The identification of progranulin mutations by Baker and colleagues is a major advance in our understanding of frontal temporal dementia (FTD). The work by both Baker and Cruts and their colleagues shows that loss of progranulin function is a major cause of FTD, at least in some populations. These findings are remarkable for several reasons: first, this is the first simple loss-of-function autosomal dominant disease; second, it suggests that the genetic linkage of two FTD loci with similar clinical features, but different pathologies, close to the same locus was just a confusing coincidence. Third, it will undoubtedly spawn a huge amount of effort to define the limits of the phenotype and to elucidate its precise function in the CNS. It will also be interesting to see whether other diseases with ubiquitin inclusions will share related pathogenic mechanisms.

View all comments by John Hardy

These studies are spectacular advances in FTD research that open up new avenues for understanding mechanisms of FTLD-U. Notably, since progranulin proteins, or derivatives thereof, were not found in the ubiquitin inclusions of these FTLD-U disorders, it will be important to identify the ubiquitinated disease protein(s) that form these hallmark lesions of FTLD-U.

View all comments by Virginia Lee
View all comments by John Trojanowski

Both of these papers represent a significant discovery of a novel mutation on progranulin, a protein with no known CNS function. It is a known growth factor in vasculo and tumorigenesis, and it may turn out to have nerve growth factor properties as well; therefore, it is reasonable to postulate that a molecular deficit caused by its mutation could produce neurodegenerative disease such as frontotemporal dementia (FTD). We published the first chromosome 17-linked ubiquitin-positive family from Ontario in 2000 and the first intranuclear ubiquitin-positive inclusions in this and other families (1,2), but these genetic teams deserve credit for finding the mutation.

What is extraordinary is that progranulin is very close to the tau gene on chromosome 17, the known culprit in the mutated form in FTD linked to 17. How the two different genes interact, if at all, to cause a very similar illness is yet to be determined. The relationship of progranulin mechanisms to chromosome 9-linked cases and the valosin mutation with FTD and myopathy also deserves attention.

References:
1. Kertesz A, Kawarai T, Rogaeva E, St George-Hyslop P, Poorkaj P, Bird TD, Munoz DG. Familial frontotemporal dementia with ubiquitin-positive, tau-negative inclusions. Neurology. 2000 Feb 22;54(4):818-27. Abstract

2. Woulfe J, Kertesz A, Munoz DG. Frontotemporal dementia with ubiquitinated cytoplasmic and intranuclear inclusions. Acta Neuropathol 2001;102:94-102. Abstract

View all comments by Andrew Kertesz

With regard to a possible genetic contribution of TDP-43 to FTLD-U, our group reported negative results in Salzburg. We conducted a high-density genetic association and linkage disequilibrium mapping analysis to investigate whether common variations at the TDP-43 locus act as a risk factor for disease in the Manchester FTLD cohort. Among 214 patients not harboring a tau or progranulin mutation, who were clinically diagnosed as having either FTD, FTD with motor neuron disease, semantic dementia, or related disorders, we observed no significant SNP or haplotype association (Rollinson et al., 2007, in press). These data suggest that common variations do not increase genetic risk in our population, but it leaves open the possibility of rare mutations yet to be found.

View all comments by Stuart Pickering-Brown

These papers represent exciting work describing a new genetic mutation associated with familial ALS. The results further highlight the importance for RNA processing in at least familial forms of motor neuron disease. Much work remains to determine the exact mechanisms by which FUS modulates motor neuron survival. It may be related to that of TDP-43. However, the lack of cytoplasmic aggregation of TDP-43, and rare ubiquitin inclusions in the patients with FUS mutations, suggest the mechanisms may be distinct. It is interesting that FUS protein did not accumulate in the cytoplasm of motor neurons in sporadic ALS patients, again suggestive that the pathogenic mechanisms of mutant FUS-induced motor neuron degeneration may be distinct from that in sporadic ALS.

View all comments by Robert Bowser

These studies raise interesting questions about whether one problem in ALS and perhaps other neurodegenerative diseases is that RNA trafficking proteins fail to properly deliver RNAs to dendritic spines. The paper by Kwiatkowski et al. reports evidence that wild-type FUS and TDP-43 may be involved in transporting RNA into dendrites, where it mediates local protein synthesis that can be stimulated by neural activity. The clumping of the mutant form described by both new papers could therefore perturb the transport of RNA. Local protein synthesis in dendrites plays a major role in the activity-dependent modulation of synaptic strength. Changes in synaptic activity have been recently reported in the mouse model of SOD1 mutation (van Zundert et al., 2008), so it will be worthwhile to examine this issue in the FUS mice that will certainly be developed by these investigators.

View all comments by Eric Frank

This is an extremely exiting story in the understanding of ALS pathogenesis. It actually it dates back to 1998—with the first description of mRNA processing errors in sporadic ALS (Lin et al., 1998), which, interestingly, was made not in the SOD1 mouse model. At the same time, the spinal muscular atrophy gene was discovered. SMA is not unlike a childhood ALS, though predominately lower motor neurons are affected in that disease. The SMA gene defect is involved in RNA metabolism. So for the next 10 years, the SMA field has investigated the pathobiology of the defective protein. At the time it made the link between sporadic ALS and the SMA story intriguing. But there was no clear genetic link (or cause for the changes in sporadic ALS).

Feed forward to 2008, when Chris Shaw and others found a true genetic defect in RNA metabolism-based protein TDP-43. (Of course more work needs to be done on that.) And now another gene by the Shaw group, and now verified by the group in Boston, does set a string of targets that all focus on RNA...  Read more

This is an extremely exiting story in the understanding of ALS pathogenesis. It actually it dates back to 1998—with the first description of mRNA processing errors in sporadic ALS (Lin et al., 1998), which, interestingly, was made not in the SOD1 mouse model. At the same time, the spinal muscular atrophy gene was discovered. SMA is not unlike a childhood ALS, though predominately lower motor neurons are affected in that disease. The SMA gene defect is involved in RNA metabolism. So for the next 10 years, the SMA field has investigated the pathobiology of the defective protein. At the time it made the link between sporadic ALS and the SMA story intriguing. But there was no clear genetic link (or cause for the changes in sporadic ALS).

Feed forward to 2008, when Chris Shaw and others found a true genetic defect in RNA metabolism-based protein TDP-43. (Of course more work needs to be done on that.) And now another gene by the Shaw group, and now verified by the group in Boston, does set a string of targets that all focus on RNA metabolism and (lower) motor neurons.

By the way, all these cases appear to predominately involve a lower motor neuron form of ALS. The hint from genetics does suggest more of a loss of function rather than gain, but cell biology will ultimately sort that out. We certainly await the generation of mouse or fly models, which are now well underway for TDP-43. However, this may be a particularly difficult target for specific, non-toxic drug therapy.

View all comments by Jeffrey D. Rothstein

These back-to-back papers on the identification of FUS (fused in sarcoma) gene as a new genetic component of ALS open a new era of research and direct our attention to mRNA biology with respect to disease. After the first identification of mRNA processing errors in ALS patients (Lin, Bristol et al., 1998), the discovery of TDP-43 (Neumann, Sampathu et al., 2006) and now the FUS gene clearly indicate the importance of mRNA management in neurodegenerative diseases. Defects in RNA transcription, splicing, and trafficking may be the reason for cell-type-specific degeneration of motor neurons in ALS. Motor neurons both in the cortex and spinal cord are very large excitatory neurons that extend long axons to their targets and require high levels of energy and protein integrity for survival and function. Defects in transcriptional mechanisms may result in splicing defects, which could give rise to formation of non-functional proteins that would deplete the pool of required proteins for cellular function, and these non-functional proteins may form aggregates that are toxic to neurons. In...  Read more

These back-to-back papers on the identification of FUS (fused in sarcoma) gene as a new genetic component of ALS open a new era of research and direct our attention to mRNA biology with respect to disease. After the first identification of mRNA processing errors in ALS patients (Lin, Bristol et al., 1998), the discovery of TDP-43 (Neumann, Sampathu et al., 2006) and now the FUS gene clearly indicate the importance of mRNA management in neurodegenerative diseases. Defects in RNA transcription, splicing, and trafficking may be the reason for cell-type-specific degeneration of motor neurons in ALS. Motor neurons both in the cortex and spinal cord are very large excitatory neurons that extend long axons to their targets and require high levels of energy and protein integrity for survival and function. Defects in transcriptional mechanisms may result in splicing defects, which could give rise to formation of non-functional proteins that would deplete the pool of required proteins for cellular function, and these non-functional proteins may form aggregates that are toxic to neurons. In addition, defects in the trafficking of mRNA may lead to depletion of key proteins that are in high demand locally for motor neuron function. But if FUS has a general function in mRNA transcription, splicing, and trafficking, why do mutations in this gene cause ALS and not other neurodegenerative diseases? What makes motor neurons more vulnerable in the presence of defective FUS? It could be true that in motor neurons FUS controls the transcription of a distinct set of mRNA that is expressed in a cell-type-specific manner in motor neurons, or that FUS controls the production of a key protein that is highly required in motor neurons when compared to other cell-types, and thus motor neurons may become vulnerable first. FUS seems to be the tip of the iceberg. Finding effectors, binding partners including mRNA, may lead to the identification of key components of both familial and sporadic ALS. More work is on the way!

References:

Kneussel M. Dynamic regulation of GABA(A) receptors at synaptic sites. Brain Res Brain Res Rev. 2002 Jun ;39(1):74-83. Abstract

Lin CL, Bristol LA, Jin L, Dykes-Hoberg M, Crawford T, Clawson L, Rothstein JD. Aberrant RNA processing in a neurodegenerative disease: the cause for absent EAAT2, a glutamate transporter, in amyotrophic lateral sclerosis. Neuron. 1998 Mar;20(3):589-602. Abstract

Neumann M, Sampathu DM, Kwong LK, Truax AC, Micsenyi MC, Chou TT, Bruce J, Schuck T, Grossman M, Clark CM, McCluskey LF, Miller BL, Masliah E, Mackenzie IR, Feldman H, Feiden W, Kretzschmar HA, Trojanowski JQ, Lee VM. Ubiquitinated TDP-43 in frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Science. 2006 Oct 6;314(5796):130-3. Abstract

Vance C, Rogelj B, Hortobágyi T, De Vos KJ, Nishimura AL, Sreedharan J, Hu X, Smith B, Ruddy D, Wright P, Ganesalingam J, Williams KL, Tripathi V, Al-Saraj S, Al-Chalabi A, Leigh PN, Blair IP, Nicholson G, de Belleroche J, Gallo JM, Miller CC, Shaw CE. Mutations in FUS, an RNA processing protein, cause familial amyotrophic lateral sclerosis type 6. Science. 2009 Feb 27;323(5918):1208-11. Abstract

View all comments by P. Hande Ozdinler