Association of folate with hearing is dependent on the 5,10-methylenetetrahdyrofolate reductase 677C-->T mutation

Cochlear Homocysteine Metabolism at the Crossroad of Nutrition and Sensorineural Hearing Loss

Hearing loss (HL) is one of the most common causes of disability, affecting 360 million people according to the World Health Organization (WHO). HL is most frequently of sensorineural origin, being caused by the irreversible loss of hair cells and/or spiral ganglion neurons. The etiology of sensorineural HL (SNHL) is multifactorial, with genetic and environmental factors such as noise, ototoxic substances and aging playing a role. The nutritional status is central in aging disability, but the interplay between nutrition and SNHL has only recently gained attention. Dietary supplementation could therefore constitute the first step for the prevention and potential repair of hearing damage before it reaches irreversibility. In this context, different epidemiological studies have shown correlations among the nutritional condition, increased total plasma homocysteine (tHcy) and SNHL. Several human genetic rare diseases are also associated with homocysteine (Hcy) metabolism and SNHL confirming this potential link. Accordingly, rodent experimental models have provided the molecular basis to understand the observed effects. Thus, increased tHcy levels and vitamin deficiencies, such as folic acid (FA), have been linked with SNHL, whereas long-term dietary supplementation with omega-3 fatty acids improved Hcy metabolism, cell survival and hearing acuity. Furthermore, pharmacological supplementations with the anti-oxidant fumaric acid that targets Hcy metabolism also improved SNHL. Overall these results strongly suggest that cochlear Hcy metabolism is a key player in the onset and progression of SNHL, opening the way for the design of prospective nutritional therapies.

Long-Term Dietary Folate Deficiency Accelerates Progressive Hearing Loss on CBA/Ca Mice

Dietary folic acid deficiency induced early hearing loss in C57BL/6J mice after 2-months, corroborates the epidemiological association previously described between vitamin deficiency and this sensory impairment. However, this strain is prone to early hearing loss, and hence we decided to analyze whether the effects exerted by folate deprivation follow the same pattern in a mouse strain such as CBA/Ca, which is resistant to hearing impairment. Here, we show results of a long-term study on hearing carried out on CBA/Ca mice subjected to dietary folate deprivation. Systemic changes included decreased serum folate levels, hyperhomocysteinemia and signs of anemia in the group fed with folate-deficient (FD) diet. Initial signs of hearing loss were detected in this strain after 8-months of vitamin deficiency, and correlated with histological damage in the cochleae. In conclusion, the data presented reinforce the importance of adequate folic acid levels for the auditory system and suggest that the impact of dietary deficiencies may depend on the genetic background.

Folic acid deficiency induces premature hearing loss through mechanisms involving cochlear oxidative stress and impairment of homocysteine metabolism

Nutritional imbalance is emerging as a causative factor of hearing loss. Epidemiologic studies have linked hearing loss to elevated plasma total homocysteine (tHcy) and folate deficiency, and have shown that folate supplementation lowers tHcy levels potentially ameliorating age-related hearing loss. The purpose of this study was to address the impact of folate deficiency on hearing loss and to examine the underlying mechanisms. For this purpose, 2-mo-old C57BL/6J mice (Animalia Chordata Mus musculus) were randomly divided into 2 groups (n = 65 each) that were fed folate-deficient (FD) or standard diets for 8 wk. HPLC analysis demonstrated a 7-fold decline in serum folate and a 3-fold increase in tHcy levels. FD mice exhibited severe hearing loss measured by auditory brainstem recordings and TUNEL-positive-apoptotic cochlear cells. RT-quantitative PCR and Western blotting showed reduced levels of enzymes catalyzing homocysteine (Hcy) production and recycling, together with a 30% increase in protein homocysteinylation. Redox stress was demonstrated by decreased expression of catalase, glutathione peroxidase 4, and glutathione synthetase genes, increased levels of manganese superoxide dismutase, and NADPH oxidase-complex adaptor cytochrome b-245, α-polypeptide (p22phox) proteins, and elevated concentrations of glutathione species. Altogether, our findings demonstrate, for the first time, that the relationship between hyperhomocysteinemia induced by folate deficiency and premature hearing loss involves impairment of cochlear Hcy metabolism and associated oxidative stress.

Progress and prospects in human genetic research into age-related hearing impairment

Age-related hearing impairment (ARHI) is a complex, multifactorial disorder that is attributable to confounding intrinsic and extrinsic factors. The degree of impairment shows substantial variation between individuals, as is also observed in the senescence of other functions. This individual variation would seem to refute the stereotypical view that hearing deterioration with age is inevitable and may indicate that there is ample scope for preventive intervention. Genetic predisposition could account for a sizable proportion of interindividual variation. Over the past decade or so, tremendous progress has been made through research into the genetics of various forms of hearing impairment, including ARHI and our knowledge of the complex mechanisms of auditory function has increased substantially. Here, we give an overview of recent investigations aimed at identifying the genetic risk factors involved in ARHI and of what we currently know about its pathophysiology. This review is divided into the following sections: (i) genes causing monogenic hearing impairment with phenotypic similarities to ARHI; (ii) genes involved in oxidative stress, biologic stress responses, and mitochondrial dysfunction; and (iii) candidate genes for senescence, other geriatric diseases, and neurodegeneration. Progress and prospects in genetic research are discussed.

Association between polymorphisms in genes encoding methylenetetrahydrofolate reductase and the risk of Ménière's disease

Folate metabolism is essential for cellular functioning. Despite extensive research on the roles of folate-metabolism-related gene polymorphisms in the pathophysiology of many diseases, such as cardiovascular disease, cancers, and sudden sensorineural hearing loss, little is known about their association with Ménière's disease (MD). The aim of this study was to investigate the effect of methylenetetrahydrofolate reductase (MTHFR) gene polymorphisms (C677T and A1298C) on the risk of MD in a Japanese population. We examined the C677T and A1298C (rs1801133 and rs1801131) polymorphisms in the MTHFR gene and compared them between 1946 adults (986 men and 960 women) participating in the National Institute for Longevity Sciences Longitudinal Study of Aging and 86 cases of MD. A multiple logistic regression was performed to obtain odds ratios (ORs) for the risk of MD regarding the MTHFR polymorphisms before (model 1) and after (model 2) adjustment for age and sex factors. The OR of MTHFR C677T for the risk of MD was 0.669 (95% confidence interval [CI], 0.479-0.934) in model 1 and 0.680 (95% CI, 0.484-0.954) in model 2. In contrast, the OR of MTHFR A1298C for the risk of MD was 1.503 (95% CI, 1.064-2.123) in model 1 and 1.505 (95% CI, 1.045-2.167) in model 2. Our results imply that the MTHFR C677T and A1298C polymorphisms are associated with the risk of MD.

Intake of specific nutrients and foods and hearing level measured 13 years later

Only a few studies have investigated the impact of nutrients and food groups on hearing level (HL) with a population-based approach. We examined the 13-year association between intake of specific nutrients and food groups and HL in a sample of French adults. A total of 1823 subjects, aged 45-60 years at baseline, participating in the Supplementation with Antioxidant Vitamins and Minerals 2 cohort were selected. Nutrient and food intake was estimated at baseline among participants who had completed at least six 24 h dietary records. HL was assessed 13 years after baseline and was defined as the pure-tone air conduction of the worse ear at the following thresholds: 0·5, 1, 2 and 4 kHz. The relationship between quartiles of energy-adjusted nutrient and food intake and HL was assessed by multivariate linear regression analyses, in men and women separately. Intakes of retinol (P-trend ¼ 0·058) and vitamin B12 (P-trend=0·068) tended to be associated with better HL in women. Intakes of meat as a whole (P-trend=0·030), red meat (P-trend=0·014) and organ meat (P-trend=0·017) were associated with better HL in women. Higher intake of seafood as a whole (P-trend=0·07) and of shellfish (P-trend=0·097) tended to be associated with better HL in men. Consumption of meat is therefore associated with a better HL in women. Further research is required to better elucidate the mechanisms behind the associations between diet and hearing.

MTHFR 677T is a strong determinant of the degree of hearing loss among Polish males with postlingual sensorineural hearing impairment

Hearing impairment (HI) is the most common sensory handicap. Congenital HI often has a genetic basis, whereas the etiology of nonsyndromic postlingual HI (npHI) usually remains unidentified. Our purpose was to test whether the MTHFR C677T (rs1801133) polymorphism affecting folate metabolism is associated with the occurrence or severity of npHI. We studied rs1801133 genotypes in 647 npHI patients (age <40, sudden sensorineural loss excluded, HI characterized as mean of better ear hearing thresholds for 0.5-8 kHz) and 3273 adult controls from the background population. Genotype distribution among patients and controls was similar, but among male cases (n = 302) we found a dose-dependent correlation of MTHFR 677T with the degree of HI (mean thresholds in dB: 38.8, 44.9, and 53.3, for CC, CT, and TT genotypes, respectively; p = 0.0013, p(cor.) = 0.017). Among male patients rs1801133 TT significantly increased the risk of severe/profound HI (odds ratio = 4.88, p = 0.001). Among controls the known effect of MTHFR 677T on plasma total homocysteine was more pronounced in men than in women (p<0.00004 for genotype-sex interaction) suggesting that in Poland folate deficiency is more prevalent in males. In conclusion, we report a novel strong effect of MTHFR 677T among males with npHI. The functional significance of rs1801133 suggests that these patients may benefit from folate supplementation-an intervention which is simple, cheap, and devoid of side effects.

The effect of 90 day administration of a high dose vitamin B-complex on work stress

OBJECTIVE

Occupational stress is increasing in Western societies and the impact is significant at a personal, organisational and community level. The present study examined for the first time the efficacy of 3 months administration of two forms of high dose vitamin B complex on mood and psychological strain associated with chronic work stress.

METHOD

Sixty participants completed the 3-month, double-blind, randomised, placebo-controlled trial in which personality, work demands, mood, anxiety and strain were assessed.

RESULTS

After individual differences in personality and work demands were statistically controlled, the vitamin B complex treatment groups reported significantly lower personal strain and a reduction in confusion and depressed/dejected mood after 12 weeks. There were no treatment-related changes in other measures of mood and anxiety.

DISCUSSION

The results of the study are consistent with two previous studies examining multivitamin supplementation and personal (non-work) feelings of strain and suggestive of significant decreases in the experience of workplace stress after 90 day supplementation of a B multivitamin.

CONCLUSION

Given the direct and indirect costs of workplace stress, these findings point to the utility of a cost-effective treatment for the mood and psychological strain effects of occupational stress. These findings may have important personal health, organisational and societal outcomes given the rising cost and incidence of workplace stress.

Hearing impairment risk and interaction of folate metabolism related gene polymorphisms in an aging study

Background

Recent investigations demonstrated many genetic contributions to the development of human age-related hearing impairment (ARHI), however, reports of factors associated with a reduction in the ARHI risk are rare. Folate metabolism is essential for cellular functioning. Despite the extensive investigations regarding the roles of folate metabolism related gene polymorphisms in the pathophysiology of complex diseases, such as cancer, cardio-cerebrovascular disease, and atherosclerosis, little is known about the association with ARHI. The aim of this study is to investigate the effects of the methionine synthase (MTR) A2756G and methylenetetrahydrofolate reductase (MTHFR) C677T gene polymorphisms on the risk of hearing impairment in middle-aged and elderly Japanese.

Methods

Data were collected from community-dwelling Japanese adults aged 40-84 years who participated in the Longitudinal Study of Aging biennially between 1997 and 2008. We analyzed cumulative data (5,167 samples in accumulated total) using generalized estimating equations.

Results

The MTHFR 677T allele was significantly associated with a reduced risk of hearing impairment only when the subjects were wild-type homozygotes for MTR A2756G. The per-T allele odds ratio of MTHFR for the risk of developing hearing impairment was 0.7609 (95% CI: 0.6178-0.9372) in the MTR AA genotype. In addition, a subgroup analysis demonstrated that the favorable effect of the MTHFR 677T allele on the risk of developing hearing impairment was independent of folate and homocysteine level, whereas plasma total homocysteine level was independently associated with an increased risk of developing hearing impairment. The interactive effect of gene polymorphisms associated with folate metabolism may modify the risk of developing hearing impairment after middle age. These results contribute to the elucidation of the causes of ARHI.

Conclusions

The present study has found that the MTHFR 677T allele has a favorable effect on a risk of hearing impairment in the middle-aged and elderly population, only when the individuals were wild-type homozygotes for MTR A2756G.

Folic acid fortification: a double-edged sword

PURPOSE OF REVIEW

To examine the impact of folic acid fortification, including its use as a functional food component, on human health.

RECENT FINDINGS

There is a consensus view that folic acid supplementation has numerous health benefits, many of which are significant in their impact. However, emerging evidence suggests that increased population exposure to folic acid may also have a negative impact with respect to certain developmental and degenerative disorders. As examples, presently much attention is focused on the role of folic acid fortification augmenting colon cancer risk, whereas earlier in the life cycle, the vitamin may additionally influence insulin resistance. Without question, conditions that are influenced by folic acid are both diverse and many - from concerns relating to cognitive decline, breast cancer and vascular disease through to preconceptional issues where maternal folate levels might conceivably alter the phenotype of offspring via epimutations.

SUMMARY

The highly complex and critical biological importance of folic acid-related molecular nutrition makes it a difficult micronutrient to deploy as a simple intervention at a population level - it has far too many biochemical spheres of influence to predict effects in a generalized way. Additionally, several gene variants and other nutrients are interactive factors. It is, therefore, hardly surprising that the scientific community does not have a true consensus view on whether mandatory fortification is appropriate as a population measure. This latter point not withstanding, any ultimate decisions on fortification should be well rooted in scientific fact rather than political expediency.

Matrix imbalance by inducing expression of metalloproteinase and oxidative stress in cochlea of hyperhomocysteinemic mice

Clinical study reports hearing loss in patients with low folic acid (FA) and elevated homocysteine (Hcy). We hypothesize that elevated Hcy induces imbalance in matrix turnover and oxidative stress in cochlea. Cystathione beta-synthase heterozygous knockout mice were used as model for hyperhomocysteinemia. Matrix remodeling induced by Hcy resulted from elevated MMP-2, -9, and -14. MMP-2 and -9 showed elevated gelatinase activity in CBS (+/-) cochlea. Tissue inhibitors of matrix metalloproteinase were significantly lower in CBS (+/-) cochlea. The expression analyses for MMPs and TIMPs were equally represented at protein and mRNA levels. Cochlea of CBS mice showed following structural changes; (1) detachment of tectorial membrane lying on hair cells (2) thinner s. vascularis (3) large fibroblast in spiral ligament. Hcy induced higher protein nitrotyrosination and cytosolic NADPHoxidase subunit p22(phox) in cochlea. It is thus suggested that Hcy induced matrix imbalance, structural changes and oxidative stress in cochlea.

Noise-Induced Permanent Threshold Shifts in the Occupational Noise and Hearing Survey: An Explanation for Elevated Risk Estimates

OBJECTIVES

To (1) estimate noise-induced permanent threshold shift (NIPTS) values from the data of the 1968-1972 Occupational Noise and Hearing Survey (ONHS); (2) compare these NIPTS estimates to NIPTS predictions from an international standard (ISO-1999); (3) determine why excess risk estimates based on the ONHS are so much higher than those based on ISO-1999.

DESIGN

The ONHS raw data were acquired from the National Institute for Occupational Safety and Health. Binaural average thresholds from 0.5 to 6 kHz were calculated for each of 1291 noise-exposed subjects (80 to 94 dBA, for up to 30 yr, all tested just before their shifts) and 665 non-noise-exposed control subjects (mostly office workers, tested throughout the work day). "Screened" subjects had had no prior significant noise exposure and had no evidence of other ear disease, whereas "excluded" subjects had failed one or more screening criteria. Twenty exposure groups were created (based on exposure level, exposure duration, and screened versus excluded status) and compared with 20 age-matched control groups. The median difference statistic estimated median NIPTS.

RESULTS

Median NIPTS estimates in the 3 to 6 kHz region were generally consistent with the NIPTS predictions of ISO-1999. At lower frequencies, especially at 0.5 and 1 kHz, the ONHS estimates were significantly larger than the ISO-1999 predictions, even for exposures below 90 dBA, but these differences did not increase systematically with exposure level and duration.

CONCLUSIONS

High-frequency median NIPTS estimates from ONHS data are consistent with the predictions of ISO-1999. Differences between exposed and control low-frequency thresholds in the ONHS are higher than predicted by ISO-1999, but these differences probably are related more to socioeconomic or test procedure effects than to occupational noise exposure. These low-frequency effects explain why excess risk estimates from the ONHS are higher than those based on ISO-1999.

Synergy of genes and nutrients: the case of homocysteine

PURPOSE OF REVIEW

Folic acid is now considered an important functional food component: it lowers potentially toxic homocysteine, prevents birth defects and modulates the risk of several cancers. The complexity of interactions involved, however, means we still have much to learn about the role of folate and homocysteine in both health and disease.

RECENT FINDINGS

This review examines the emergence of homocysteine as a public health issue, and places this in context by exploring recent developments in the field of homocysteine as a vasculo, neuro and embryotoxic thiol. The paper also examines the homocysteine nexus in relation to mood disorders and cancer. It ends with an assessment of the issues associated with government-mandated folate fortification.

SUMMARY

Folate fortification as a population measure may mask B12 deficiency, affect antiepileptic drug seizure control, and influence the genetic selection of a potentially deleterious genotype, albeit over a number of generations. It is likely that only large studies with a comprehensive battery of endpoints that fully address the complexity of nutrient-gene and gene-gene interactions will be able to answer all the necessary questions fully.