31].
In this paper, we included a large number of patients with PSP and CBS phenotypes, usually defined as sporadic; genetic PSP has also been associated with mutations in parkinsonism-related genes [103,104] and very rarely with C9orf72 and GRN mutations.PSP patients from our review had a mean age at onset earlier than that observed in sporadic forms or in PSP associated with other FTD genes [105], and almost 40% of PSP linked to MAPT had a very early age at onset, before their 40s, an important feature that can lead to suspect a MAPT mutation in PSP phenotypes.Among patients with CBS phenotype, relatively "pure" CBS was found in more than 50% of patients, while the others showed mixed clinical symptoms, classifiable as PSP-CBS according to MDS criteria [106].Therefore, we could suggest that co-occurrence of PSP and CBS features and early age at onset may orient the diagnosis toward a genetic tauopathy.
We identified 30 patients with an AD-like phenotype presenting with memory deficits but later developing behavioural modifications with or without parkinsonism, thus orienting the diagnosis towards FTD.The MAPT mutation more frequently associated with this phenotype was the R406W, but we also found two patients carrying P301L, two IVS10+16C>T, one V363I, one Q336H and one carrying Q351R mutations.As expected, age at onset in the AD group was significantly higher than in all other groups, with almost the totality of patients with onset after 50's.While in the R406W mutation, a mixed 3R/4R pathology is often described, similarly to AD, in AD-like cases with different mutations, the pathology is not specified, and thus a mixed proteinopathy cannot be excluded.
Several mutations are associated with multiple phenotypes, showing no clear genotype-phenotype correlation.However, some mutations are predominantly associated with atypical forms such as AD-like presentations or parkinsonisms (Fig. 2).However, a positive family history, the presence of other associated symptoms, and an early age of onset may be suggestive of a MAPT mutation.
MAPT mutations can have different impacts on tau protein, altering its ability to promote microtubule assembly, affecting its aggregation properties or determining the ratio between 4R and 3R isoforms [107], such as mutations associated with PSP and CBS phenotypes that often lead to 4R pathology, similarly to sporadic cases (see Supplementary Fig. 1).However, the mechanisms by which the mutations can give origin to different clinical forms are not known yet.A recent interesting study on iPS cells showed that one of the more frequent mutations associated with parkinsonism (N279K) has an impact on mitochondrial functions at variance with other mutations [108].This suggests that the clinical variability could be explained by mutation-related specific biological mechanisms that need to be further explored.
Our review has several limitations.First, the research terms included the name of the diseases and of the gene but not every single symptom or tau pathology.In addition, some papers lacked detailed clinical descriptions and neuropsychological assessment.However, to include the majority of articles, we searched in depth in the reference list of every paper and, in some articles written before the appearance of current clinical criteria, we deduced the diagnoses based on clinical description.In addition, due to the heterogeneity of the clinical phenotypes, except for age at onset, we did not perform a metanalysis of the results but a narrative descriptions, and we discussed the results in light of previous findings on other genetic forms.Despite these limitations, we could provide an overview and an update of clinical-genetic associations of the cases described in the literature, analysing the phenotypes of a large number of MAPT patients.We underlined the high clinical heterogeneity of FTD associated with MAPT mutations without clear phenotype-genotype correlations in most cases.Deep phenotyping of patients, together with the study of the pathogenetic mechanisms of single mutations, particularly those associated with atypical phenotypes, are necessary to better understand the biological basis of the clinical and neuropathological phenotypic variability of neurodegenerative diseases associated with MAPT mutations.