Structural Brain Changes in Pre-Clinical FTD MAPT Mutation Carriers

INTRODUCTION

Frontotemporal dementia (FTD) is the second most common cause of early-onset neurodegenerative dementia [1]. Up to 40% of FTD cases are associated with an autosomal dominant pattern of inheritance. Mutations in over eight genes have been identified in FTD, including progranulin (GRN), chromosome 9 open reading frame (C9orf72), and microtubule-associated protein tau (MAPT) genes [2, 3]. Neuroimaging has been explored as a potential biomarker to identify patients in initial phases of neurodegenerative diseases and to measure biological change over time [4]. Several studies have focused on early imaging changes in FTD patients [5–10], but once subjects meet full criteria for FTD diagnosis, structural changes are generally widespread [11]. Studying mutation carriers who are asymptomatic or transitioning from asymptomatic to symptomatic (what we will refer to as “preclinical”) [12] may allow for characterization of the earliest neuroimaging changes in disease [13–17].

Studies specifically addressing preclinical neuroimaging features in MAPT mutation carriers are scarce. Some studies [18, 19] have demonstrated early insular atrophy, and others early medial temporal degeneration [20, 21]. Also, some studies have reported early white matter changes in preclinical FTD patients [22–25], but those performed specifically in MAPT mutation carriers have not [26]. Our institution has access to a relatively large, broadly phenotyped group of MAPT mutation carriers and familial matched controls that brings the opportunity to study early imaging changes in a well characterized population to address the inconsistencies found in previous studies. With that purpose, we compare cortical and subcortical gray matter volumes as well as white matter hyperintensities and tract-integrity between MAPT mutation carriers and demographically-matched familial controls.

MATERIALS AND METHODS

RESULTS

We analyzed data from 56 participants belonging to five families carrying MAPT mutations. Twelve subjects were determined to be carriers of the following mutations: P301L (one subject), Exon 10 + 16 C > T (one subject), Exon 10 + 15 C > T (two subjects), V337M (c.2014G>A) (four subjects), and Exon 10 + 14 C > T (four subjects). Forty-four subjects were non-carriers. Demographic characteristics of the sample and performance in neuropsychological testing are displayed in Table 1. Subjects were considered preclinical if they did not fulfill FTD diagnostic criteria. We use the term “preclinical” rather than “presymptomatic” as this group includes those that scored 0 and 0.5 in CDR. CDR = 0 carriers had no cognitive or behavioral impairment, but all CDR = 0.5 carriers had one or more abnormal neuropsychological score, and most also had questionable or mild behavioral impairment as indicated on the Behavior, Comportment, and Personality rating of the CDR Supplemental scores (FTLD-CDR) [39]. However, as these abnormalities were considered as questionable by raters and they did not fulfill diagnostic criteria for FTD, patients with CDR score of 0.5 were included in the preclinical group. There were no significant differences in age, gender, and years of education between the groups.

For each ROI, Freesurfer-derived raw volumes were converted to percentage of TIV prior to the analyses. The resulting data were analyzed using two models, a Wilcoxon analysis and a multivariate elastic-net model. In Wilcoxon analysis, 32 of 89 measures show significant between-group differences after multiplicity adjustment (Table 2), including caudate, putamen, hippocampus, amygdala, nucleus accumbens, and several regions of the frontal and temporal lobes. Over 500 iteration bootstrapping, five measures were constantly selected (with over 80% selection rate), demonstrating a potential strong association with mutation status (Table 2, Fig. 1). These five ROIs are left amygdala (selection percentage 0.83), rostral anterior cingulate (selection percentage 0.91), posterior cingulate (selection percentage 0.81), temporal pole (selection percentage 0.92), and lingual volume (selection percentage 0.83). Among these five, the lingual area and posterior cingulate did not come out significant in Wilcoxon’s test, likely because they are only strong predictors of mutation status when other ROIs are controlled. No significant difference was found in whole brain volume between mutation carriers and non-carriers. Results remained the same after recalculating bilateral selection percentage over 500 bootstrap iterations as when considering both hemispheres separately. Therefore, for those ROIs, the left and right sides were not competing. When considering the sum of ROIs on both sides, four measures were selected over 80% selection rate: white matter hyperintensities in the occipital lobe (selection percentage 0.862), temporal pole volume (selection percentage 0.97), lingual volume (selection percentage 0.876), and posterior cingulate volume (selection percentage 0.84).

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Fig.1

Multivariate analysis: Elastic net. Percentage of selection over 500 Bootstrap iterations. In Elastic-net model, 10/89 ROIs are selected into the model. Over 500 iteration bootstrapping, 5 measures are constantly selected (with over 80% selection rate), demonstrating a potential strong association with mutation status. This is a recalculation of selection percentage based on the bootstrap results. The column ‘Either’ is a surrogate measure of the % selection for either left or right hemisphere of the specific ROI. When considering bilateral selection %, the results remain the same as considering both hemispheres separately. It could be shown from the graph that for those ROIs, the bilateral ROIs seems not competing with each other.

We performed a subgroup analysis comparing CDR 0 and CDR 0.5 carriers (2-sided Wilcoxon Test after adjustment for multiple comparisons). We found significant differences in 18/89 measures after multiplicity adjustment with larger volumes in CDR 0 subgroup (shown in Table 2 with a 1, values available in the Supplementary Table 1). Most of these ROIs are the same that were significantly different between carriers and non-carriers. Only left superior frontal gyrus (CDR0 = 1.26 versus CDR0.5 = 0.97, p = 0.04) and right precentral gyrus (CDR0 = 0.77 versus CDR0.5 = 0.63, p = 0.049) showed differences between CDR subgroups that were not present when comparing carriers and non-carriers. These results should be considered with caution, as the number of subjects for the subgroup analysis is very low.

We did not find significant differences between carriers and non-carriers in the volume of white matter hyperintensities. There was only a weak difference after adjusting results by age and TIV, which did not survive after adjustment for multiple comparisons and elastic-net analysis (Table 2). We found a significant difference for white matter hyperintensities in the basal ganglia between CDR 0 and CDR 0.5 preclinical carriers (CDR 0 = 2.69E–05 versus CDR 0.5 = 4.49E–06; p = 0.03) after adjustment for multiple comparisons.

DTI measures were available for 9 carriers and 40 controls due to technical issues during the neuroimaging exam. After comparing DTI results between groups, we found significant differences between carriers and non-carriers for the left cingulum at the cingulate gyrus (0.579±0.055 versus 0.609±0.034; p = 0.041), right cingulum at the cingulate gyrus (0.531±0.053 versus 0.562±0.038; p = 0.045), and left cingulum at the hippocampus (0.522±0.074 versus 0.563±0.047; p = 0.039). However, after adjusting these results for age, the differences were no longer significant.

DISCUSSION

In this study, we report early structural changes in pre-clinical MAPT mutation carriers measured using voxel-based morphometry and DTI analysis. Prior to overt symptom onset, MAPT mutation carriers in our sample showed volume differences in almost 30% of the 89 regions explored, including basal ganglia (caudate, putamen), temporal lobe (in particular medial temporal lobe), and some areas of the cingulate gyrus and the medial frontal lobe. Most areas were equally affected in both hemispheres, with a symmetrical distribution that has been previously described in MAPT mutation patients [19, 40]. Using a much more restrictive statistical analysis five regions were consistently associated with mutation status including the left temporal lobe (left amygdala, left temporal pole), bilateral cingulate cortex (left rostral anterior cingulate gyrus, right posterior cingulate), and the lingual gyrus in the occipital lobe.

Until now, studies regarding early structural changes in asymptomatic MAPT carriers have shown inconsistent results: while several small case series had reported neuroimaging differences in asymptomatic subjects, other studies did not find any structural change [40, 41]. Those studies that found differences point to early degeneration in the temporal lobe, medial frontal lobe, and cingulate cortex. An earlier study, published in 2010 by Miyoshi et al. [21], found the medial temporal lobe and cingulate gyrus affected (hippocampal atrophy and striatal dopaminergic dysfunction). The most recent work shows also emerging grey matter temporal lobe changes in MAPT asymptomatic mutation carriers [42]. Our results agree partially with these findings as well as with the largest study in asymptomatic mutation carriers to date [19, 20], which found differences in hippocampal and amygdala volumes as early as 15 years prior to expected symptom onset, in the temporal lobe 10 years before expected onset and in insula 5 years before expected symptom onset. However, after correction for multiple comparisons, only changes in the insular area remained significant. Ours study supports these previous results, confirming differences in hippocampus, amygdala, the insular area, and temporal lobe between carriers and non-carriers, which survived correction for multiple comparisons. Moreover, we found significant differences between basal ganglia volume (caudate, putamen), some areas of the frontal lobe, and the cingulate gyrus. Our results survived multiple comparison correction even though we applied a much more restrictive statistical method in order to determine the strongest association with mutation status. If we consider only the elastic-net model results, the amygdala and temporal lobe remain affected in our sample, whereas there were no differences for hippocampus and insula volumes. Considering that the insula was the last structure affected in the GENFI study, it is possible that the carriers in our cohort had not yet reached that stage of progression. Both studies have important differences regarding sample characteristics, statistical methodology, and imaging analysis. Our sample includes fewer asymptomatic carriers than the GENFI study; however, subjects came from only five families carrying five mutations, as opposed to the GENFI 17 different families of MAPT carriers carrying ten different mutations. Considering that FTD is a protean pathology [11] in which disease phenotypes may vary substantially between families and individuals with different mutations [43], our sample’s characteristics can add to the body of knowledge currently available. Regarding neuroimaging, our study was performed at only three sites and we used only one type of MRI scan, which increases the consistency of the results. We examined the volume not only of the brain lobes as a whole, but also of specific areas in each lobe. Regarding white matter changes, our results agree with the findings of the GENFI cohort [26], which did not report any difference in white matter hyperintensities either in clinical and preclinical MAPT mutation carriers when compared to controls. Interestingly, our analysis was performed using FLAIR sequence, which is the standard for the study of white matter hyperintensities and is usually available in clinical practice, while the initial GENFI cohort used T2-weighted images. Since white matter hyperintensities do not reflect white matter integrity, we performed DTI analysis in our sample, where we did not find significant differences in our groups after adjusting for age. Previous studies have found white matter involvement in MAPT mutation patients [22–25], particularly in the uncinate fasciculus [42, 44]. A recent study published in 2019 showed a disproportional volume loss of the right temporal lobe and more fractional anisotropy decline in the uncinate fasciculus of MAPT carriers converting to clinical FTD [45]. This study includes a follow-up phase and compares converters, non-converters, and non-carriers. Differences between groups were only evident 2 years before symptom onset, while 4 years before symptom onset these differences did not exist. In our study we did not differentiate converters and non-converters and we performed one cross-sectional analysis, without controlling for time to onset. These methodological disparities could explain the different results regarding white matter integrity.

Studies performed in symptomatic subjects have been much more numerous, although once the disease is clinically noticeable, structural changes are generally widespread. Moreover, many of these studies are performed in sporadic forms of the disease, which may differ from genetic cases. However, neuropathological studies performed in sporadic FTD patients who died early in the course of their illness [5] show atrophy in some of the areas affected in our study: the frontal lobe, the medial temporal lobe (hippocampus, amygdala), and anterior cingulate gyrus. Some authors studying specifically neuroimaging changes in MAPT mutation patients [8, 40] report predominant gray matter loss in the temporal lobe, particularly the anterior and medial temporal lobe, with varying degrees of frontal and parietal lobe involvement in clinically diagnosed FTD. The orbitofrontal cortex, ventral insula, and anterior cingulate have also been found affected [7]. When subjects with mutations in the MAPT gene show a widespread pattern of frontotemporal gray matter loss, the most severely affected regions are the anteromedial temporal lobes, suggesting that this may be the first area affected by the disease [8]. MAPT patients tend to show symmetric patterns of atrophy [46], with no differences between left and right hemisphere when comparing bilateral regions of interest. This is consistent with our sample of preclinical mutation carriers where most areas in both hemispheres were also equally affected, suggesting that the disease begins and progresses symmetrically.

Overall, the atrophy profile observed in MAPT patients involves a ventral orbitofrontal-medial temporal-ventral insula network [9]. Dysfunction of this network has been associated with poor performance on memory and naming, executive dysfunction, and language deficits, widely recognized in FTD [47]. Patients commonly develop semantic impairment later in the disease, as well as prominent episodic memory difficulties [9]. Other structures affected in our sample (amygdala and cingulate cortex) belong to the rostral limbic system, which has been suggested to underlie FTD symptoms [47]. This system integrates limbic structures with output-related structures: the amygdala processes the value of internal and external stimuli, represents that value in the form of emotion to the brain and associates this emotion to external stimuli. Moreover, the amygdala, in close connection with the ventromedial prefrontal and anterior cingulate cortices, contributes to other higher order functions such as decision-making, theory of mind, and emotional processing [48, 49], while the anterior section of the cingulate cortex detects conflict within ongoing information processing and integrates information from different structures of the circuit [50].

There is evidence that early changes in connectivity could precede the occurrence of regional atrophy. Some authors studied asymptomatic mutation carriers using functional neuroimaging and reported changes in connectivity, metabolic structure, and blood flow without structural changes [18], suggesting that structural imaging changes may appear after deficits in functional networks have been going on for some time. Alberici et al. [51] reported significant reductions of frontal lobe blood flow (dorsolateral frontal cortex, frontal poles, and mesial frontal cortex) in an asymptomatic P301L mutation carrier using SPECT, although these changes were not evident in structural brain imaging. Dopper and colleagues [41] reported frontal, posterior temporal, and parietal hypoperfusion in asymptomatic MAPT and GRN mutation carriers. Whitwell et al. [18] compared functional connectivity in MAPT mutation carriers, healthy controls, and bvFTD patients. Although there was no significant reduction of salience network connectivity in MAPT carriers, there was a suggestion of reduced connectivity in the anterior cingulate, one of the areas affected in our cohort’s MAPT mutation carriers. The aforementioned studies suggest that once changes are noticeable using structural neuroimaging, deficits in functional brain networks may have been going on for some time.

This study was performed in a well-characterized and broadly phenotyped group of asymptomatic MAPT mutation carriers and familial matched controls. We would like to remark the strength of our findings: MAPT carriers and controls were recruited from the offspring generation of only five families, and were matched by age and family for analyses. Age, sex, and education were included in all analyses as covariates to further reduce potential confounds. The differences we found between carriers and non-carriers survived multiple comparisons and elastic/net analysis. Nevertheless, there were limitations to our study. First, our cohort is small when compared to previous studies. Second, although none of our carriers met diagnostic criteria for FTD, some of them received a CDR score of 0.5 for questionable symptoms and it is arguable if we can describe these patients as pre-symptomatic or if defining an MCI-FTD stage subgroup could be more appropriate. A CDR 0.5 allows a suspicion of early dementia, meaning that the subject shows consistent changes in cognitive function or functional impairments, even if they do not fulfill diagnostic criteria for FTD. It would be also desirable to know the expected time to onset for each subject, as it was reported in previous studies, and it would also help clarify the meaning of the CDR 0.5 subjects in this sample. As the specific underlying mutation may affect the pattern of atrophy [40], a subgroup analysis would have been desirable, but this was not possible due to the size of our sample. Specifically, patients carrying IVS10 + 16, IVS10 + 3, N279K, and S305N mutations show the most severe grey matter loss in the anterior temporal lobe, especially the medial structures. Patients with P301L or V337M mutations also show severe gray matter loss in the anterior temporal lobe, but unlike in our study, with a relative sparing of the medial temporal lobe and greater atrophy observed in more inferior and lateral temporal regions [40]. In addition, there is evidence that P301L and V337M FTD patients exhibit severe atrophy of the basal ganglia, a finding that was observed in our cohort of MAPT mutation carriers as a whole. These differences in patterns of atrophy between MAPT mutations may be secondary to their effects in the splicing of exon 10 and in the structural and functional properties of the resulting tau protein [52]. Larger samples exploring how different mutations result in diverse atrophy profiles are needed.

In conclusion, this study provides additional data regarding early structural changes in a homogeneous sample of preclinical MAPT mutation carriers, adding to previous reports [20]. In our sample, atrophy was detected in preclinical mutation carriers compared to related non-carriers. Temporal lobe (left amygdala, left temporal pole), cingulate cortex (left rostral anterior cingulate gyrus, right posterior cingulate), and the lingual gyrus seem to be early targets of the disease. Regarding white matter, we did not find differences in white matter hyperintensities or DTI analysis after adjusting for age.

Although this cross-sectional study offers valuable information, we continue to follow these patients in a longitudinal study design so we can assess atrophy rates across time. The degree to which FTD spreads between neighboring regions of the brain versus following a functional network comprised of spatially separated brain regions is still under investigation.