Introduction
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder that affects neurones. Tar DNA-binding protein 43 (TDP-43) is a protein coded by the TARDBP gene (a gene with mutations that are known to cause ALS). TDP-43 is typically found in affected neurones, even in non-genetically caused cases. TDP-43 is found within all cells, this applies to all motor neurone diseases (MND) except MND cases caused by SOD1 mutations. In health human cells TDP-43 shuttles between the nucleus and cytoplasm of a cell (the liquid within a cell) and is mainly located within the nucleus of a cell. In MND cases this ‘shuttling’ does not occur causing large aggregated to form within the cytoplasm resulting in TDP-proteinopathy.TDP-43 has also been found within the peripheral blood of mononuclear cells (white blood cells) in ALS patients, this is detected though the aforementioned TDP aggregates. This has raised suspicion about a functional imbalance of immune cells in disease (ALS) which would support previous reported inflammation within the ALS nerves. Abnormal inflammatory responses are sustained by protein aggregates (DAMPs). DAMPs stand’s for danger associated molecular patterns, these are ‘danger’ signals that are discharged to the extracellular space (space outside of the cells) in response to damage be this trauma or pathological. DAMP’s when released result in an inflammatory response of which is not infectious, this is done through a binding pattern.
The adaptive immune response against TDP-43 inclusions could be responsible for the production of naturally occurring TDP-43 targeting antibodies (defined anti TDP-43 Nabs). This has been demonstrated with many neurodegenerative disorders, such as Alzheimer’s disease (AD) and Parkinson’s disease.
This study looks at whether anti-TDP-43 Nabs levels change within ALS patients, compared to controls and if they have a pathogenic role in ALS onset and progression. A hypothesis has been formed, that lower levels of anti-DAMP serum Nabs could be related to increased protein aggregation, explaining the seen neuro-degradation.
Besides intracellular (within cells) aggregates of TDP-43, serum levels of soluble TDP-43 have been reported to be moderately increased in ALS patients. It could be hypothesized, that increased soluble TDP-43 in serum, could contribute to an increased production of anti-TDP-43 Nabs. To test these hypotheses, serum was analysed from the following groups: Amyotrophic lateral sclerosis (ALS), Frontotemporal lobar degeneration (FTLD), Alzheimer’s Disease (AD), motor neuron disorder mimics (MN-m) and healthy controls (CTRL).
Results
Anti-TDP-43 Nab were detected in the serum of all subjects with higher serum levels in greater than 85% of ALS patients compared to controls. There was also 2-3-fold increase in ALS patients, compared to AD and FTLD patients. Anti-TDP-43 Nab was detected four-fold lower in MN-m than in ALS patients and two fold lower in MN-m patients than controls. These levels did not change very much when considering total serum IgG.
Serum levels of TDP-43 protein were widely different. ALS patients displayed higher serum levels of TDP-43 (fivefold) than detected in Controls and FTLD patients, but lower levels than those of MN-m (around 50%) patients. AD patients displayed slightly increased values with respects to controls.
No correlation between TDP-43 protein and anti-TDP-43 Nab levels in serum could be found for any group, but a positive correlation between TDP-43 protein levels in serum and the disease progression index was found.
Discussion
This paper highlights that detection of higher levels anti-TDP-43 Nab in serum of ALS patients result from immunoreactivity to the TDP-43 aggregates. Increased levels were detected in ALS patients with or without TARDBP mutations, which concurs with the observations of the abnormal localisation of TDP-43 in most ALS cases. Interestingly, this increased level of TDP-43 Nab was further amplified by the inclusion of other disease states which had lower levels than the control group. Serum TDP-43 protein levels also increased in ALS as well, confirming previous observations. However, the levels showed a positive asymmetry, suggesting they may not be as useful as previously thought in terms of developing a biomarker assay to assist detection of ALS from blood serum.
While an important initial study of the role of anti-TDP-43 Nab in ALS disease progression and detection. It is important to remember that sample size is limited, especially for the other pathological groups (AD, FTLD and MN-m) and a further larger study is currently being recruited for to confirm these initial observations. This study will collect more data on the ALS clinical phenotypes of patients to study any potential relationship of blood based TDP-43 markers (both protein and anti-TDP-43 Nab) with disease signs, progression and with degree of cognitive impairment.
The authors conclude that this data and that of the larger follow up study may be useful in providing a good starting point for possible earlier diagnosis and the development of possible anti TDP 43 immunotherapies.