While identification of TDP-43 aggregates was the initiating breakthrough, the pathology alone left unclear if TDP-43 aggregation had a primary role in pathogenesis rather than a by-product of the disease process. Accumulation of intracellular or extracellular misfolded or misprocessed proteins in the central nervous system had previously been found in most neurodegenerative conditions. Recognizing that rare mutations in the genes encoding the misfolded proteins had been reported in Alzheimer's and Parkinson's diseases, the tauopathies and prion diseases, TARDBP, the gene encoding TDP-43 on chromosome 1, became an excellent candidate for direct sequencing in cohorts of patients with motor neuron disease and/or frontotemporal dementia.
Starting in early 2008, dominant mutations in the TARDBP
gene were reported by several groups as a primary cause of ALS (see (Banks et al., 2008
) for review and (Corrado et al., 2009
; Daoud et al., 2009
; Del Bo et al., 2009; Kuhnlein et al., 2008
; Lemmens et al., 2009
; Rutherford et al., 2008
), collectively providing persuasive evidence that aberrant TDP-43 can directly trigger neurodegeneration. A total of thirty different mutations are now known in 22 unrelated families (~3% of familial ALS cases) and in 29 sporadic cases (~1.5% of those cases) (). Since linkage of familial ALS to chromosome 1 had not previously been identified, key among these genetic efforts was a retrospective analysis in a large family where the TDP-43M337V
change had been identified through direct sequencing of TDP-43. This study (Sreedharan et al., 2008
) identified linkage between the disease and only one genomic region -- an 8.2 Mb region on chromosome 1p36 that contains the TARDBP
gene. Although unconventional, this approach provided a strong additional support for the pathogenic effect of the TDP-43M337V
Widely expressed and predominantly nuclear, TDP-43 is a 414 amino acid protein encoded by six exons and containing two R
otifs (RRM1 and 2) and a C-terminal glycine rich region that is proposed to mediate interactions with other proteins. All but one of the mutations identified so far are localized in the C-terminal region encoded by exon 6 of TARDBP
(). All are dominantly inherited missense changes (Daoud et al., 2009
). The pathogenicity of these missense changes is strongly supported by several lines of evidence. First, they affect amino acids highly conserved through evolution. Second, they have not been found in large cohorts of controls. Third, in the cases where the DNAs are available the mutations segregate with the disease and no mutations have been found in unaffected family members (except those below the typical age of onset). This indicates a high penetrance in these families, although further studies on this point are needed since TDP-43 mutants were also identified in apparently sporadic patients. Collectively, the evidence is now overwhelming that aberrant TDP-43 can trigger ALS.
Patients with TDP-43 mutations develop typical ALS with some variability in the site and age of onset within families. While up to 50% of ALS patients develop cognitive impairment of various severities, only one of the patients with TDP-43 mutations has been reported to develop cognitive deficits (Corrado et al., 2009
), despite the presence of TDP-43 inclusions not only in neurons and glial cells within the spinal cord but also throughout the brain (Banks et al., 2008
), pathologic abnormalities similar that described in sporadic cases (Banks et al., 2008
). Diffuse granular cytoplasmic staining (which may represent an earlier stage in inclusion development) and nuclear clearing have also been described (). It should be emphasized that it is not known whether TDP-43 mutations lead to neurodegeneration through a gain of one or more toxic properties and/or a loss of normal function arising from its sequestration into either nuclear or cytoplasmic aggregates and the corresponding disruption in its interactions with protein partners and/or RNA targets.