Microcystins and Daily Sunlight: Predictors of Chronic Liver Disease and Cirrhosis Mortality

Rajesh Melaram

School of Health Sciences, Walden University, Minnesota, USA

^*Corresponding Author: Rajesh Melaram, School of Health Sciences, Walden University, Minneapolis, Minnesota, USA

Received: 15 May 2021; Accepted: 20 May 2021; Published: 11 June 2021

Abbreviations

CDC WONDER-Centers for Disease Control and Prevention Wide-ranging Online Data for Epidemiologic Research; ELISA-Enzyme-linked immunosorbent assay; ICD-10-International Classification of Disease, Tenth Revision; NLDAS-North America Land Data Assimilation System; SPSS-Statistical Package for the Social Sciences; USEPA-United States Environmental Protection Agency; WHO-World Health Organization

1. Introduction

In the United States (US), chronic liver disease (CLD) and cirrhosis represent a significant cause of mortality. CLD and cirrhosis was the 12th leading cause of death in 2013, resulting in more than 36,000 deaths [1]. The figure has been underestimated for over two decades as researchers have indicated an annual mortality near 66,000 deaths [2]. CLD is a debilitating condition in which the liver progressively worsens for six months or greater, terminating with cirrhosis. Major causes of CLD in the US include alcoholic liver disease and hepatitis C, though non-alcoholic fatty liver disease has become the most common etiology [3, 4]. Racial/ethnic differences in CLD and cirrhosis prevalence are evident in the US, despite limited and restricted data on few racial/ethnic groups [5]. In developing countries, varied etiologies for cirrhosis have been reported, such as malnutrition, tropical infections, and toxins [6]. While certain risk factors may be region-specific, identifying CLD and cirrhosis risk factors constitutes an essential step in exposure prevention.

Microcystins are cyclic heptapeptide structures produced by cyanobacteria in aquatic environments [7, 8]. They may be released into the water column via cell rupture or after bloom senescence [9-11]. Colonial Microcystis primarily manufactures microcystin, although other freshwater cyanobacterial genera can synthesize the biotoxin [12-15]. Microcystin is a contaminant of water sources used for agriculture, drinking water, and recreation [16, 17]. Additionally, microcystin fatalities are common in animals, livestock, and pets [18, 19].

Though rare, the largest microcystin episode in humans occurred in Caruaru, Brazil, killing 52 hemodialysis patients. Cylindrospermopsins were also considered a contributing factor in hemodialysis deaths [20-24].  However, the most common exposure route for microcystin is the consumption of contaminated drinking water [25, 26]. Following oral ingestion, microcystin is transported by a bile acid transport system into the liver. Organic anion transporting polypeptides (OATP) 1A2, 1B1, and 1B3 transport microcystin into hepatocytes [27]. Herein, microcystin covalently binds protein phosphatases PP1 and PP2A, inactivating dephosphorylation capabilities. High microcystin concentrations, therefore, causes hyperphosphorylation among cytoskeletal proteins, subsequently disrupting hepatocyte structure and function [28, 29]. For example, microcystin-leucine arginine was found to stimulate hyperphosphorylation of cytokeratin 8 and 18 in primary cultured rat hepatocytes, a process associated with liver tumor promotion [30].

Furthermore, several epidemiologic investigations linked microcystin exposure to increased liver disease. Two surveys in China identified hepatotoxic blue-green algae toxins in drinking water sources as one potential risk factor for primary liver cancer [31]. In Florida, an increased risk for primary hepatocellular carcinoma occurred among persons within a serviced area of a surface water treatment plant [32]. Chronic exposure to freshwater microcystin in Three Gorges Region, China, may have induced liver damage in children [33]. On the contrary, one study determined that liver cancer was not associated with toxic cyanobacterial exposure [34]. These studies support a probable association between microcystin and liver disease, but none considered meteorological factors in their assessment.

Apart from anthropogenic eutrophication, global climate change is a key driver of cyanobacterial expansion worldwide [35]. Combustion of fossil fuels and concomitant air temperatures may enhance algae productivity. Similarly, variations in weather patterns, resulting in severe droughts and rainfall, can leach nitrates and phosphate into eutrophic waters [36]. Research has indicated a synergistic interaction between climate-related changes and increased nutrients [37]. Thus, climatic factors and nutrient levels may enhance the frequency and severity of potentially toxic blooms in freshwater ecosystems. Since climate is long-term and can trigger bloom formation and ensuing health risks, little is known about short-term meteorologic factors on liver disease. Furthermore, the influence of meteorological patterns on liver diseases has not been extensively studied in the field [38]. A population-based study determined that acute-on-chronic liver disease prevalence was influenced by lower temperatures [39]. Therefore, we conducted an ecological study to examine whether meteorological factors, including daily sunlight, daily maximum temperature, and daily precipitation, in conjunction with microcystin, associated with liver disease mortality. Understanding if meteorological factors increase a secondary factor of liver disease mortality, such as microcystin, further warranted examination.

2. Materials and Methods

Secondary data on total microcystins were collected from the 2007 United States Environmental Protection Agency (USEPA) National Lakes Assessment. An enzyme-linked immunosorbent assay (ELISA) (Abraxis, LLC, Warminster, PA) analyzed total microcystins in lake samples (limit of detection < 0.10 μg/L). A composite average of total microcystins was computed for each state by averaging two or more repeated measurements from the same county with individual measurements from different counties. Non-detectable levels of total microcystins or lack of toxin data in the dataset resulted in the exclusion of seven states (Alaska, Hawaii, New Hampshire, New Mexico, South Carolina, Vermont, Wyoming). Mean total microcystins in each state were compared against the WHO relative probable health risk due to microcystins (low, moderate, high).

Environmental data on annual precipitation, average daily max temperature, daily precipitation, and daily sunlight, derived from the North America Land Data Assimilation System (NLDAS) (1979-2011), were gathered from the Centers for Disease Control and Prevention Wide-ranging Online Data for Epidemiologic Research (CDC WONDER). Data from 2007 were used to coincide with concentrations of total microcystins. The Underlying Cause of Death database was used to retrieve age-adjusted CLD and cirrhosis death rates of the United States between 2003 and 2007. The International Classification of Disease, Tenth Revision (ICD-10) 113 Cause List was utilized to examine records of age-adjusted CLD and cirrhosis death rates (K70, K73-K74). All ages, genders, origins, and races were selected in the demographics of age-adjusted CLD and cirrhosis death rates (Table A1). Multiple linear regressions were performed in Statistical Package for the Social Sciences (SPSS) version 25. Normality was achieved by log-transforming (base 10) all variables in the analysis.

Further examination identified extraneous variables within the dataset. The removal of outliers resulted in 35 states in the final analysis (Table A2). Statistical significance was based on p < 0.05. Descriptive statistics on mean total microcystins and mean meteorological factors were grouped by census region and state. Inferential statistics were applied to aggregate national data to assess the ecological association between total microcystins, meteorological factors, and age-adjusted CLD and cirrhosis death rates.

3. Results

3.1 Mean total microcystins and meteorological factors by census region

Table 1 displays a summary of mean total microcystins and meteorological factors by census region in 2007. Mean total microcystins was highest in the Midwest, with a concentration of 3.90 μg/L. The South and West had mean total microcystins of 0.858 μg/L and 1.99 μg/L, respectively. Mean total microcystins was lowest in the Northeast, at 0.688 μg/L. Mean daily maximum temperature ranged between 15.44 C in the Midwest to 22.10 C in the South. The West received the least mean daily precipitation at 1.28 mm, while the Northeast received the most at 3.06 mm. Mean daily sunlight ranged from 15064.91 KJ/m² in the Northeast to 17635.69 KJ/m² in the West.

Table 1: Summary of mean total microcystins and mean meteorological factors by census region in 2007. n = number of states, σ = standard deviation, WHO = World Health Organization, Low = 0.10 µg/L ≤ 10 μg/L. C = Celsius, mm = millimeters, KJ/m² = Kilojoule per square meter.

Table 2: Summary of mean total microcystins above 0.10 μg/L and mean meteorological factors by state in 2007. n = number of measurements ≥ 0.10 µg/L, WHO = World Health Organization, Low = 0.10 µg/L ≤ 10 μg/L, Moderate = 10 ug/L ≤ 20 µg/L. C = Celsius, mm = millimeters, KJ/m² = Kilojoule per square meter.

3.2 Mean total microcystins and meteorological factors by census region

Mean total microcystins and mean meteorological factors by state in 2007 are depicted in Table 2. The mean total microcystins for all 43 states was 1.91 μg/L. The lowest mean total microcystins occurred in Missouri (0.20 μg/L), and the highest mean total microcystins occurred in North Dakota (18.18 μg/L). 41 states (95.35%) had a low relative probable health risk, while 2 states (4.65%) had a moderate relative probable health risk. For meteorological factors, mean daily maximum temperature reached 17.87 C, mean daily precipitation 2.36 mm, and mean daily sunlight was 16434.07 KJ/m².

Table 3: Age-adjusted chronic liver disease and cirrhosis death rates per 100,000 from 2003 to 2007 by state.

3.3 Regression models

Multiple linear regressions were run to assess the predictive function of meteorological factors and total microcystins on age-adjusted CLD and cirrhosis death rates. All predictors were initially merged into the model. A positive association was observed between mean total microcystins, meteorological factors, and liver disease mortality (R = 0.726). Approximately 46.4% (R² = 0.464) of variance in age-adjusted CLD was explained by the predictors. The simultaneous model partially supported the hypothesis that meteorological factors in concurrence with total microcystins predict liver disease mortality (Table 4). The stepwise method was selected to determine which explanatory variables fitted the regression model. In Table 4, the final model revealed a positive association between mean daily sunlight, total microcystins, and age-adjusted CLD and cirrhosis death rates (R = 0.676 and R² = 0.423). Mean daily maximum temperature and mean daily precipitation were not statistically significant predictors (p > 0.05) (Table 5).

Table 4: Multiple linear regressions of exposure correlates and liver disease mortality.

Table 5: Coefficients of predictors for liver disease mortality.

4. Discussion

Liver disease is a serious health problem in the United States. In 2013, CLD and cirrhosis claimed over 33,000 lives, making it the 12^th leading cause of death [1]. Liver disease mortality estimates remain conservative, although research suggests that nearly 66,000 individuals die from CLD and cirrhosis each year [2]. The condition is largely preventable, with alcohol, obesity, and viral hepatitis being three major risk factors [40]. Other risk factors, such as toxins, tropical infections, and malnutrition occur in developing countries [6]. These environmental factors may increase in developed countries due to constant lifestyle and weather changes.

Microcystin is a blue-green algal hepatotoxin secreted by freshwater cyanobacteria. When favorable environmental conditions persist within stagnant waters, cyanobacteria multiply to create thick blooms. Bloom senescence can promote microcystin release as cells lyse in water. Microcystin is hepatotoxic since it targets protein phosphates in hepatocytes upon oral ingestion of contaminated drinking water. Inactivation of protein phosphatases PP1 and PP2A stimulates hyperphosphorylation, which can drastically affect liver function [28-30].

Moreover, epidemiological studies indicate microcystin exposure may associate with liver disease. Yet, limited knowledge exists on meteorological factors and chronic liver disease. For instance, average humidity and temperature were shown to correlate with acute-on-chronic liver failure positively and negatively, respectively [39]. The present study explored the ecological association between microcystins, meteorological factors, and liver disease mortality. Study results demonstrated a positive association between daily sunlight, total microcystins, and age-adjusted CLD and cirrhosis death rates. The observed association mirrored previous epidemiological investigations connecting freshwater microcystins with liver disease [31-33]. Daily sunlight exposure was a strong predictor of age-adjusted CLD and cirrhosis death rates. In a different study examining sunlight exposure and end stage renal disease, low amounts of daily sunlight increased the risk of all-cause mortality in dialysis patients, especially among diabetics and individuals age 75 and older [41].

There were a few limitations inherent within the study. First, the study was ecological in design, meaning interpreted results strictly concerned populations, not individuals. The ecological fallacy is a major limitation of an ecological study in which inferences from group data generalize among individuals. In the study context, people who consume drinking water contaminated with microcystin and receive sunlight exposure over their lifetime may die from liver disease. However, one should note that people drink treated water sourced from potable supplies, and CLD and cirrhosis may result from a combination of environmental and health risk factors. Another limitation involves confounding bias, which is not uncommon in ecological studies. The current study lacked known risk factors for CLD and cirrhosis, such as alcohol consumption and obesity. The variables were omitted due to incomplete data of risk factors in the collected secondary data sources. Missing pertinent variables in regression models can increase or decrease the predictivity of exposure variables on a health outcome. Hence, the effect of total microcystin and daily sunlight exposure is perhaps higher than expected if other risk factors were embedded in the model. A third limitation rests with ELISA for total microcystins quantitation. The method is subject to cross-reactivity, matrix interference, and low specificity despite screening multiple samples concurrently. Consequently, the identity of specific congeners and their associated concentrations in lake samples is unknown. Microcystin detection via liquid chromatography mass spectrometry can offer valuable information by distinguishing various congeners in the environment [42].  

Liver disease is a major public health problem and continues to grow as population growth increases and people age. The study identified a positive association between mean total microcystins, daily sunlight exposure, and age-adjusted CLD and cirrhosis death rates. The association, however, does not imply causation. Future research should consider behavioral and environmental lifestyle factors when exploring associations between meteorological factors, hepatotoxins, and liver disease mortality. Findings may encourage routine biomonitoring practices in endemic areas of liver disease where lakes bloom. Health and medical professionals can use the results to aid in the prevention, diagnosis, and treatment of liver diseases.

Acknowledgments

This research received no external funding.

Conflict of Interest

The author declares no conflict of interest.

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Appendix

Table A1: Demographics of age-adjusted chronic liver disease and cirrhosis death rates per 100,000 in the United States from 2003 to 2007. Death rates with a numerator of 20 or less are flagged as unreliable.

Table A2: Mean total microcystins and mean meteorological factors from 2007 and age-adjusted chronic liver disease and cirrhosis death rates from 2003 to 2007 in the United States.