In FTD-RisC, we follow healthy 50% at-risk family members from genetic FTD families on a 2-year basis. In the current study, we included 87 participants from MAPT or GRN families with study entries between December 2009 and January 2013 [8, 9, 13]. The follow-up period was 4 years, in which we acquired neuropsychological assessments at study entry, follow-up after 2 years and follow-up after 4 years. DNA genotyping (see “Procedure”) assigned participants either to the presymptomatic mutation carrier (n = 46; 31 GRN, 15 MAPT), or control group (n = 39; 29 GRN, 10 MAPT family members). We excluded two controls as they had cognitive disorders (≥ 2 SD below mean) on multiple domains, ultimately including 85 participants (46 mutation carriers, 37 controls; Fig. 1).
Standard protocol approvals, registrations, and patient consents
Clinical investigators were blind for participants’ genetic status if they had not undergone predictive testing. In case of conversion to clinical FTD, we offered the patient and family members genetic counselling and unblinding of genetic status, to confirm the presence of the pathogenic mutation. At study entry, all participants gave written informed consent. The study was approved by the Medical and Ethical Review Committee of the Erasmus Medical Center.
Every 2 years, participants underwent a standardized assessment consisting of a neuropsychological test battery, neurological examination, and MR imaging of the brain. DNA sequencing was performed at study entry. All participants were asymptomatic according to established diagnostic criteria for bvFTD  or PPA  at baseline. Knowledgeable informants were asked about cognitive and/or behavioural deterioration at each study visit by means of a structured interview and a well-validated questionnaire (Neuropsychiatric Inventory; NPI) .
Eight mutation carriers became symptomatic within the study time window (“converters”). Symptom onset was determined by means of the above mentioned assessment (anamnesis, MR imaging of the brain, neuropsychological assessment, heteroanamnestic information and unblinding of genetic status). Conversion was determined in a multidisciplinary consensus meeting of the Erasmus MC FTD Expertise Centre, involving neurologists (LDK, JCvsS), neuropsychologists (LCJ, JLP, SF, EvdB, JMP), medical doctors (LHM, ELvdE), as well as neuroradiologists, geriatricians, a clinical geneticist (RvM), and a care consultant. Six converters (5 MAPT, 1 GRN) presented with progressive behaviour deterioration, functional decline, and frontal and/or temporal lobe atrophy on MRI, fulfilling the international diagnostic consensus criteria of Rascovsky et al.  for bvFTD with definite FTLD pathology. Two converters (both GRN) presented with isolated language difficulties and no impairments in daily living activities, thereby fulfilling the diagnostic criteria for PPA of Gorno-Tempini et al. . Both developed nfvPPA, as they showed a non-fluent, halting speech, with sound errors and agrammatism. See Supplementary Table 1 for demographic, clinical and neuropsychological data of the converters. We defined mutation carriers remaining without FTD symptoms as non-converters (n = 38; 28 GRN, 10 MAPT).
We screened global cognitive functioning by means of the Mini-Mental State Examination  (MMSE) and frontal assessment battery  (FAB). Experienced neuropsychologists (LCJ, JLP, SF) administered neuropsychological tests within six cognitive domains: language, attention and mental processing speed, executive functioning, social cognition, memory, and visuoconstruction. We rated language with the 60-item Boston Naming Test (BNT) , verbal Semantic Association Test (SAT) , ScreeLing phonology , and categorical fluency . We assessed attention and mental processing speed by means of Trail making Test (TMT)-A , Stroop Color-Word Test I and II , Wechsler Adult Intelligence Scale III (WAIS-III) Digit Span forwards , and Letter Digit Substitution Test (LDST) . Executive functioning was evaluated using TMT-B , Stroop Color-Word Test III , WAIS-III Digit Span backwards , modified Wisconsin Card Sorting Test (WCST) concepts , letter fluency , and WAIS-III Similarities . Happé cartoons  and Ekman Faces  measured social cognition. We assessed memory using the Dutch Rey Auditory Verbal Learning Test (RAVLT)  and Visual Association Test (VAT) . We evaluated visuoconstruction by means of clock drawing  and WAIS-III Block Design . Alternate forms were used at follow-up visits, when applicable (letter fluency, RAVLT, VAT). Depressive symptoms were rated with the Beck’s Depression Inventory (BDI) .
In converters, we restructured the three original time points within our study window (i.e. baseline, follow-up after 2 years, follow-up after 4 years) into the following three new time points (Fig. 1):
4 years before symptom onset: we used the data of the study visit 4 years before diagnosis. Analyses could were performed in six converters, as two (1 GRN, 1 MAPT—2 bvFTD) developed symptoms between baseline and first follow-up (i.e. at 2 years follow-up), and therefore no data 4 years prior to symptom onset were available.
2 years before symptom onset: we used the data of the study visit 2 years before diagnosis. Analyses included all eight converters.
After symptom onset: we used the data of the diagnosis visit. Analyses included all eight converters.
In non-converters and controls, we used the original time points: baseline (data were compared to “4 years before symptom onset” data of converters), follow-up after 2 years (data were compared to “2 years before symptom onset data of converters) and follow-up after 4 years (data were compared to “after symptom onset data of converters).
Statistical analyses were performed using SPSS Statistics 21.0 (IBM Corp., Armonk, NY) and GraphPad Prism 7 (La Jolla, California, USA), with the significance level at p < 0.05 (two-tailed) across all comparisons. We compared demographic data between MAPT mutation carriers, GRN mutation carriers and controls, and between converters, non-converters and controls by means of one-way ANOVAs. We performed Pearson Χ2 tests to investigate differences in sex. Longitudinal comparisons of clinical data were performed with repeated measures ANOVAs. We standardized all raw neuropsychological test scores by converting them into z-scores (i.e. individual test score minus the baseline mean of the controls, divided by the baseline SD of the controls) per time point, after which we calculated composite z-scores for the respective six cognitive domains by averaging the z-scores of the individual tests per domain. For the longitudinal comparisons we used multilevel linear regression modeling. This analysis corrects for bias when data absence is dependent on characteristics present in the model, and can therefore efficiently handle missing and unbalanced time points. There were two levels in the models: the participants constituted the upper level; their repeated measures the lower level. We ran two analyses to assess cognitive decline per mutation (1) and clinical status (2):
We entered mutation status (MAPT mutation carrier, GRN mutation carrier or control), time (4 years before symptom onset, 2 years before symptom onset, and after symptom onset), and first-order interactions, with age, gender and educational level as covariates. We reran the analyses excluding the converters to exclude converters driving the cognitive decline in the mutation carrier groups;
We split the converter group according to genotype (MAPT or GRN) and phenotype (bvFTD or nfvPPA) to investigate specific profiles of cognitive decline over time. We then entered clinical status (converter, non-converter or control), time, and first-order interactions, with age, gender and educational level as covariates.
Third, to investigate the prognostic abilities of cognitive decline in discriminating between converters and non-converters, we determined the area under the curve (AUC) by receiver operating characteristic (ROC) analyses on the neuropsychological trajectories between visits. For this, we calculated deltas between test scores; one between 4 and 2 years before symptom onset and one between 2 years before symptom onset and symptom onset. Optimal cut-off levels were given by the highest Youden’s index . Again, we split the converter group according to genotype (MAPT or GRN) and phenotype (bvFTD or nfvPPA). Next, we performed logistic regression analyses, taking group (converter vs. non-converter) as the dependent variable and the deltas (tests with significant diagnostic performance in abovementioned ROC analyses) as the independent variables. The models were selected with a forward stepwise method according to the likelihood ratio test and applying the standard p values for variable inclusion (0.05) and exclusion (0.10), with age, sex and education as covariates. Goodness of fit was evaluated with the HL Χ2 test. Nagelkerke R2 is reported as measure of effect size. We checked predictor variables for multicollinearity. All models were corrected for multiple comparisons (Bonferroni).