Granulin mutation drives brain damage and reorganization from preclinical to symptomatic FTLD

Abstract

Granulin (GRN) mutations have been identified as a major cause of frontotemporal lobar degeneration (FTLD) by haploinsufficiency mechanism, although their effects on brain tissue dysfunction and damage still remain to be clarified. In this study, we investigated the pattern of neuroimaging abnormalities in FTLD patients, carriers and noncarriers of GRN Thr272fs mutation, and in presymptomatic carriers. We assessed regional gray matter (GM) atrophy, and resting (RS)-functional magnetic resonance imaging (fMRI). The functional connectivity maps of the salience (SN) and the default mode (DMN) networks were considered. Frontotemporal gray matter atrophy was found in all FTLD patients (more remarkably in those GRN Thr272fs carriers), but not in presymptomatic carriers. Functional connectivity within the SN was reduced in all FTLD patients (again more remarkably in those mutation carriers), while it was enhanced in the DMN. Conversely, presymptomatic carriers showed increased connectivity in the SN, with no changes in the DMN. Our findings suggest that compensatory mechanisms of brain plasticity are present in GRN-related FTLD, but with different patterns at a preclinical and symptomatic disease stage.

Introduction

Frontotemporal lobar degeneration (FTLD) is a neurodegenerative disorder characterized by behavioral abnormalities, language impairment, and deficits of executive functions as most typical clinical features (McKhann et al., 2001, Neary et al., 1998). FTLD has a strong genetic background with about 50% of patients showing a positive family history for dementia (Rademakers and Rovelet-Lecrux, 2009). FTLD pathophysiology has long been referred to the presence of mutations in microtubule-associated protein tau (MAPT) (Hutton et al., 1998, Spillantini and Goedert, 1998), which were first identified in families with FTLD-Parkinsonism and tau-positive inclusions, as assessed by postmortem investigation. For a decade, gene mutations for MAPT have been regarded as the key player for monogenic FTLD, with more than 40 mutations that have been identified so far (http://www.molgen.ua.ac.be/FTDmutations). More recently, an exciting breakthrough in the search of novel causal FTLD genes was provided by identification of loss-of-function mutations for Granulin (GRN) (Baker et al., 2006, Cruts et al., 2006). To be considered pathogenetic, these mutations are expected to induce a loss of 50% functional progranulin (PGRN), with a mechanism of haploinsufficiency (Rademakers and Rovelet-Lecrux, 2009). In less than 4 years, more than 60 different pathogenetic mutations for GRN have been reported in literature (http://www.molgen.ua.ac.be/FTDmutations). In the presence of GRN gene mutation, FTLD segregates in a Mendelian fashion, which is compatible with an autosomal dominant inheritance (Cruts and Van, 2008). The physiological role, as well as the effect of reduction of PGRN in the brain tissue are still largely unknown, although it has been recently suggested that PGRN might act as a neurotrophic factor (Van Damme et al., 2008). Soon after the discovery of GRN mutations, the nuclear protein TAR DNA-binding protein 43 (TDP-43) was identified as the major protein that plays a pathogenetic role in all FTLD cases associated with GRN mutations (Neumann et al., 2006). The underlying mechanism from which PGRN haploinsufficiency determines TDP-43 inclusions and, subsequently, brain damage and the clinical onset of disease is unknown. The behavioral (bvFTD) and the progressive nonfluent aphasia (PNFA) variants of FTLD are the most typical presentations in GRN mutation carriers, with a clinical onset in the 5th and 6th decades of life (LeBer et al., 2008, Masellis et al., 2006, Mesulam et al., 2007, Rademakers et al., 2007, Snowden et al., 2006, Van Deerlin et al., 2007).

Against this large background of improvements in characterizing FTLD genetics, the relationship between molecular aspects of pathogenesis, and structural and functional modifications of the brain tissue still remains to be clarified. Imaging genetics is a rapidly emerging field that is opening up a new landscape of discovery in neuroscience (Thompson et al., 2010). In this context, magnetic resonance imaging (MRI) has become an increasingly popular tool for human brain investigation in vivo. MRI has the unique ability to provide quantitative information on both brain tissue structure and functioning. Voxel-based morphometry (VBM) is currently regarded as a robust magnetic resonance technique suitable for assessing structural gray matter (GM) modifications in an unbiased fashion (Bozzali et al., 2008, Gorno-Tempini et al., 2004). On the other hand, resting state functional MRI (fMRI) has shown the ability to provide measures of functional brain connectivity (Biswal et al., 1995, De Luca et al., 2005, Fox and Raichle, 2007). Functional connectivity is a concept based on the evidence that different brain regions present with synchronous patterns of activity at rest. Those regions are likely to be part of common networks subserving complex brain functions. In the presence of pathology, the loss of brain connectivity may account for some cognitive disabilities, and even for some gray matter loss secondary to neuronal disconnection (Gili et al., 2011). From resting state fMRI data (i.e., fMRI time series collected while subjects lie vigilant but at rest in the scanner), several networks can be extracted at the same time in a data-driven fashion, using independent component analysis (Greicius et al., 2003). The default mode network (DMN) is by far the most extensively studied network. This is believed to be relevant for specific higher level functions, such as the working memory, mind wandering, and goal-directed behaviors (Fox and Raichle, 2007). The so-called salience network (SN) is another interesting resting state fMRI component, which is believed to be particularly informative when investigating patients with FTLD (Zhou et al., 2010). It is characterized by a more anterior anatomical distribution, and it has been related to behavioral and emotional functions. In a recent work, Zhou and coworkers (Zhou et al., 2010) have assessed changes in both the DMN and the SN in patients with FTLD and Alzheimer's disease (AD), demonstrating a reversed pattern of abnormalities in the 2 diseases. AD, as also demonstrated by others (Gili et al., 2011, Greicius et al., 2004), is characterized by a remarkable disruption of the DMN. In contrast, SN has been reported to be selectively damaged in patients with FTLD. The hypothesis of a selective involvement of a specific network in either form of dementia is supported by the observation that the pattern of atrophy typically observed in AD overlaps with the DMN in healthy subjects (Seeley et al., 2009), while the pattern of atrophy observed in FTLD overlaps with the SN (Zhou et al., 2010). Furthermore, an increase of connectivity within the DMN (Zhou et al., 2010) has been reported in FTLD, a result which could be suggestive of a compensatory mechanism. However, this interpretation would need to be corroborated by data obtained in patients at early (or preclinical) stages.

Monogenic FTLD represents a unique opportunity to investigate the pathophysiology of FTLD in its preclinical stages, thanks to the possibility to identify carriers of pathogenetic mutations as at-risk individuals. Our group has previously identified a genetically coalescent cohort of families with GRN Thr272fs in Italian patients with FTLD (Borroni et al., 2008a, Borroni et al., 2008b, Borroni et al., 2011a), and has demonstrated that these families harbor a common ancestor dating back to the Neolithic era (Borroni et al., 2011a).

Taking advantage from a unique large pedigree with different generations available, principal aims of the current study were: (1) to confirm on a larger population of subjects, structural and functional changes that have been previously reported in patients with FTLD byZhou et al. (2010); and (2) to investigate the effect of GRN mutation on both brain tissue structure and function, moving from the preclinical to the manifest stage of FTLD.