Teratogenicity and underlying mechanisms of homocysteine in animal models: A review

Abstract

Background

Hyperhomocysteinemia in humans is a risk factor for adverse pregnancy outcome, especially congenital malformations. This review summarizes the studies directed on the teratogenicity of homocysteine carried out in animal studies, and elaborates on the underlying mechanisms.

Methods

Literature was searched in Pubmed (NCBI) through January 2010 and selected manually. Keywords comprised homocysteine, congenital abnormalities and animals.

Results

Increased frequencies of a wide range of congenital malformations are reported especially in the chicken embryo after exposure to homocysteine (Hcy) in various dosages and forms. Reduced embryonic growth and abnormalities of the vascularization of the yolk sac are described in mouse studies. A study in rats revealed a reduced development of blastocysts. The congenital malformations observed in the chicken embryo model share the mutual involvement of Hcy sensitive neural crest cells. Derangements in the behavior of these cells by interactions between Hcy and pathways involved in vascularization, growth, metabolism, signaling, and DNA synthesis and methylation may explain the wide range of effects on embryonic organs, the yolk sac and placental tissues.

Conclusions

The associations between human hyperhomocysteinemia and congenital malformations are substantiated by chicken and rodent studies. Moreover, derangements of several pathways induced by Hcy are demonstrated with adverse effects on both reproduction and long term health. Because of the high prevalence of hyperhomocysteinemia in both the reproductive and general population, research on underlying epigenetic mechanisms is warranted.

Introduction

Congenital malformations are the most important cause of perinatal morbidity and mortality and affect over 8 million children worldwide each year [1]. More than 80% of congenital malformations have a complex etiology, in which interactions between subtle structural genetic, i.e., single nucleotide polymorphisms (SNP) and environmental exposures such as periconception malnutrition and unhealthy lifestyles are implicated.

Three decades ago the pediatrician Professor Smithells was the first to suggest that a maternal folate shortage plays a part in the development of neural tube defects (NTD). His group showed that a low maternal folate status in the first trimester of pregnancy, which coincides with the period of neural tube closure, i.e., 21–28 days after conception, was associated with an increased risk of NTD offspring [2]. In a nonrandomised trial they also showed that periconception multivitamin use, including 360 μg of folic acid per day, reduced the recurrence risk of NTD by 86% [3]. These findings stimulated both human and animal studies on associations between folate and other complex congenital malformations and on the clarification of underlying mechanisms, e.g. the homocysteine (Hcy) pathway (Fig. 1) [4], [5], [6]. Moreover, worldwide this has resulted in the recommendation of a daily intake of 400 μg of folic acid in the periconception period by governments and international organizations, such as the World Health Organization and the Food and Drug Administration from 1991 onwards. Additionally, this has led to the stimulation of folic acid use through local and national campaigns, and to the launch of food fortification programs in several countries. These programs have already shown to significantly reduce birth prevalence rates of NTD [7], [8].

The most recent meta-analysis of systematically reviewed human intervention studies revealed that, dependent on study design, i.e., case control (cc) or cohort studies and randomised controlled trials (c-rct), folic acid-containing multivitamins protect against NTD (cc 33% and c-rct 48%), cardiovascular defects (cc 22% and rct 39%), limb defects (cc 52% and rct 43%), orofacial clefts (cc 24%–37%), urinary tract anomalies (cc 52%) and congenital hydrocephalus (cc 63%) [9].

In the early nineties Steegers-Theunissen et al. were the first to suggest that plasma Hcy is a more sensitive marker of the folate status than serum or plasma folate. This hypothesis was based on case–control studies in mothers with and without NTD offspring showing that a fasting Hcy concentration above approximately 14 μmol/L was associated with a 2–3-fold enhanced risk of NTD offspring [10]. This finding has been substantiated by studies of others [11], [12]. The question remained, however, whether Hcy should be considered a sensitive biomarker of a compromised folate, vitamin B12, and vitamin B6 status, or a direct teratogen [5].

Experimental animal studies were performed to explore the direct effects of Hcy after exogenous Hcy application and showed varying effects [13], [14], [15], [16], [17], [18]. The effects were different between species and models used and appeared to be dependent on the period of Hcy exposure and the chemical structure of Hcy used. The focus of this review is to describe the congenital malformations reported in chicken, mouse and rat studies after exogenous Hcy application in animal models without a known genetic contribution (mutation, knock out) and to elaborate on the suggested associations and underlying mechanisms.