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Allman PH, Aban IB, Tiwari HK, Cutter GR. An introduction to Mendelian randomization with applications in neurology. Mult Scler Relat Disord 2018; 24:72-78. [PMID: 29960142 DOI: 10.1016/j.msard.2018.06.017] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2018] [Accepted: 06/19/2018] [Indexed: 10/28/2022]
Abstract
Mendelian randomization studies have become increasingly common due to the maturation of genome-wide association studies and its potential to ascertain causal relationships. With the increasing use of this method comes the need for medical practitioners and clinicians to develop an understanding of its rationale, limitations, and interpretation. Mendelian randomization attempts to ascertain a causal relationship between some risk factor of interest and some outcome or disease of interest. It exploits Mendel's law on the random assortment of genetic variants. This random assortment of genetic variants mimics the main principle of randomization used in clinical trials; with the genetic variant replacing the randomly allocated treatment. In this paper we provide a readable introduction to the rationale behind Mendelian randomization and its limitations. We also discuss and interpret several examples of Mendelian randomization analyses which pertain to neurological diseases.
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Affiliation(s)
- Phillip H Allman
- University of Alabama at Birmingham, Department of Biostatistics. Ryals Public Health Bldg. RPHB 327, AL, USA.
| | - Inmaculada B Aban
- University of Alabama at Birmingham, Department of Biostatistics. Ryals Public Health Bldg. RPHB 327, AL, USA
| | - Hemant K Tiwari
- University of Alabama at Birmingham, Department of Biostatistics. Ryals Public Health Bldg. RPHB 327, AL, USA
| | - Gary R Cutter
- University of Alabama at Birmingham, Department of Biostatistics. Ryals Public Health Bldg. RPHB 327, AL, USA.
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2
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Moretti E, Collodel G, Fiaschi AI, Micheli L, Iacoponi F, Cerretani D. Nitric oxide, malondialdheyde and non-enzymatic antioxidants assessed in viable spermatozoa from selected infertile men. Reprod Biol 2017; 17:370-375. [PMID: 29055788 DOI: 10.1016/j.repbio.2017.10.003] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2017] [Revised: 10/09/2017] [Accepted: 10/11/2017] [Indexed: 01/30/2023]
Abstract
There are growing evidences that the semen of infertile male population shows higher reactive oxygen species (ROS) levels concomitant with lower antioxidant capacity compared to those detected in semen of fertile population. The plasma membrane of the sperm cell, which has high levels of polyunsaturated fatty acids, renders it particularly sensitive to ROS. The aim of this study was to compare the sperm parameters (concentration, motility, morphology and vitality) and the levels of malondialdehyde (MDA), as marker of lipid peroxidation (LPO), nitric oxide (NO), ascorbic acid (AA), total (GSHt) and oxidized glutathione (GSSG) in viable sperm in a group of 38 infertile patients and in a group of 55 control subjects with unknown reproductive potential. The comparison between variables in infertile patients and controls revealed that the sperm quality was reduced in the infertile group, whereas the levels of NO, AA and GSH were significantly increased in viable spermatozoa from infertile men; however, the endogenous levels of MDA were similar in infertile and control groups. Based on our results, we could speculate that the rise of GSHt and AA levels in viable sperm of infertile group help partially to counteract the damaging effect of ROS and partly prevent a substantial LPO. The observation of the concomitant increase of NO and antioxidant indices in viable spermatozoa of infertile subjects is a novel finding and we think that these results can be useful since the viable sperm population is conceivably used in assisted reproductive technology.
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Affiliation(s)
- Elena Moretti
- Dept. of Molecular and Developmental Medicine, Policlinico Le Scotte, Viale Bracci 14, Siena, Italy
| | - Giulia Collodel
- Dept. of Molecular and Developmental Medicine, Policlinico Le Scotte, Viale Bracci 14, Siena, Italy.
| | - Anna Ida Fiaschi
- Dept. of Medical and Surgical Sciences and Neurosciences, Policlinico Le Scotte, Viale Bracci 14, Siena, Italy
| | - Lucia Micheli
- Dept. of Medical and Surgical Sciences and Neurosciences, Policlinico Le Scotte, Viale Bracci 14, Siena, Italy
| | - Francesca Iacoponi
- Istituto Zooprofilattico Sperimentale del Lazio e della Toscana "M. Aleandri", via Appia Nuova 1411, Rome, Italy
| | - Daniela Cerretani
- Dept. of Medical and Surgical Sciences and Neurosciences, Policlinico Le Scotte, Viale Bracci 14, Siena, Italy
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3
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Ascorbic acid and the brain: rationale for the use against cognitive decline. Nutrients 2014; 6:1752-81. [PMID: 24763117 PMCID: PMC4011065 DOI: 10.3390/nu6041752] [Citation(s) in RCA: 57] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2013] [Revised: 03/24/2014] [Accepted: 04/10/2014] [Indexed: 12/21/2022] Open
Abstract
This review is focused upon the role of ascorbic acid (AA, vitamin C) in the promotion of healthy brain aging. Particular attention is attributed to the biochemistry and neuronal metabolism interface, transport across tissues, animal models that are useful for this area of research, and the human studies that implicate AA in the continuum between normal cognitive aging and age-related cognitive decline up to Alzheimer’s disease. Vascular risk factors and comorbidity relationships with cognitive decline and AA are discussed to facilitate strategies for advancing AA research in the area of brain health and neurodegeneration.
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Schwartz B. New criteria for supplementation of selected micronutrients in the era of nutrigenetics and nutrigenomics. Int J Food Sci Nutr 2014; 65:529-38. [PMID: 24625102 DOI: 10.3109/09637486.2014.898258] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Advances in molecular biology, emergence of novel techniques and huge amount of information generated in the post-Human Genome Project era have fostered the emergence of new disciplines in the field of nutritional research: Nutrigenomics deals with the effect of diet on gene expression whereas nutrigenetics refers to the impact of inherited traits on the response to a specific dietary pattern, functional food or supplement. Understanding the role of micronutrient supplementation with specific genetic backgrounds may provide an important contribution to a new optimum health strategy based on individualized nutritional treatment and may provide the strategies for the development of safer and more effective dietary interventions. This overview of the various aspects of supplementation of micronutrients in the era of nutrigenetics and nutrigenomics may provide a better understanding of novel nutritional research approach and provide an additional insight that can be applied to the daily dietary practice.
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Affiliation(s)
- Betty Schwartz
- The Robert H. Smith Faculty of Agriculture, Food and Environment, Institute of Biochemistry, Food Science and Nutrition, The Hebrew University of Jerusalem , Rehovot , Israel
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Davey Smith G. Use of genetic markers and gene-diet interactions for interrogating population-level causal influences of diet on health. GENES & NUTRITION 2011; 6:27-43. [PMID: 21437028 PMCID: PMC3040803 DOI: 10.1007/s12263-010-0181-y] [Citation(s) in RCA: 90] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/30/2010] [Accepted: 08/07/2010] [Indexed: 01/20/2023]
Abstract
Differences in diet appear to contribute substantially to the burden of disease in populations, and therefore changes in diet could lead to major improvements in public health. This is predicated on the reliable identification of causal effects of nutrition on health, and unfortunately nutritional epidemiology has deficiencies in terms of identifying these. This is reflected in the many cases where observational studies have suggested that a nutritional factor is protective against disease, and randomized controlled trials have failed to verify this. The use of genetic variants as proxy measures of nutritional exposure-an application of the Mendelian randomization principle-can contribute to strengthening causal inference in this field. Genetic variants are not subject to bias due to reverse causation (disease processes influencing exposure, rather than vice versa) or recall bias, and if obvious precautions are applied are not influenced by confounding or attenuation by errors. This is illustrated in the case of epidemiological studies of alcohol intake and various health outcomes, through the use of genetic variants related to alcohol metabolism (in ALDH2 and ADH1B). Examples from other areas of nutritional epidemiology and of the informative nature of gene-environment interactions interpreted within the Mendelian randomization framework are presented, and the potential limitations of the approach addressed.
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Affiliation(s)
- George Davey Smith
- MRC Centre for Causal Analyses in Translational Epidemiology, Department of Social Medicine, University of Bristol, Oakfield House, Oakfield Grove, Bristol, BS8 2BN UK
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Smith GD. Mendelian Randomization for Strengthening Causal Inference in Observational Studies: Application to Gene × Environment Interactions. PERSPECTIVES ON PSYCHOLOGICAL SCIENCE 2010; 5:527-45. [PMID: 26162196 DOI: 10.1177/1745691610383505] [Citation(s) in RCA: 93] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
Identification of environmentally modifiable factors causally influencing disease risk is fundamental to public-health improvement strategies. Unfortunately, observational epidemiological studies are limited in their ability to reliably identify such causal associations, reflected in the many cases in which conventional epidemiological studies have apparently identified associations that randomized controlled trials have failed to verify. The use of genetic variants as proxy measures of exposure -an application of the Mendelian randomization principle-can contribute to strengthening causal inference. Genetic variants are not subject to bias due to reverse causation (disease processes influencing exposure, rather than vice versa) or recall bias, and if simple precautions are applied, they are not influenced by confounding or attenuation by errors. The principles of Mendelian randomization are illustrated with specific reference to studies of the effects of alcohol intake on various health-related outcomes through the utilization of genetic variants related to alcohol metabolism (in ALDH2 and ADH1B). Ways of incorporating Gene × Environment interactions into the Mendelian randomization framework are developed, and the strengths and limitations of the approach discussed.
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Affiliation(s)
- George Davey Smith
- School of Social and Community Medicine, University of Bristol, Bristol, United Kingdom
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7
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Reidling JC, Rubin SA. Promoter analysis of the human ascorbic acid transporters SVCT1 and 2: mechanisms of adaptive regulation in liver epithelial cells. J Nutr Biochem 2010; 22:344-50. [PMID: 20471816 DOI: 10.1016/j.jnutbio.2010.03.001] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2009] [Revised: 02/22/2010] [Accepted: 03/02/2010] [Indexed: 11/28/2022]
Abstract
Ascorbic acid, the active form of vitamin C, is a vital antioxidant in the human liver, yet the molecular mechanisms involved in the regulation of ascorbic acid transporters [human sodium-dependent vitamin C transporters (hSVCT) 1 and 2] in liver cells are poorly understood. Therefore, we characterized the minimal promoter regions of hSVCT1 and 2 in cultured human liver epithelial cells (HepG2) and examined the effects of ascorbic acid deprivation and supplementation on activity and regulation of the transport systems. Identified minimal promoters required for basal activity were found to include multiple cis regulatory elements, whereas mutational analysis demonstrated that HNF-1 sites in the hSVCT1 promoter and KLF/Sp1 sites in the hSVCT2 promoter were essential for activities. When cultured in ascorbic acid deficient or supplemented media, HepG2 cells demonstrated significant (P<.01) and specific reciprocal changes in [(14)C]-Ascorbic acid uptake, and in hSVCT1 mRNA and protein levels as well as hSVCT1 promoter activity. However, no significant changes in hSVCT2 expression or promoter activity were observed during ascorbic acid deficient or supplemented conditions. We mapped the ascorbic acid responsive region in the hSVCT1 promoter and determined that HNF-1 sites are important for the adaptive regulation response. The results of these studies further characterize the hSVCT1 and 2 promoters establish that ascorbic acid uptake by human liver epithelial cells is adaptively regulated and show that transcriptional mechanisms via HNF-1 in the hSVCT1 promoter may, in part, be involved in this regulation.
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Michels AJ, Hagen TM. Hepatocyte nuclear factor 1 is essential for transcription of sodium-dependent vitamin C transporter protein 1. Am J Physiol Cell Physiol 2009; 297:C1220-7. [PMID: 19741195 DOI: 10.1152/ajpcell.00348.2009] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Transport and distribution of vitamin C is primarily regulated by the function of sodium-dependent vitamin C transporters (SVCTs). SVCT1 is expressed in the small intestine, liver, and kidney, organs that play a vital role in whole body vitamin C homeostasis. Despite the importance of this protein, little is known about regulation of the gene encoding SVCT1, Slc23a1. In this study, we present the first investigation of the transcriptional regulation of human Slc23a1, identifying transcription factors that may influence its expression. A 1,239-bp genomic DNA fragment corresponding to the 5'-flanking region of Slc23a1 was isolated from a human hepatocarcinoma cell line (HepG2) and sequenced. When cloned into a reporter gene construct, robust transcriptional activity was seen in this sequence, nearly 25-fold above the control vector. Deletion analysis of the SVCT1 reporter gene vector defined the minimal active promoter as a small 135-bp region upstream of the transcriptional start site. While several transcription factor binding sites were identified within this sequence, reporter constructs showed that basal transcription required the binding of hepatic nuclear factor 1 (HNF-1) to its cognate sequence. Furthermore, mutation of this HNF-1 binding site resulted in complete loss of luciferase expression, even in the context of the whole promoter. Additionally, small interfering RNA knockdown of both members of the HNF-1 family, HNF-1alpha and HNF-1beta, resulted in a significant decline in SVCT1 transcription. Together, these data suggest that HNF-1alpha and/or HNF-1beta binding is required for SVCT1 expression and may be involved in the coordinate regulation of whole body vitamin C status.
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Affiliation(s)
- Alexander J Michels
- Linus Pauling Institute and the Department of Biochemistry and Biophysics, Oregon State University, Corvallis, Oregon 97331, USA
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9
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Velho AM, Jarvis SM. Topological studies of hSVCT1, the human sodium-dependent vitamin C transporter and the influence of N-glycosylation on its intracellular targeting. Exp Cell Res 2009; 315:2312-21. [PMID: 19379732 DOI: 10.1016/j.yexcr.2009.04.007] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2009] [Accepted: 04/12/2009] [Indexed: 10/20/2022]
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10
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Smith GD, Timpson N, Ebrahim S. Strengthening causal inference in cardiovascular epidemiology through Mendelian randomization. Ann Med 2008; 40:524-41. [PMID: 18608114 DOI: 10.1080/07853890802010709] [Citation(s) in RCA: 75] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Observational studies have contributed in a major way to understanding modifiable determinants of cardiovascular disease risk, but several examples exist of factors that were identified in observational studies as potentially protecting against coronary heart disease, that in randomized controlled trials had no such effect. The likely reason for misleading findings from observational epidemiological studies is that associations are influenced by confounding, bias, and reverse causation--where disease influences a risk factor, rather than vice versa. Mendelian randomization utilizes genetic variants that serve as proxy measures for modifiable risk factors to allow estimation of the causal influence of the modifiable risk factor in question. We present examples of the use of the Mendelian randomization approach and discuss both the limitations and potentials of this strategy.
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Affiliation(s)
- George Davey Smith
- MRC Centre for Causal Analyses in Translational Epidemiology, Department of Social Medicine, University of Bristol, Bristol, UK.
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11
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Gournas C, Papageorgiou I, Diallinas G. The nucleobase–ascorbate transporter (NAT) family: genomics, evolution, structure–function relationships and physiological role. MOLECULAR BIOSYSTEMS 2008; 4:404-16. [PMID: 18414738 DOI: 10.1039/b719777b] [Citation(s) in RCA: 85] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Affiliation(s)
- Christos Gournas
- Faculty of Biology, Department of Botany, University of Athens, Panepistimioupolis, Athens, Greece
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12
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Savini I, Rossi A, Pierro C, Avigliano L, Catani MV. SVCT1 and SVCT2: key proteins for vitamin C uptake. Amino Acids 2007; 34:347-55. [PMID: 17541511 DOI: 10.1007/s00726-007-0555-7] [Citation(s) in RCA: 242] [Impact Index Per Article: 14.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2007] [Accepted: 04/18/2007] [Indexed: 12/13/2022]
Abstract
Vitamin C is accumulated in mammalian cells by two types of proteins: sodium-ascorbate co-transporters (SVCTs) and hexose transporters (GLUTs); in particular, SVCTs actively import ascorbate, the reduced form of this vitamin. SVCTs are surface glycoproteins encoded by two different genes, very similar in structure. They show distinct tissue distribution and functional characteristics, which indicate different physiological roles. SVCT1 is involved in whole-body homeostasis of vitamin C, while SVCT2 protects metabolically active cells against oxidative stress. Regulation at mRNA or protein level may serve for preferential accumulation of ascorbic acid at sites where it is needed. This review will summarize the present knowledge on structure, function and regulation of the SVCT transporters. Understanding the physiological role of SVCT1 and SVCT2 may lead to develop new therapeutic strategies to control intracellular vitamin C content or to promote tissue-specific delivery of vitamin C-drug conjugates.
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Affiliation(s)
- I Savini
- Department of Experimental Medicine and Biochemical Sciences, University of Rome Tor Vergata, Rome, Italy.
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13
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Rubin SA, Dey S, Reidling JC. Functional analysis of two regulatory regions of the human Na+-dependent vitamin C transporter 2, SLC23A2, in human vascular smooth muscle cells. ACTA ACUST UNITED AC 2005; 1732:76-81. [PMID: 16380174 DOI: 10.1016/j.bbaexp.2005.11.002] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2005] [Revised: 11/08/2005] [Accepted: 11/09/2005] [Indexed: 11/26/2022]
Abstract
Uptake of vitamin C occurs through the Na+ -dependent vitamin C transporters (SVCT1 and 2), the products of two separate genes. In cultured human vascular smooth muscle cells (hVSMC), we found expression of only the hSVCT2 transcript and identified an additional 5'-UTR transcript variant that we termed exon 1b, in addition to the previously described exon 1a. We cloned and tested the promoter functionality of the two genomic regions of the hSVCT2 upstream of these alternative first exons in hVSMC. Both demonstrated activity, and deletion constructs demonstrated that the minimal promoter regions were within approximately 100 bp relative to their adjacent exons.
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MESH Headings
- 5' Untranslated Regions/genetics
- Base Sequence
- Caco-2 Cells
- Cells, Cultured
- Exons/genetics
- Humans
- Molecular Sequence Data
- Muscle, Smooth, Vascular/cytology
- Muscle, Smooth, Vascular/metabolism
- Organic Anion Transporters, Sodium-Dependent/genetics
- Promoter Regions, Genetic/genetics
- RNA, Messenger/genetics
- RNA, Messenger/metabolism
- Regulatory Sequences, Nucleic Acid/genetics
- Sodium-Coupled Vitamin C Transporters
- Symporters/genetics
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Affiliation(s)
- Stanley A Rubin
- The Greater Los Angeles Veterans Affairs Medical Center, Los Angeles, CA and the Department of Medicine, School of Medicine, University of California, 11301 Wilshire Blvd., Los Angeles, CA 99073, USA.
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Eck P, Erichsen HC, Taylor JG, Yeager M, Hughes AL, Levine M, Chanock S. Comparison of the genomic structure and variation in the two human sodium-dependent vitamin C transporters, SLC23A1 and SLC23A2. Hum Genet 2005; 115:285-94. [PMID: 15316768 DOI: 10.1007/s00439-004-1167-x] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Vitamin C (L-ascorbic acid) is an essential co-factor for eight mammalian enzymes and quenches reactive oxygen species. Sodium-dependent vitamin C transport is mediated by two transporters, SVCT 1 and SVCT 2, encoded by SLC23A1 and SLC23A2. We characterized the genomic structures of SLC23A1 and SLC23A2, determined the extent of genetic variation and linkage disequilibrium across each gene, analyzed nucleotide diversity to estimate the effect of selective pressure, and compared sequence variation across species. In SLC23A1, the majority of single nucleotide polymorphisms (SNPs) are population-specific in either African Americans or Caucasians, including three of four non-synonymous SNPs. In contrast, most SNPs in SLC23A2 are shared between African Americans and Caucasians, and there are no non-synonymous SNPs in SLC23A2. Our analysis, combined with previous in vitro and in vivo studies, suggests that non-synonymous variation appears to be tolerated in SLC23A1 but not SLC23A2, and that this may be a consequence of different selective pressures following past gene duplication of the sodium-dependent vitamin C transporters. Genetic association studies of these two genes will need to account for the differences in haplotype structure and the population-specific variants. Our data represent a fundamental step toward the application of genetics to refining nutrient recommendations, specifically for vitamin C, and may serve as a paradigm for other vitamins.
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Affiliation(s)
- Peter Eck
- Molecular and Clinical Nutrition Section, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892, USA
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15
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Takanaga H, Mackenzie B, Hediger MA. Sodium-dependent ascorbic acid transporter family SLC23. Pflugers Arch 2004; 447:677-82. [PMID: 12845532 DOI: 10.1007/s00424-003-1104-1] [Citation(s) in RCA: 104] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2003] [Revised: 05/02/2003] [Accepted: 05/03/2003] [Indexed: 10/26/2022]
Abstract
l-Ascorbic acid (vitamin C) is an effective antioxidant and an essential cofactor in numerous enzymatic reactions. Two Na(+)-dependent vitamin C transporters (SVCT1 and SVCT2) are members of the SLC23 human gene family, which also contains two orphan members. SVCT1 and SVCT2 display similar properties, including high affinity for l-ascorbic acid, but are discretely distributed. SVCT1 is confined to epithelial systems including intestine, kidney, and liver, whereas SVCT2 serves a host of metabolically active and specialized cells and tissues including neurons, the eye, lung, and placenta, and a range of neuroendocrine, exocrine, and endothelial tissues. An SVCT2-knockout mouse reveals an obligatory requirement for SVCT2, but many of the specific roles of this transporter remain unclear.
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Affiliation(s)
- Hitomi Takanaga
- Membrane Biology Program and Renal Division, Brigham & Women's Hospital and Harvard Medical School, 77 Avenue Louis Pasteur, Boston, MA 02115, USA
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Smith GD, Ebrahim S. 'Mendelian randomization': can genetic epidemiology contribute to understanding environmental determinants of disease? Int J Epidemiol 2003; 32:1-22. [PMID: 12689998 DOI: 10.1093/ije/dyg070] [Citation(s) in RCA: 3061] [Impact Index Per Article: 145.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
Associations between modifiable exposures and disease seen in observational epidemiology are sometimes confounded and thus misleading, despite our best efforts to improve the design and analysis of studies. Mendelian randomization-the random assortment of genes from parents to offspring that occurs during gamete formation and conception-provides one method for assessing the causal nature of some environmental exposures. The association between a disease and a polymorphism that mimics the biological link between a proposed exposure and disease is not generally susceptible to the reverse causation or confounding that may distort interpretations of conventional observational studies. Several examples where the phenotypic effects of polymorphisms are well documented provide encouraging evidence of the explanatory power of Mendelian randomization and are described. The limitations of the approach include confounding by polymorphisms in linkage disequilibrium with the polymorphism under study, that polymorphisms may have several phenotypic effects associated with disease, the lack of suitable polymorphisms for studying modifiable exposures of interest, and canalization-the buffering of the effects of genetic variation during development. Nevertheless, Mendelian randomization provides new opportunities to test causality and demonstrates how investment in the human genome project may contribute to understanding and preventing the adverse effects on human health of modifiable exposures.
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Affiliation(s)
- George Davey Smith
- University of Bristol, Department of Social Medicine, Canynge Hall, Whiteladies Road, Bristol BS8 2PR, UK
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17
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Michels AJ, Joisher N, Hagen TM. Age-related decline of sodium-dependent ascorbic acid transport in isolated rat hepatocytes. Arch Biochem Biophys 2003; 410:112-20. [PMID: 12559983 DOI: 10.1016/s0003-9861(02)00678-1] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
This study investigated whether the age-related decline in hepatic ascorbic acid (AA) levels in rats was due to altered AA uptake. AA concentrations were 68% lower in freshly isolated hepatocytes from old (24-26 months) versus young (3-5 months; p<0.0005) Fischer 344 rats. When incubated with 100 microM AA, cells from old as compared to young rats showed a 66% decline in both the rate of AA transport and the steady state intracellular levels. Sodium-free media significantly reduced AA uptake, suggesting that the sodium-dependent vitamin C transporter (SVCT) was largely responsible for declines in AA transport. Analysis of SVCT messenger RNA (mRNA) levels shows that one isoform of this transport protein, SVCT1, declines 45% with age, with no significant changes in SVCT2 mRNA levels. These results show for the first time that sodium-dependent AA transport declines during the aging process, which may account for much of the loss in tissue AA content.
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Affiliation(s)
- Alexander J Michels
- Linus Pauling Institute and Department of Biochemistry and Biophysics, Oregon State University, 571 Weniger Hall, Corvallis, OR 97331, USA
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