Examining the possible causal relationship between lung function, COPD and Alzheimer's disease: a Mendelian randomisation study

Rationale

Large retrospective case-control studies have reported an association between chronic obstructive pulmonary disease (COPD), reduced lung function and an increased risk of Alzheimer’s disease. However, it remains unclear if these diseases are causally linked, or due to shared risk factors. Conventional observational epidemiology suffers from unmeasured confounding and reverse causation. Additional analyses addressing causality are required.

Objectives

To examine a causal relationship between COPD, lung function and Alzheimer’s disease.

Methods

Using two-sample Mendelian randomisation, we used single nucleotide polymorphisms (SNPs) identified in a genome wide association study (GWAS) for lung function as instrumental variables (exposure). Additionally, we used SNPs discovered in a GWAS for COPD in those with moderate to very severe obstruction. The effect of these SNPs on Alzheimer’s disease (outcome) was taken from a GWAS based on a sample of 24 807 patients and 55 058 controls.

Results

We found minimal evidence for an effect of either lung function (OR: 1.02 per SD; 95% CI 0.91 to 1.13; p value 0.68) or liability for COPD on Alzheimer’s disease (OR: 0.97 per SD; 95% CI 0.92 to 1.03; p value 0.40).

Conclusion

Neither reduced lung function nor liability COPD are likely to be causally associated with an increased risk of Alzheimer’s, any observed association is likely due to unmeasured confounding. Scientific attention and health prevention policy may be better focused on overlapping risk factors, rather than attempts to reduce risk of Alzheimer’s disease by targeting impaired lung function or COPD directly.

The impact of the first COVID-19 surge on severe asthma patients in the UK. Which is worse: the virus or the lockdown?

Asthma therapy, including monoclonal antibodies, was not associated with #COVID19 infection or hospitalisation in a UK severe asthma population. Shielding led to a reported worsening of mental health in nearly half of patients contacted (47%). https://bit.ly/3jImUsG.

Cytology and cell-block immunohistochemistry of circulating tumour cells

Objective

The study set out to assess the feasibility of using Parsortix^TM circulating tumour cell (CTC) extraction and CytoFoam Disc cell‐block immunohistochemistry to diagnose metastatic carcinoma from blood samples in a National Health Service district general hospital.

Methods

Blood samples were taken from 50 patients with metastatic carcinoma and 50 healthy volunteers and processed, using a previously published method, to extract CTCs and collect them in a cell‐block for routine formalin‐fixed paraffin sectioning and immunohistochemistry. The extracted cells were compared with the patients’ routine diagnostic samples.

Results

The samples from the 50 carcinoma patients showed cytokeratin‐positive cells in 19 cases. In eight of these, the cytokeratin‐positive cells had a similar immunoprofile to the carcinoma in the conventional biopsy or cytology specimen. Some carcinoma patients also had circulating cytokeratin‐positive cells that were probably benign epithelial cells and circulating megakaryocytes. Both of these types of cells were also found in healthy volunteers. Processing and initial examination could be completed in 2 days. The full processing cost was approximately £316 per case.

Conclusions

CTCs could be extracted from the blood of some patients with metastatic carcinoma and formed into a formalin‐fixed cell‐block for routine paraffin processing and immunohistochemistry. The specificity of this approach is constrained by the observation that some patients with metastatic carcinoma had circulating cytokeratin‐positive cells that were probably benign, and these were also found in healthy volunteers. Circulating megakaryocytes were present in carcinoma patients and healthy volunteers.

Circulating tumour cells were extracted from the blood of patients with metastatic carcinoma and formed into formalin fixed cell blocks for routine paraffin processing followed by immunohistochemistry that showed a match with the patient's carcinoma. Some patients with metastatic carcinoma had circulating cytokeratin positive cells that were probably benign cells and similar cells were also found in healthy volunteers.

Similar molecules spatially correlate with lipofuscin and N-retinylidene-N-retinylethanolamine in the mouse but not in the human retinal pigment epithelium

The accumulation of lipofuscin in the retinal pigment epithelium (RPE) has been implicated in the development of age-related macular degeneration (AMD) in humans. The exact composition of lipofuscin is not known but its best characterized component is N-retinylidene-N-retinylethanolamine (A2E), a byproduct of the retinoid visual cycle. Utilizing our recently developed matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS)-based technique to determine the spatial distribution of A2E, this study compares the relationships of lipofuscin fluorescence and A2E in the murine and human RPE on representative normal tissue. To identify molecules with similar spatial patterns, the images of A2E and lipofuscin were correlated with all the individual images in the MALDI-IMS dataset. In the murine RPE, there was a remarkable correlation between A2E and lipofuscin. In the human RPE, however, minimal correlation was detected. These results were reflected in the marked distinctions between the molecules that spatially correlated with the images of lipofuscin and A2E in the human RPE. While the distribution of murine lipofuscin showed highest similarities with some of the known A2E-adducts, the composition of human lipofuscin was significantly different. These results indicate that A2E metabolism may be altered in the human compared to the murine RPE.

Lack of correlation between the spatial distribution of A2E and lipofuscin fluorescence in the human retinal pigment epithelium

PURPOSE

The accumulation of lipofuscin in the RPE is a hallmark of aging in the eye. The best characterized component of lipofuscin is A2E, a bis-retinoid byproduct of the normal retinoid visual cycle, which exhibits a broad spectrum of cytotoxic effects in vitro. The purpose of our study was to correlate the distribution of lipofuscin and A2E across the human RPE.

METHODS

Lipofuscin fluorescence was imaged in flat-mounted RPE from human donors of various ages. The spatial distributions of A2E and its oxides were determined using matrix-assisted laser desorption-ionization imaging mass spectrometry (MALDI-IMS) on flat-mounted RPE tissue sections and retinal cross-sections.

RESULTS

Our data support the clinical observations of strong RPE fluorescence, increasing with age, in the central area of the RPE. However, there was no correlation between the distribution of A2E and lipofuscin, as the levels of A2E were highest in the far periphery and decreased toward the central region. High-resolution MALDI-IMS of retinal cross-sections confirmed the A2E localization data obtained in RPE flat-mounts. Singly- and doubly-oxidized A2E had distributions similar to A2E, but represented <10% of the A2E levels.

CONCLUSIONS

This report to our knowledge is the first description of the spatial distribution of A2E in the human RPE by imaging mass spectrometry. These data demonstrate that the accumulation of A2E is not responsible for the increase in lipofuscin fluorescence observed in the central RPE with aging.

Lipofuscin and N-retinylidene-N-retinylethanolamine (A2E) accumulate in retinal pigment epithelium in absence of light exposure: their origin is 11-cis-retinal

The age-dependent accumulation of lipofuscin in the retinal pigment epithelium (RPE) has been associated with the development of retinal diseases, particularly age-related macular degeneration and Stargardt disease. A major component of lipofuscin is the bis-retinoid N-retinylidene-N-retinylethanolamine (A2E). The current model for the formation of A2E requires photoactivation of rhodopsin and subsequent release of all-trans-retinal. To understand the role of light exposure in the accumulation of lipofuscin and A2E, we analyzed RPEs and isolated rod photoreceptors from mice of different ages and strains, reared either in darkness or cyclic light. Lipofuscin levels were determined by fluorescence imaging, whereas A2E levels were quantified by HPLC and UV-visible absorption spectroscopy. The identity of A2E was confirmed by tandem mass spectrometry. Lipofuscin and A2E levels in the RPE increased with age and more so in the Stargardt model Abca4(-/-) than in the wild type strains 129/sv and C57Bl/6. For each strain, the levels of lipofuscin precursor fluorophores in dark-adapted rods and the levels and rates of increase of RPE lipofuscin and A2E were not different between dark-reared and cyclic light-reared animals. Both 11-cis- and all-trans-retinal generated lipofuscin-like fluorophores when added to metabolically compromised rod outer segments; however, it was only 11-cis-retinal that generated such fluorophores when added to metabolically intact rods. The results suggest that lipofuscin originates from the free 11-cis-retinal that is continuously supplied to the rod for rhodopsin regeneration and outer segment renewal. The physiological role of Abca4 may include the translocation of 11-cis-retinal complexes across the disk membrane.

Lipofuscin and A2E accumulate with age in the retinal pigment epithelium of Nrl-/- mice

Lipofuscin is a fluorescent material with significant phototoxic potential that accumulates with age in the retinal pigment epithelium (RPE) of the eye. It is thought to be a factor in retinal degeneration diseases. The most extensively characterized lipofuscin component, N-retinylidene-N-retinylethanolamine (A2E), has been proposed to be a byproduct of reactions involving the visual pigment chromophore. To examine the impact of the visual pigment and photoreceptor cell type on lipofuscin accumulation, we analyzed the RPE from Nrl(-/-) mice of various ages for lipofuscin fluorescence and A2E levels. The photoreceptor cells of the Nrl(-/-) retina contain only cone-like pigments, and produce cone-like responses to photostimulation. The cone-like nature of these cells was confirmed by the presence of RPE65. Lipofuscin was measured with fluorescence imaging, whereas A2E was quantified by UV/VIS absorbance spectroscopy coupled to HPLC. The identity of A2E was corroborated with tandem mass spectrometry. Lipofuscin and A2E accumulated with age, albeit to lower levels compared with wild type mice. The emission spectra of RPE lipofuscin granules from Nrl(-/-) mice were similar to those from wild type mice, with λ(max) ca 610 nm. These results demonstrate that cone visual pigments can contribute to the production of lipofuscin and A2E.