The human lens is an avascular transparent tissue with unique protein composition and cellular organisation. The physiological function of the lens in combination with the cornea is to focus an image onto the retina. A high refractive index, the ability of the lens to change shape and lens transparency are all critical factors in producing this clear, focussed image; however, with advancing age, lens transparency is compromised and in the elderly this usually manifests itself as age-related nuclear (ARN) cataract. Age-related nuclear cataract is associated with oxidative damage and is characterised by cross-linking, oxidation and modifications of lens proteins that produce light scattering and loss of lens transparency. Globally, approximately 50 per cent of blindness is due to cataracts.
Lens cataract is the leading cause of blindness in developing countries. While cataract is primarily a disease of old age and is relatively rare in children, accounting for only four per cent of global blindness, childhood cataract is responsible for a third of the economic cost of blindness. While many of the causes of cataract in children are known, over half of childhood cataracts are idiopathic with no known cause. The incidence of idiopathic cataract is highest in developing countries and studies have discovered that low birth weight is a risk factor in the development of idiopathic childhood cataract. As low birth weight is a reflection of poor foetal growth, it is possible that maternal malnutrition, which is endemic in some developing countries, results in the altered physiology of the foetal lens. We have conducted a review of the literature that provides evidence for a link between maternal malnutrition, low birth weight and the development of childhood cataract. Using our accumulated knowledge on the pathways that deliver nutrients to the adult lens, we propose a cellular mechanism, by which oxidative stress caused by maternal malnutrition affects the development of antioxidant defence pathways in the embryonic lens, leading to an accelerated onset of nuclear cataract in childhood.
The Prevalence of Childhood Cataracts
While ARN cataract is the world's major cause of blindness, which can be treated relatively easily with surgery, an estimated 1.4 million children throughout the world are blind or severely visually impaired. Over three-quarters of the world's blind children live in Africa and Asia, where prevalence is high, the child population large and resources severely limited. Blindness in children, caused predominantly by cataracts and corneal problems, is particularly devastating as up to 60 per cent of children die within a year of becoming blind and the rest live on average an additional 40 years without vision. By definition, childhood or paediatric cataract appears after birth but before 16 years of age. As a result, childhood blindness is thought to be responsible for about one-third of the total economic cost of blindness.
The majority of childhood cataracts are idiopathic
The major causes of blindness in children vary widely from region to region due to differences in socioeconomic development and the availability of primary health care. In all regions of the world, retinal disease, cornea disease, cataracts and congenital abnormalities are the major causes of blindness, with corneal problems and cataracts being the predominant causes of blindness in developing countries.[5, 10] Investigation into the aetiology of childhood cataracts reveals that approximately 40 per cent of cases are linked to rubella infection, genetic/hereditary causes or are secondary to other ocular diseases or syndromes;[6, 9, 11] however, the majority of childhood cataracts have no known cause and are classed as idiopathic (Figure 1). A study by Eckstein and colleagues found that nearly half of bilateral cataracts in children in southern India were idiopathic. Although this finding has been reported in several other studies,[12-14] surprisingly little has been done to investigate the possible explanation for idiopathic childhood cataract, so that preventative strategies may be put into place.
What are the possible explanations for idiopathic childhood cataracts? The location of the opacity can provide vital clues as to possible aetiology. In the elderly, the most common type of cataract is a nuclear cataract, as a result of chronic oxidative damage to lens proteins in the nucleus or centre of the lens; however, in children, a dense nuclear cataract can be due to congenital rubella. Anterior polar cataracts can arise from abnormal separation of the lens vesicle in early pregnancy or persistent pupillary membranes. Problems of the hyaloid vascular system can cause lens opacities, which can be small (Mittendorf's dot) or extremely large, causing posterior polar cataracts. Despite limited available evidence, it appears that in idiopathic childhood cataract, there is a trend toward a higher incidence of total/nuclear cataracts followed by mixed cataracts.[6, 13, 14] This suggests that for this type of cataract, an inner lens or nuclear origin exists (Table 1).
|STUDY||Total/nuclear %||Mixed %||Cortical/lamella %||Other %|
Maternal Malnutrition and Oxidative Stress: A Novel Hypothesis to Explain Idiopathic Childhood Cataract?
One known risk factor for the development of idiopathic childhood cataract is low birth weight, with idiopathic childhood cataract 3.8 times more likely in children born at weights at or below 2,500 g when compared to those above this birth weight. There are several causes of low birth weight but the primary cause is maternal malnutrition due to lack of food resources, which is prolific in developing countries. Conversely, in developed countries, eating disorders are of concern. Studies in humans have shown that low birth weight is associated with a global increase in oxidative stress. Superoxide dismutase, catalase and reduced glutathione are all lower in the cord blood of low birth weight babies. Furthermore, maternal malnutrition in goats can reduce the antioxidant capacity of neonatal kids, resulting in decreased plasma and tissue activities of superoxide dismutase, glutathione peroxidase and catalase as well as decreased gene expression of antioxidant enzymes. While the effects on ocular tissues in both the aforementioned studies were not investigated, it is likely that reduced antioxidant capacity and an increase in global oxidative stress would increase the susceptibility of the lens to cataract development. Animal studies conducted in rats showed that females fed a diet low in tryptophan and vitamin E throughout gestation and lactation, bore progeny that developed either unilateral or bilateral cataracts, indicating an association between tryptophan and vitamin E relative to foetal lens development.[22, 23] In contrast, while body weight was reduced by 50 per cent, no cataracts were observed in progeny from female rats fed a diet low in either phenylalanine/tyrosine or methionine/cystine. In another study, rats malnourished during the foetal stage also exhibited signs of lens damage and cataracts. Interestingly, lens damage could be reversed after prolonged nutritional rehabilitation. Taken together, these studies support the idea that maternal malnutrition results in offspring with altered antioxidant pathways, increased oxidative stress and cataracts.
The incidence and severity of age-related nuclear cataract has been associated with a decrease in glutathione, specifically in the lens nucleus that exposes this region of the lens to oxidative stress resulting in protein aggregation and loss of lens transparency. It is known that the development of the eye starts in early pregnancy, raising some intriguing questions. Does oxidative stress caused by maternal malnutrition affect the development of antioxidant defence pathways in the embryonic lens, causing an accelerated onset of nuclear cataract in childhood?
To support this hypothesis, we will first briefly review the intrauterine development of the lens before using our accumulated knowledge of antioxidant delivery and metabolism pathways in the adult lens[25-31] to advance a working hypothesis that attempts to explain the link between maternal malnutrition and the early onset of cataract in children.
Intrauterine development of the lens
To better appreciate the processes leading to childhood cataract formation as a result of oxidative stress, a comprehensive understanding of the growth, structure and properties of the lens, especially the nucleus, is essential. The lens is unique, as it grows throughout the lifetime of the individual due to the addition of new cells at the surface. The older cells are not discarded but instead are packed into the centre of the lens. Human lens growth takes place in two distinct phases. An asymptotic growth phase followed by a linear growth phase. Asymptotic growth occurs throughout gestation and stops approximately three months after birth, after which lens growth becomes linear. Two distinct and different lens compartments are generated due to this biphasic lens growth. The prenatal growth phase leads to the formation of the lens nucleus of fixed dimensions and properties in the adult lens. The postnatal linear growth produces the ever-expanding cortex with physical and biochemical properties that are distinct from those of the nucleus.
The development of the eye starts early in pregnancy (Figure 2). The first morphological sign of embryonic lens development occurs at around 28 days after conception as the optic vesicle (OV) approaches the lens ectoderm (Figure 2.1). Upon contact of the optic vesicle with the lens ectoderm, the ectoderm elongates to form the lens placode (Figure 2.2). At six weeks gestation, the lens placode invaginates to form the lens pit and contact with the overlying surface of the ectoderm is lost to form the lens vesicle (Figure 2.3). The cells at the anterior portion of the lens vesicle give rise to the lens epithelium, while cells at the posterior of the lens vesicle elongate to form primary fibre cells (Figure 2.4). Between the seventh to eighth week of gestation, primary fibre cells fill the lumen of the lens vesicle (Figure 2.5 and 2.6) which become the embryonic nucleus in the mature lens (Figure 2.7). Secondary fibre cells are derived from epithelial cells located at the lens equator and are continually added as outer layers after birth (Figure 2.7).[34, 35] As the fibre cells are laid down, the different optical densities of the fibres produce several zones of discontinuity that can be observed clinically with a slitlamp:
- the embryonic nucleus
- the foetal nucleus
- the juvenile nucleus
- the adult nucleus and
- the outer cortex.
This process of lens development results in a uniquely transparent structure with many interesting properties. During elongation and differentiation, fibre cells over-express crystallin proteins. Crystallins are normally soluble proteins that exist at very high concentrations and their concentrations increase two- to three-fold from lens surface to centre, creating a gradient of refractive index, which is essential for reducing spherical aberration.[1, 36] To maintain its transparency, crystallins must not aggregate, precipitate or undergo phase separations that reduce their solubility.[37, 38] To further diminish light scattering, fibre cells lose their nuclei, mitochondria and endoplasmic reticulum. Consequently, mature fibre cells (adult through to embryonic nucleus) use anaerobic metabolism, exhibit a reduced metabolic activity and are not able to synthesise new proteins or turnover existing proteins. Hence, lens proteins laid down in the embryo are required to last the lifetime of the individual. Therefore, oxidative stress in utero due to maternal malnutrition has the potential to affect lens function after birth and throughout life.
Delivery of nutrients and antioxidants to the lens nucleus
The function of the lens is to focus light onto the retina and to produce a clear image; it must maintain its transparency throughout life. To help avoid light scattering, the adult lens lacks blood vessels. Soon after the lens forms in the foetus, it becomes surrounded with a network of capillaries arising mainly from the hyaloid artery to meet the nutritional demands of the developing lens; however, it regresses shortly before birth leaving the lens without a blood supply for the rest of its life. This raises the problem of how do cells in the lens nucleus receive appropriate supplies of key nutrients and antioxidants after birth to protect against oxidative stresses that lead to the aggregation of crystallins that ultimately manifests as nuclear cataract. Due to the large size of the lens, it is not feasible for the delivery of nutrients and antioxidants to occur by passive diffusion alone. In fact it has been hypothesised that a specialised lens microcirculation system operates to deliver nutrients and remove waste products from the lens nucleus (Figure 3A).
The working model suggests that ionic currents, carried primarily by Na+, enter the lens predominantly at both the anterior and posterior poles via an extracellular space between adjacent fibre cells. Once in the deeper lens, Na+ crosses the fibre cell membrane and flows from cell to cell toward the surface via an intercellular pathway mediated by gap junction channels (Figure 3B). The gap junctions direct Na+ to the lens equator, where Na+ pumps that actively remove Na+ from the lens are concentrated. This circulating current of Na+ generates an extracellular fluid flow, which in turn transports nutrients including glucose, amino acids and antioxidants toward the deeper fibre cells faster than would occur by passive diffusion alone. Consistent with this model, ion channels and transporters are found in distinct fibre cell membrane domains that allow the uptake of solutes delivered to them via the circulation system. A number of these ion channels and transporters either directly or indirectly use energy stored in the form of the Na+ gradient to drive the accumulation of amino acids or antioxidants against their concentration gradients. Therefore, it can be predicted that any dissipation of the Na+ gradient in the nucleus of the lens will selectively reduce amino acid/nutrient and antioxidant uptake in this region of the lens. A failure of the circulation system to maintain appropriate nutrient and antioxidant levels specifically in the lens nucleus results in an increased susceptibility of this region to oxidative damage to crystallin proteins that precipitates cataract formation.
This view is supported by studies that show that with advancing age, the levels of glutathione (GSH) decline specifically in the lens nucleus relative to the cortex. Glutathione is the principal antioxidant in the lens. It is found in high levels in the lens and is essential for the detoxification of reactive oxygen species and is vital for maintaining lens transparency. The level of glutathione in the lens is maintained by a balance between synthesis from its precursor amino acids (cysteine, glutamate and glycine), uptake of dietary glutathione from the aqueous humour, export and regeneration from oxidised glutathione (GSSG). In the lens, there are regional differences in glutathione metabolism between the outer cortex which contains differentiating fibre cells and the nucleus which contains fibre cells laid down during foetal life. The outer cortex has higher levels of glutathione relative to the nucleus. With increasing age, glutathione levels specifically in the nucleus decrease. When levels of glutathione in the nucleus fall below a critical level (approximately 1.0 mmol/l), older lenses (65 to 75 years) become susceptible to oxidative damage and cataract formation. Thus, the lens nucleus, which is not capable of continual glutathione synthesis, depends on glutathione initially produced during embryonic development and/or delivery of glutathione from the aqueous humour for its protection against oxidative stress. If there is a reduced level of glutathione at birth or a problem with the delivery pathway for glutathione and reducing equivalents to the lens centre, then the lens nucleus may be more susceptible to oxidative damage, causing an acceleration of the ageing process that leads to cataract formation in early childhood.
Nutritional and antioxidant strategies to prevent childhood cataract
It is obvious that a major risk factor for loss of nutrients and antioxidants to the lens nucleus is age. A normal young lens has substantial reserves of antioxidants; however, this may not be the case in low birth weight children. It is conceivable that as a result of maternal malnutrition, babies will be born with reduced levels of antioxidants in the lens nucleus, making them more susceptible to the early onset of cataract. In animal studies using rat lenses, mapping of glutathione-related enzymes such as glutathione reductase (GR), involved in the recycling of glutathione from GSSG and glutathione peroxidase (GPx), which reduces peroxides using GSH, showed the importance of these antioxidant defence systems in prenatal and early postnatal stages.[48, 49] Glutathione reductase immunoreactivity was first detected during day 10 of embryonic development with strongest immunoreactivity detected at postnatal days 9 to 16 and decreased immunoreactivity at postnatal day 21. On the other hand, glutathione peroxidase immunoreactivity was first detected at postnatal day 3 and was strongest at postnatal day 9, where it was maintained through to adult age. Eye opening in rats occurs at approximately postnatal days 13 to 16 and therefore, exposure of the lens to ultraviolet (UV) light will be a major contributor of oxidative stress. These data suggest an important role of these enzymes in protecting the lens against oxidative stress in these early stages of life. Therefore, the prenatal reduction of glutathione levels and glutathione-related enzyme activity will predispose newborns to endogenous (oxidative metabolism) and exogenous sources (UV light exposure, environmental toxins) of oxidative stress.
As cells laid down in the embryo are retained throughout life and lose their ability to synthesise new protein in the lens nucleus, the activity of glutathione metabolism pathways is determined prenatally. Postnatally, the delivery of glutathione to the lens nucleus is driven by the microcirculation system and if glutathione levels are maintained above a threshold level, the lens nucleus is protected from oxidative damage. In low birth weight children, failure to deliver to and maintain appropriate glutathione levels in the lens nucleus after birth, will accelerate the initiation of lens opacification leading to childhood nuclear cataract. Therefore, it is our hypothesis that prenatal oxidative stress results in depletion of glutathione in the lens nucleus that cannot be replenished by delivery of glutathione via the microcirculation system after birth due to continued poor nutrition, thereby providing a mechanistic link between maternal malnutrition, low birth weight and a higher incidence of idiopathic childhood cataract.
In summary, the development of the eye starts early in pregnancy with lens cells that form the nucleus being laid down early in life and retained throughout the lifetime of the individual. This means that glutathione levels critical for protection from age-related nuclear cataract are predetermined at birth. As there is a strong link between low birth weight and childhood cataracts, we have suggested that maternal malnutrition results in oxidative stress that leads to reduced prenatal levels of glutathione and associated glutathione metabolic enzymes in the lens nucleus. This in turn predisposes the lens to the accelerated development of nuclear cataracts in early childhood. Diets are often poor in developing countries and so young lenses already vulnerable to oxidative damage due to low prenatal glutathione levels are further exposed to oxidative damage and an increased likelihood of early cataract formation. To test whether a link between maternal malnutrition and reduced glutathione levels exists, future studies are planned using animal models of maternal global nutrient manipulation[50, 51] to assess the resultant effects on cataract development in malnourished pups.
From a public health point of view, maternal nutrition is obviously an important factor because it is modifiable by health interventions. An increasing number of public health programs have begun in developing countries to improve access to diagnostic, therapeutic and surgical services as well as better nutrition, education, immunisation and eye-health screening programs. If future studies can prove maternal nutrition is vital for the correct prenatal development of a robust antioxidant defence system that protects newborns against the subsequent development of childhood cataract, then we may be able to augment existing nutritional strategies to improve ocular health in children. In view of this, improved diet before pregnancy deserves greater attention and may be the key to reducing the incidence of idiopathic childhood cataract.