Understanding Gene Expression: Case Study

Aim

To find out whether maternal diet during pregnancy affects the age at which offspring enter puberty.

Method 

• Pregnant Wistar rats were fed one of three diets (balanced, undernourished or high-fat)
• At the age of 22 days (weaning), the offspring were separated from their mothers and split into two groups ‐ balanced diet or high-fat diet
• The phenotype of the offspring was observed and recorded
• Dexa scans measured body fat; plasma and tissue samples were taken to measure physiological factors

Results

Both maternal high-fat and maternal under-nutrition during pregnancy lead to changes in phenotype.

Key changes in the phenotype were:

• Pups born to mothers who had a balanced diet, but were fed a high-fat diet from weaning (22 days) became fat and started puberty early (Fig 4 and 5, dark grey bars)
• Pups born to mothers who were undernourished during pregnancy were born small, grew quickly, started puberty early and became obese (Fig 4, pink and red bars; Fig 6; Fig 7)
• Pups from mothers who were undernourished during pregnancy were more likely to eat more, exercise less, show signs of type two diabetes and high blood pressure as adults (graphs not shown)
• Pups born to mothers who ate high-fat diets during pregnancy were born small, grew quickly, started puberty early and became obese (Fig 5; Fig 8)

Free Radicals, Anti‐Oxidants and Oxidative Stress

In an attempt to find out why these changes were occurring, the science team measured the levels of a number of different markers that can indicate oxidative stress.

Oxidative stress is the term used to describe damage to cells, tissues and organs that is caused by a group of chemicals known as reactive oxygen species. These are chemicals such as free radicals or peroxidises which are produced as a result of the metabolism of oxygen and therefore exists in all aerobic species. Free radicals are highly reactive because they have at least one unpaired electron. The level of oxidative stress is determined by the balance between the rate at which free radicals cause damage and the rate at which anti‐oxidants repair damage.

The science team found that the offspring of undernourished mothers had high levels of a molecule called Protein Carbonyl, a marker of oxidative stress, in their ovaries (Fig 10). They also found that the offspring of both undernourished and high-fat-fed mothers had altered levels of an enzyme that regulates levels of hydrogen peroxide - a chemical that is known to cause oxidative stress.

Developmental Programming: The Predictive Adaptive Response Theory

The prenatal environment is having an effect on adult phenotype. Professor Sir Peter Gluckman of the Liggins Institute and Professor Mark Hanson of the University of Southampton have proposed that the effects that scientists have observed are due to a mismatch between the environment that the fetus experiences in the womb and the environment that the child experiences once born. In the womb, the fetus has experienced “hard times” (i.e. there is not enough food) and will make a series of metabolic adaptations in order to survive. As a result of this, the fetus is well adapted for a life of under-nutrition. When it is born into a world where there is plenty of food, the child is not well adapted. This can result in a series of potential problems including obesity, diabetes, heart disease and early puberty.  

Predictive Adaptive Responses

We know from examples in nature that some animals are capable of predicting their adult environment and adjusting their phenotype during development to ensure they are well adapted for the predicted adult environment, improving survival. These animals demonstrate a PREDICTIVE ADAPTIVE RESPONSE. An example of this is the meadow vole, a small animal (similar to a mouse) found in Alaska, Canada and the northern United States. With a life span of around six months, the meadow vole will experience either summer or winter conditions in its life - not both. Coat thickness is determined before birth by maternal signals (the hormone melatonin), related to whether the day length is shortening or lengthening. Meadow voles born at a time when they will live through winter (short days) develop a thick coat, whereas those born to live during summer (long days) will develop a thin coat. This is an example of DEVELOPMENTAL PLASTICITY.  

The phenotype is being determined by gene‐environment interactions, determining which genes are turned on to produce the phenotype most suitable for the predicted environment.  

Developmental plasticity offers a survival, and therefore an evolutionary advantage if the predicted environment matches the actual adult environment as in the case of the meadow vole. However, what we have seen in the evidence relating to fetal origins of adult disease is a mismatch between predicted environment and actual adult environment. When there is this mismatch, problems occur and the results of the plasticity are animals that are maladapted to their environment and therefore have reduced chances of survival for the individual. The animals that experienced poor fetal nutrition are programmed to have an increased chance of early puberty and adult disease. 

Developmental Plasticity

These changes in phenotype are permanent, yet they do not change the genotype. It is likely that these changes in the phenotype have been made in order to maximise the potential of the individual to survive. The fetus develops in a way which will best fit the environment, maximising its potential to survive and reproduce. Scientists believe that there are probably only a few stages during development where the environment is capable of influencing the phenotype in this way. During these stages, we say the development is very “plastic”, it is mouldable. In early life, when the differentiation of cells into specialised tissues is not complete, there tends to be a high potential for plasticity. 

Gene Environment Interactions and Epigenetics

The genotype of an organism, found in the DNA, is expressed when genes are turned on and proteins are synthesised. In order to be expressed, a gene must be unpacked from the condensed state in which it sits wrapped around histone proteins within the chromosome (the DNA—Protein complex is chromatin). This allows the transcription factors access to the gene so that the process of transcription can occur.   

In some gene‐environment interactions, a process called EPIGENESIS is occurring. This process involves non‐genetic factors that do not change the genes themselves but can change the behaviour of the genes. These factors can change the ability of a gene to be accessed within the chromatin and expressed. Epigenetic factors can include changes to the chromatin structure such as the addition of methyl (CH3) groups to the DNA (methylation). Scientists know that environmental influences early in life in a number of organisms cause epigenetic changes to the DNA and therefore change the phenotype, without changing the sequence of bases in the DNA. There is evidence that the packaging of the DNA in the chromosome is changed. This alters the ability of the gene to be unpackaged and transcribed, therefore changing the phenotype.

It is possible that the responses seen in the animal models of poor fetal nutrition and increased likelihood of early puberty and adult disease are caused by epigenetic changes to the DNA. If this is found to be correct, the poor fetal environment is causing an EPIGENETIC change in the DNA packaging which is altering the ability of specific genes to be expressed and therefore altering the phenotype.