29 February 2008. Working independently, a U.S. and a British-Australian team of geneticists both report this week that they have found mutations of the TAR DNA binding protein (TDP-43) in people with amyotrophic lateral sclerosis (ALS). Also known as Lou Gehrig’s disease, ALS afflicts some 30,000 Americans and often leads to death within a few years of diagnosis. TDP-43 gene mutations were rare in both sporadic and familial ALS in the studies, but coupled with the recent finding that TDP-43 is a main ingredient of aggregates seen in motor neurons of ALS patients, and in the neurodegenerative disease frontotemporal lobar dementia with ubiquitin inclusions (FTLD-U), the new discovery places TDP-43 at the cutting edge of ALS research.
Some patients develop symptoms of either disease, or a combination of the two, leading to the emerging hypothesis that they are part of the same spectrum. ALS is inherited in around 5-10 percent of cases, with an autosomal-dominant pattern in some families. The first mutation linked to familial ALS was found in the superoxide dismutase-1 (SOD1) protein 15 years ago, and ever since then scientists have been trying to use SOD1 mutations to model sporadic ALS. Interestingly, abnormal accumulations of TDP-43 in motor neurons and glial cells show up in the majority of ALS cases (and also in FTLD-U) except where SOD1 is mutated, suggesting the possibility that they may be two different disorders.
Michael Gitcho, Alison Goate, and Nigel Cairns at the Alzheimer’s Disease Research Center at the Washington University School of Medicine in St. Louis, Missouri, and colleagues published their findings in the February 20 online edition of Annals of Neurology. They analyzed DNA from eight families with ALS and 30 families with FTLD-U and found a missense mutation in exon 6 of the TDP-43 gene, where a threonine appeared instead of an alanine in amino acid position 315. This A315T mutation segregated in all four afflicted family members but not in their healthy relatives. None of 1,505 controls had the mutation. This is the first time scientists have found a mutation in TDP-43 linked to a dominantly inherited form of ALS.
“We have this missense mutation, but we don't know how it affects the function of TDP-43 and leads to the degeneration of motor neurons,” said Goate. “Two areas that need to be developed are an animal model and establishing the precise mechanism of the disease.”
The second study, out today in Science, by scientists at King's College London, the University of Sydney, and the Northcott Neuroscience Laboratory at the ANZAC Research Institute in Australia, also found missense mutations within exon 6 of the TDP-43 gene. These authors describe two different mutations in two sporadic cases, and a third mutation in familial ALS that segregated within one family. This mutation, M337V, showed up in five relatives with ALS but not in nine unaffected siblings and, according to the authors, most likely substitutes the amino acid valine for methionine.
Led by Christopher Shaw at King’s College, the British-Australian researchers sequenced all six exons of the TDP-43 gene in 500 controls and 200 sporadic ALS cases (all of British descent) and found in one a missense mutation in exon 6, substituting lysine for glutamine (Q331K). They also screened 172 Australian sporadic cases, 172 controls, and an additional 200 British controls, and found a mutation in one patient's exon 6 that substituted alanine for glycine (G294A). No mutations showed up in exon 6 of 390 further British and Australian controls. Overall, the researchers found mutations in three out of 526 ALS cases (one in a kindred out of 154 familial and two out of 372 sporadic cases) and none in 1,262 controls.
“These studies place TDP-43 at the center of our hypothesis as to what's causing the disease,” said Ian Mackenzie from the University of British Columbia and Vancouver General Hospital. “They are incredibly important because they provide extremely strong evidence that the protein plays a central role in the disease pathogenesis. Even in cases where the gene itself is normal, there's still something going wrong with the protein.” Cell biologist Leonard Petrucelli at the Mayo Clinic in Jacksonville, Florida, added, “These papers are going to be critical to the field; they legitimize the pathological significance of TDP-43 in ALS.”
All these mutations fell into a highly conserved region of TDP-43, near the glycerin-rich C terminal that scientists think is involved in protein-protein interactions. According to Mackenzie, this makes them even more interesting, and will help researchers zoom in on this region and look for further mutations that are harder to detect. “All mutations are in exon 6, so it raises the hypothesis that there's something particular about that part of the protein that will affect processing or some other important aspect,” Mackenzie said.
While TDP-43's exact function remains a mystery, it is known to be involved in regulating messenger RNA splicing. Previous papers have shown that when TDP-43 fragments show up in the cytoplasm of neurons in FTLD and ALS, the protein is noticeably absent from the nucleus (Neumann et al., 2006; Arai et al., 2006; Mackenzie et al., 2007).
Moving toward a mechanistic dissection of these mutations, the British-Australian team tested the familial TDP-43 mutation M337V and the sporadic mutation Q331K in Chinese hamster ovary cells, where they found no difference in cell death compared to wild-type TDP-43. But when they expressed the two mutations in the spinal cord of chick embryos, they found that the TDP-43 mutations induced a dramatic failure in the embryos to develop normal limb and tail buds. Only 5 to 15 percent of those body parts grew normally after 24 hours compared to the controls of normal TDP-43, normal SOD1, and mutant SOD1. TUNEL staining showed that those embryos had a greater number of apoptotic nuclei, hinting that mutant TDP-43 leads to a toxic gain of function or a dominant-negative effect.
Studies exploring how TDP-43 functions are sure to come out at a rapid clip in the next few years. For example, a paper published February 27 in PNAS online by Youhna M. Ayala and Francisco Baralle at the International Centre for Genetic Engineering and Biotechnology in Trieste, Italy, and Tom Misteli at the National Institutes of Health in Bethesda, Maryland, examined the function of TDP-43 in human cell lines. These researchers have been studying TDP-43 before other scientists linked it to neurodegenerative diseases in 2006. Removing the protein from those human cells caused apoptosis, altered the shape of the nucleus, and caused misregulation of the cell cycle. (Other research groups have previously implicated the cell cycle in neurodegenerative diseases.) The absence of TDP-43 led to an increase in cyclin-dependent kinase 6 protein, which correlated with increased phosphorylation of the retinoblastoma protein pRb and pRb-related protein pRb2/p130. Baralle and colleagues proposed that TDP-43 is intimately tied in with essential cellular metabolic processes. For more on this fast-moving area of science, stay tuned.—Kristina Grifantini.
Kristina Grifantini is a freelance science writer in Boston, Massachusetts.
Gitcho MA, Baloh RH, Chakraverty S, Mayo K, Norton JB, Levitch D, Hatanpaa KJ, White CL, Bigio EH, Caselli R, Baker M, Al-Lozi MT, Morris JC, Pestronk A, Rademakers R, Goate AM, Cairns NJ. TDP-43 A315T mutation in familial motor neuron disease. Ann Neurol. 2008 Feb 20; Abstract
Sreedharan J, Blair IP, Tripathi VB, Hu X, Vance C, Rogelj B, Ackerley S, Durnall JC, Williams KL, Buratti E, Baralle F, de Belleroche J, Mitchell JD, Leigh PN, Al-Chalabi A, Miller CC, Nicholson G, Shaw CE. TDP-43 Mutations in Familial and Sporadic Amyotrophic Lateral Sclerosis Sciencexpress. Abstract
Ayala YM, Misteli T, Baralle FE. TDP-43 regulates retinoblastoma protein phosphorylation through the repression of cyclin-dependent kinase 6 expression. Proc Natl Acad Sci U S A. 2008 Feb 27; Abstract