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The “somatic-spread” hypothesis for sporadic neurodegenerative diseases
Medical Hypotheses 77 (2011) 544–547
Contents lists available at ScienceDirect
Medical Hypotheses journal homepage: www.elsevier.com/locate/mehy
The ‘‘somatic-spread’’ hypothesis for sporadic neurodegenerative diseases Roger Pamphlett ⇑ The Stacey Motor Neuron Disease Laboratory, Department of Pathology D06, Sydney Medical School, The University of Sydney, New South Wales 2006, Australia
a r t i c l e
i n f o
Article history: Received 27 April 2011 Accepted 9 June 2011
a b s t r a c t The major neurodegenerative diseases (Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis) share in common a mostly sporadic occurrence, a focal onset of pathology, and spread from the initial site of injury to adjacent regions of the nervous system. The sporadic nature and focal onset of these diseases can be explained either by somatic mutations (arising in either of two models of cell lineage) or environmental agents, both of which affect a small number of neurons. The genetic or environmental agent then changes the conformation of a vital protein in these neurons. Spread of the diseases occurs by the misfolded proteins being transferred to adjacent neurons. Clinical and pathological details of one neurodegenerative disorder, amyotrophic lateral sclerosis, are presented to show how the pathogenesis of a typical neurodegenerative disease can be explained by this ‘‘somatic-spread’’ hypothesis. Ultrasensitive techniques will be needed to detect the initiating genetic or environmental differences that are predicted to be present in only a few cells. Ó 2011 Elsevier Ltd. All rights reserved.
Introduction Any hypothesis that aims to explain the pathogenesis of the major neurodegenerative diseases (Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis) must take into account three major features of these diseases: their mostly sporadic occurrence, the focal or multifocal onset of pathology, and the progressive spread of the pathology to adjacent regions of the nervous system. In amyotrophic lateral sclerosis (ALS), a prototypical neurodegenerative disorder, a progressive loss of motor neurons leads to weakness and death within a few years after diagnosis. The cause of ALS in the great majority of patients remains unknown [1]. ALS is usually a sporadic disease (SALS), with only about 5% of cases occurring within families (FALS) [2]. While gene mutations have been found in about 20% of patients with FALS, the cause of SALS remains a mystery. A sporadic disease usually brings to mind some environmental agent, but genetic variants, for example somatic mutations [3], can also produce a disease that does not run in families. Two cell lineage models can be affected by somatic mutations Germline mutations affect all the cells of the body and so have a chance of being passed onto the next generation through the gametes. In early embryonic development, the germline progenitor cells separate from the dividing zygote before the somatic progen⇑ Tel.: +61 2 9351 3318; fax: +61 2 9351 3429. E-mail address: [email protected] 0306-9877/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mehy.2011.06.027
itors [4,5] (Fig. 1). This means that a somatic progenitor cell can suffer a mutation that does not affect the germline [3]. These somatic mutations can consist of single nucleotide variants, structural variants, or epigenetic changes. The traditional way of depicting human cell lineages is to portray somatic progenitor cells as dividing in an orderly manner until a number of stem cells are produced, each of which then gives rise to a particular cell line [3,6,7] (Fig. 1A). In this schema, a mutation in a later progenitor cell or a stem cell would affect only one or a few cell lines, such as the motor neurons or neuroepithelial-derived cells. The earlier in development a mutation occurs, the more daughter cells will carry the mutation. The above ‘‘orderly-division’’ representation may be an oversimplification, however. In the mouse, for example, about eight early somatic founder cells intermingle in the different tissues of the body [4] (Fig. 1B). There is no reason to expect the situation in other mammals such as humans is not similar. This mixed-progenitor cell model implies that a mutation in an early somatic progenitor cell will be found in a proportion of cells in all the tissues of the body, and not just in one or a few cell lines. The mixing of progenitor cells within the tissues also means that the number of cells in any particular tissue carrying the progenitor mutation is likely to be quite small. Environmental variations may underlie some cases of SALS Numerous environmental toxins and viruses have been implicated in SALS. Motor neurons may be particularly susceptible to these agents, for example, because they have viral receptors that allow entry into the neurons [8] or because the neurons take up
545
R. Pamphlett / Medical Hypotheses 77 (2011) 544–547
A
B
Fig. 1. Two models of human cell lineages have been proposed. In both, the gamete-producing germline progenitor cells are formed before the somatic cell lineages start dividing. In model A, progenitor cells progressively divide in an orderly hierarchy, until finally stem cells for individual cell lines result. In model B, a small group of early progenitor cells are mixed together and so are all represented in each cell line.
toxins selectively [9]. A proposal that is commonly put forward is that SALS is due to a genetic susceptibility to an environmental agent such as a toxic heavy metal [10].
A
B
C
Motor neuron damage in SALS spreads through the CNS Clinical observations indicate that motor neuron loss in SALS starts unifocally or multifocally in the CNS, and then spreads to involve adjacent motor neurons [11–14]. For example, a study of 100 ALS patients showed that the disease started in a single body region in 98% of patients, and that over time both upper and lower motor neuron degeneration advanced to contiguous regions [15]. This clinical impression has been confirmed pathologically by showing that the motor neuron loss of SALS is graded away from the region of disease onset [16].
Misfolded proteins can spread disease through the nervous system The three genes which account for the majority of known germline mutations in FALS are SOD1, TDP-43, and FUS. When the CNS of FALS patients with these mutations is examined post mortem, abnormal inclusions of the proteins expressed by these genes are found in both motor neurons and glial cells [17–21]. Patients with SALS also show alterations in motor neuron and glial TDP-43 [19,20,22] and well as in SOD1 [23]. It has long been known some neurodegenerative diseases are spread through the CNS by prion proteins which have undergone pathological conformational changes. A growing body of evidence now suggests that misfolded proteins also have the capacity to spread SALS through the nervous system [24,25]. SOD1 [26,27], TDP-43 [28,29] and FUS [30] have certain similarities to prion proteins, suggesting that these proteins when misfolded can spread motor neuron damage to adjacent regions of the nervous system.
Fig. 2. Three pathways can lead to a sporadic and spreading disease of motor neurons. In pathway A, a mutation in a late progenitor cell affects a proportion (dark borders) of triangular motor neurons, but no other cell lines, such as the starshaped glia. In pathway B, a mutation in an early progenitor cell is admixed in different cell lines and so affects a proportion of both motor neurons and glia. In pathway C, an environmental agent affects some motor neurons preferentially. After each of these insults, an altered protein (red cytoplasm) arises within the affected motor neurons and spreads (arrow heads) to other motor neurons, which are damaged or destroyed (dashed borders). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Testing the hypothesis The hypothesis A combination of two mechanisms underlies sporadic neurodegenerative diseases (Fig. 2). Firstly, the sporadic occurrence of the disease is explained by the primary insult being either a somatic mutation or an environmental agent that affects a focal group of neurons. Secondly, the spread of the disease is explained by the primary insult altering the conformation of a protein in this group of neurons, with the misfolded protein spreading the disease to related neurons in the nervous system.
It is possible that each of the three initial insult pathways described above could be responsible for a proportion of cases of SALS. Each pathway will need a different experimental approach to assess its importance. Late progenitor cell (cell line) mutations If only a single cell line (such as the motor neurons) contains the somatic mutation, very sensitive mutation-detecting methods will
546
R. Pamphlett / Medical Hypotheses 77 (2011) 544–547
be needed to detect it, since motor neurons make up a small proportion of any CNS tissue sample. A further difficulty in human SALS research is that motor neurons have largely disappeared by the time of death, and those that survive may be the ones that do not contain the pathogenic agent (a ‘‘survivor effect’’). This makes techniques such as laser capture microdissection less likely to be useful in finding somatic mutations, even though it can be used to detect germline mutations in motor neurons [31]. Sanger sequencing can only detect mutations that occur in 20% or more cells of a tissue, and even methods such as DHPLC that can detect mutations in smaller percentages of cells [32] may not be sensitive enough to discover mutations that are only present in a few surviving cells. Massively parallel sequencing on the other hand with its great depths of coverage is now able to detect mutations in ever smaller proportions of cells [33], so this is likely to become the method of choice to detect somatic mutations. Early progenitor cell (admixed) mutations Sensitive mutation-detection techniques such as those mentioned above will be needed to find any of these admixed somatic mutations since they will be present in only a proportion of the cells. However, there will be a better chance of finding these admixed mutations since they will be present in a proportion of cells within all tissues. This means that techniques that can detect mutations in a small proportion of a tissue will be able to find the admixed mutations in nervous tissue, even in the absence of motor neurons, since they will also be found in a proportion of glial cells (Fig. 2). An important clinical point is that admixed somatic mutations would also be present in clinically-available samples such as blood or hair, so the diagnosis of a SALS-causing somatic mutation could be made during life. Environmental agents Finding an environment agent that affects a small number of nervous system cells will be challenging, especially if these cells die after passing on their misfolded protein to other cells. The environmental agent may leave a fingerprint, however, such as SOD1 oxidation [23], which would be more easily detectible than finding the agent itself. With the recent availability of transgenic mice that express wildtype human proteins such as SOD1 and TDP-43, it will be possible to administer these mice a variety of toxins or infectious agents to see if they cause the protein changes that are characteristically found in SALS.
Other sporadic neurodegenerative disorders The brain region that is first affected by Alzheimer’s disease is the transentorhinal layer in the medial temporal lobe, with subsequent spread of the disease to the hippocampus and neocortex [34]. Hyperphosphorylated tau is the major component of neurofibrillary tangles of Alzheimer’s disease, and of interest is the finding that mutant tau injected into mouse brain spreads the pathology from the site of injection into neighboring brain regions [35]. In Parkinson’s disease the neurons first affected seem to be those of the dorsal vagal nucleus in the medulla oblongata, with later spread into the upper brain stem and cerebral cortex [36]. The major protein involved in Parkinson’s disease, alpha synuclein, can be transferred from cell to cell when misfolded [37]. Hence a number of similarities exist between the major sporadic neurodegenerative diseases as regards focality of disease onset and later spread of pathology. The ‘‘somatic-spread’’ hypothesis therefore seems to be applicable to these diseases as well.
Conclusion Evidence is mounting that misfolded proteins can spread the pathology of sporadic neurodegenerative disorders from a focal site, which has been injured by some genetic or environmental mechanism. The challenges in the future are to find out which somatic insults are causing the initial protein change (so that these can be treated or avoided), and to discover ways of preventing the misfolded proteins from spreading though the nervous system. Conflict of interest None declared. Acknowledgment The Stacey MND Laboratory is supported by the Stacey and Burnett Motor Neuron Disease Bequests and by the Australian Motor Neuron Disease Research Institute. References [1] Kiernan MC, Vucic S, Cheah BC, et al. Amyotrophic lateral sclerosis. Lancet 2011;377:942–55. [2] Byrne S, Walsh C, Lynch C, et al. Rate of familial amyotrophic lateral sclerosis: a systematic review and meta-analysis. J Neurol Neurosurg Psychiatry 2011;82:623–7. [3] Youssoufian H, Pyeritz RE. Mechanisms and consequences of somatic mosaicism in humans. Nat Rev Genet 2002;3:748–58. [4] Soriano P, Jaenisch R. Retroviruses as probes for mammalian development: allocation of cells to the somatic and germ cell lineages. Cell 1986;46:19–29. [5] Pang AW, MacDonald JR, Pinto D, et al. Towards a comprehensive structural variation map of an individual human genome. Genome Biol 2010;11:R52. [6] Pamphlett R. Somatic mutation: a cause of sporadic neurodegenerative diseases? Med Hypotheses 2004;62:679–82. [7] Frank SA. Evolution in Health and Medicine Sackler Colloquium: somatic evolutionary genomics: mutations during development cause highly variable genetic mosaicism with risk of cancer and neurodegeneration. Proc Natl Acad Sci USA 2010;107(Suppl. 1):1725–30. [8] Saunderson R, Yu B, Trent RJ, Pamphlett R. A polymorphism in the poliovirus receptor gene differs in motor neuron disease. Neuroreport 2004;15:383–6. [9] Pamphlett R, Waley P. Motor neuron uptake of low dose inorganic mercury. J Neurol Sci 1996;135:63–7. [10] Morahan JM, Yu B, Trent RJ, Pamphlett R. Genetic susceptibility to environmental toxicants in ALS. Am J Med Genet B Neuropsychiatr Genet 2007;144:885–90. [11] Caroscio JT, Mulvihill MN, Sterling R, Abrams B. Amyotrophic lateral sclerosis. Its natural history. Neurol Clin 1987;5:1–8. [12] Brooks BR. The role of axonal transport in neurodegenerative disease spread: a meta-analysis of experimental and clinical poliomyelitis compares with amyotrophic lateral sclerosis. Can J Neurol Sci 1991;18:435–8. [13] Turner MR, Brockington A, Scaber J, et al. Pattern of spread and prognosis in lower limb-onset ALS. Amyotroph Lateral Scler 2010;11:369–73. [14] Korner S, Kollewe K, Fahlbusch M, et al. Onset and spreading patterns of upper and lower motor neuron symptoms in amyotrophic lateral sclerosis. Muscle Nerve 2011;43:636–42. [15] Ravits J, Paul P, Jorg C. Focality of upper and lower motor neuron degeneration at the clinical onset of ALS. Neurology 2007;68:1571–5. [16] Ravits J, Laurie P, Fan Y, Moore DH. Implications of ALS focality: rostral–caudal distribution of lower motor neuron loss postmortem. Neurology 2007;68:1576–82. [17] Jonsson PA, Ernhill K, Andersen PM, et al. Minute quantities of misfolded mutant superoxide dismutase-1 cause amyotrophic lateral sclerosis. Brain 2004;127:73–88. [18] Kato S, Hayashi H, Nakashima K, et al. Pathological characterization of astrocytic hyaline inclusions in familial amyotrophic lateral sclerosis. Am J Pathol 1997;151:611–20. [19] Tan CF, Eguchi H, Tagawa A, et al. TDP-43 immunoreactivity in neuronal inclusions in familial amyotrophic lateral sclerosis with or without SOD1 gene mutation. Acta Neuropathol 2007;113:535–42. [20] Mackenzie IR, Bigio EH, Ince PG, et al. Pathological TDP-43 distinguishes sporadic amyotrophic lateral sclerosis from amyotrophic lateral sclerosis with SOD1 mutations. Ann Neurol 2007;61:427–34. [21] Kobayashi Z, Tsuchiya K, Arai T, et al. Occurrence of basophilic inclusions and FUS-immunoreactive neuronal and glial inclusions in a case of familial amyotrophic lateral sclerosis. J Neurol Sci 2010;293:6–11. [22] Hasegawa M, Arai T, Nonaka T, et al. Phosphorylated TDP-43 in frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Ann Neurol 2008;64:60–70.
R. Pamphlett / Medical Hypotheses 77 (2011) 544–547 [23] Bosco DA, Morfini G, Karabacak NM, et al. Wild-type and mutant SOD1 share an aberrant conformation and a common pathogenic pathway in ALS. Nat Neurosci 2010;13:1396–403. [24] Dangond F, Hwang D, Camelo S, et al. Molecular signature of late-stage human ALS revealed by expression profiling of postmortem spinal cord gray matter. Physiol Genom 2004;16:229–39. [25] Ross CA, Poirier MA. Opinion: what is the role of protein aggregation in neurodegeneration? Nat Rev Mol Cell Biol 2005;6:891–8. [26] Chia R, Tattum MH, Jones S, Collinge J, Fisher EM, Jackson GS. Superoxide dismutase 1 and tgSOD1 mouse spinal cord seed fibrils, suggesting a propagative cell death mechanism in amyotrophic lateral sclerosis. PLoS One 2010;5:e10627. [27] Munch C, O’Brien J, Bertolotti A. Prion-like propagation of mutant superoxide dismutase-1 misfolding in neuronal cells. Proc Natl Acad Sci USA 2011;108:3548–53. [28] Fuentealba RA, Udan M, Bell S, et al. Interaction with polyglutamine aggregates reveals a Q/N-rich domain in TDP-43. J Biol Chem 2010;285:26304–14. [29] Liu-Yesucevitz L, Bilgutay A, Zhang YJ, et al. Tar DNA binding protein-43 (TDP43) associates with stress granules: analysis of cultured cells, pathological brain tissue. PLoS One 2010;5:e13250.
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[30] Udan M, Baloh RH. Implications of the prion-related Q/N domains in TDP-43 and FUS. Prion 2011;5:1–5. [31] Pamphlett R, Heath PR, Holden H, Ince PG, Shaw PJ. Detection of mutations in whole genome-amplified DNA from laser-microdissected neurons. J Neurosci Meth 2005;147:65–7. [32] Luquin N, Yu B, Trent RJ, Pamphlett R. DHPLC can be used to detect low-level mutations in amyotrophic lateral sclerosis. Amyotroph Lateral Scler 2010;11:76–82. [33] Gottlieb B, Beitel LK, Alvarado C, Trifiro MA. Selection and mutation in the ‘‘new’’ genetics: an emerging hypothesis. Hum Genet 2010;127:491–501. [34] Braak H, Braak E. Neuropathological stageing of Alzheimer-related changes. Acta Neuropathol 1991;82:239–59. [35] Clavaguera F, Bolmont T, Crowther RA, et al. Transmission and spreading of tauopathy in transgenic mouse brain. Nat Cell Biol 2009;11:909–13. [36] Braak H, Rub U, Gai WP, Del Tredici K. Idiopathic Parkinson’s disease: possible routes by which vulnerable neuronal types may be subject to neuroinvasion by an unknown pathogen. J Neural Transm 2003;110:517–36. [37] Hansen C, Angot E, Bergstrom AL, et al. Alpha-Synuclein propagates from mouse brain to grafted dopaminergic neurons and seeds aggregation in cultured human cells. J Clin Invest 2011;121:715–25.
Contents lists available at ScienceDirect
Medical Hypotheses journal homepage: www.elsevier.com/locate/mehy
The ‘‘somatic-spread’’ hypothesis for sporadic neurodegenerative diseases Roger Pamphlett ⇑ The Stacey Motor Neuron Disease Laboratory, Department of Pathology D06, Sydney Medical School, The University of Sydney, New South Wales 2006, Australia
a r t i c l e
i n f o
Article history: Received 27 April 2011 Accepted 9 June 2011
a b s t r a c t The major neurodegenerative diseases (Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis) share in common a mostly sporadic occurrence, a focal onset of pathology, and spread from the initial site of injury to adjacent regions of the nervous system. The sporadic nature and focal onset of these diseases can be explained either by somatic mutations (arising in either of two models of cell lineage) or environmental agents, both of which affect a small number of neurons. The genetic or environmental agent then changes the conformation of a vital protein in these neurons. Spread of the diseases occurs by the misfolded proteins being transferred to adjacent neurons. Clinical and pathological details of one neurodegenerative disorder, amyotrophic lateral sclerosis, are presented to show how the pathogenesis of a typical neurodegenerative disease can be explained by this ‘‘somatic-spread’’ hypothesis. Ultrasensitive techniques will be needed to detect the initiating genetic or environmental differences that are predicted to be present in only a few cells. Ó 2011 Elsevier Ltd. All rights reserved.
Introduction Any hypothesis that aims to explain the pathogenesis of the major neurodegenerative diseases (Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis) must take into account three major features of these diseases: their mostly sporadic occurrence, the focal or multifocal onset of pathology, and the progressive spread of the pathology to adjacent regions of the nervous system. In amyotrophic lateral sclerosis (ALS), a prototypical neurodegenerative disorder, a progressive loss of motor neurons leads to weakness and death within a few years after diagnosis. The cause of ALS in the great majority of patients remains unknown [1]. ALS is usually a sporadic disease (SALS), with only about 5% of cases occurring within families (FALS) [2]. While gene mutations have been found in about 20% of patients with FALS, the cause of SALS remains a mystery. A sporadic disease usually brings to mind some environmental agent, but genetic variants, for example somatic mutations [3], can also produce a disease that does not run in families. Two cell lineage models can be affected by somatic mutations Germline mutations affect all the cells of the body and so have a chance of being passed onto the next generation through the gametes. In early embryonic development, the germline progenitor cells separate from the dividing zygote before the somatic progen⇑ Tel.: +61 2 9351 3318; fax: +61 2 9351 3429. E-mail address: [email protected] 0306-9877/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mehy.2011.06.027
itors [4,5] (Fig. 1). This means that a somatic progenitor cell can suffer a mutation that does not affect the germline [3]. These somatic mutations can consist of single nucleotide variants, structural variants, or epigenetic changes. The traditional way of depicting human cell lineages is to portray somatic progenitor cells as dividing in an orderly manner until a number of stem cells are produced, each of which then gives rise to a particular cell line [3,6,7] (Fig. 1A). In this schema, a mutation in a later progenitor cell or a stem cell would affect only one or a few cell lines, such as the motor neurons or neuroepithelial-derived cells. The earlier in development a mutation occurs, the more daughter cells will carry the mutation. The above ‘‘orderly-division’’ representation may be an oversimplification, however. In the mouse, for example, about eight early somatic founder cells intermingle in the different tissues of the body [4] (Fig. 1B). There is no reason to expect the situation in other mammals such as humans is not similar. This mixed-progenitor cell model implies that a mutation in an early somatic progenitor cell will be found in a proportion of cells in all the tissues of the body, and not just in one or a few cell lines. The mixing of progenitor cells within the tissues also means that the number of cells in any particular tissue carrying the progenitor mutation is likely to be quite small. Environmental variations may underlie some cases of SALS Numerous environmental toxins and viruses have been implicated in SALS. Motor neurons may be particularly susceptible to these agents, for example, because they have viral receptors that allow entry into the neurons [8] or because the neurons take up
545
R. Pamphlett / Medical Hypotheses 77 (2011) 544–547
A
B
Fig. 1. Two models of human cell lineages have been proposed. In both, the gamete-producing germline progenitor cells are formed before the somatic cell lineages start dividing. In model A, progenitor cells progressively divide in an orderly hierarchy, until finally stem cells for individual cell lines result. In model B, a small group of early progenitor cells are mixed together and so are all represented in each cell line.
toxins selectively [9]. A proposal that is commonly put forward is that SALS is due to a genetic susceptibility to an environmental agent such as a toxic heavy metal [10].
A
B
C
Motor neuron damage in SALS spreads through the CNS Clinical observations indicate that motor neuron loss in SALS starts unifocally or multifocally in the CNS, and then spreads to involve adjacent motor neurons [11–14]. For example, a study of 100 ALS patients showed that the disease started in a single body region in 98% of patients, and that over time both upper and lower motor neuron degeneration advanced to contiguous regions [15]. This clinical impression has been confirmed pathologically by showing that the motor neuron loss of SALS is graded away from the region of disease onset [16].
Misfolded proteins can spread disease through the nervous system The three genes which account for the majority of known germline mutations in FALS are SOD1, TDP-43, and FUS. When the CNS of FALS patients with these mutations is examined post mortem, abnormal inclusions of the proteins expressed by these genes are found in both motor neurons and glial cells [17–21]. Patients with SALS also show alterations in motor neuron and glial TDP-43 [19,20,22] and well as in SOD1 [23]. It has long been known some neurodegenerative diseases are spread through the CNS by prion proteins which have undergone pathological conformational changes. A growing body of evidence now suggests that misfolded proteins also have the capacity to spread SALS through the nervous system [24,25]. SOD1 [26,27], TDP-43 [28,29] and FUS [30] have certain similarities to prion proteins, suggesting that these proteins when misfolded can spread motor neuron damage to adjacent regions of the nervous system.
Fig. 2. Three pathways can lead to a sporadic and spreading disease of motor neurons. In pathway A, a mutation in a late progenitor cell affects a proportion (dark borders) of triangular motor neurons, but no other cell lines, such as the starshaped glia. In pathway B, a mutation in an early progenitor cell is admixed in different cell lines and so affects a proportion of both motor neurons and glia. In pathway C, an environmental agent affects some motor neurons preferentially. After each of these insults, an altered protein (red cytoplasm) arises within the affected motor neurons and spreads (arrow heads) to other motor neurons, which are damaged or destroyed (dashed borders). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Testing the hypothesis The hypothesis A combination of two mechanisms underlies sporadic neurodegenerative diseases (Fig. 2). Firstly, the sporadic occurrence of the disease is explained by the primary insult being either a somatic mutation or an environmental agent that affects a focal group of neurons. Secondly, the spread of the disease is explained by the primary insult altering the conformation of a protein in this group of neurons, with the misfolded protein spreading the disease to related neurons in the nervous system.
It is possible that each of the three initial insult pathways described above could be responsible for a proportion of cases of SALS. Each pathway will need a different experimental approach to assess its importance. Late progenitor cell (cell line) mutations If only a single cell line (such as the motor neurons) contains the somatic mutation, very sensitive mutation-detecting methods will
546
R. Pamphlett / Medical Hypotheses 77 (2011) 544–547
be needed to detect it, since motor neurons make up a small proportion of any CNS tissue sample. A further difficulty in human SALS research is that motor neurons have largely disappeared by the time of death, and those that survive may be the ones that do not contain the pathogenic agent (a ‘‘survivor effect’’). This makes techniques such as laser capture microdissection less likely to be useful in finding somatic mutations, even though it can be used to detect germline mutations in motor neurons [31]. Sanger sequencing can only detect mutations that occur in 20% or more cells of a tissue, and even methods such as DHPLC that can detect mutations in smaller percentages of cells [32] may not be sensitive enough to discover mutations that are only present in a few surviving cells. Massively parallel sequencing on the other hand with its great depths of coverage is now able to detect mutations in ever smaller proportions of cells [33], so this is likely to become the method of choice to detect somatic mutations. Early progenitor cell (admixed) mutations Sensitive mutation-detection techniques such as those mentioned above will be needed to find any of these admixed somatic mutations since they will be present in only a proportion of the cells. However, there will be a better chance of finding these admixed mutations since they will be present in a proportion of cells within all tissues. This means that techniques that can detect mutations in a small proportion of a tissue will be able to find the admixed mutations in nervous tissue, even in the absence of motor neurons, since they will also be found in a proportion of glial cells (Fig. 2). An important clinical point is that admixed somatic mutations would also be present in clinically-available samples such as blood or hair, so the diagnosis of a SALS-causing somatic mutation could be made during life. Environmental agents Finding an environment agent that affects a small number of nervous system cells will be challenging, especially if these cells die after passing on their misfolded protein to other cells. The environmental agent may leave a fingerprint, however, such as SOD1 oxidation [23], which would be more easily detectible than finding the agent itself. With the recent availability of transgenic mice that express wildtype human proteins such as SOD1 and TDP-43, it will be possible to administer these mice a variety of toxins or infectious agents to see if they cause the protein changes that are characteristically found in SALS.
Other sporadic neurodegenerative disorders The brain region that is first affected by Alzheimer’s disease is the transentorhinal layer in the medial temporal lobe, with subsequent spread of the disease to the hippocampus and neocortex [34]. Hyperphosphorylated tau is the major component of neurofibrillary tangles of Alzheimer’s disease, and of interest is the finding that mutant tau injected into mouse brain spreads the pathology from the site of injection into neighboring brain regions [35]. In Parkinson’s disease the neurons first affected seem to be those of the dorsal vagal nucleus in the medulla oblongata, with later spread into the upper brain stem and cerebral cortex [36]. The major protein involved in Parkinson’s disease, alpha synuclein, can be transferred from cell to cell when misfolded [37]. Hence a number of similarities exist between the major sporadic neurodegenerative diseases as regards focality of disease onset and later spread of pathology. The ‘‘somatic-spread’’ hypothesis therefore seems to be applicable to these diseases as well.
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