Molecular Markers

A common phrase in the vocabulary of many geneticists is molecular markers. Molecular markers are DNA fragments that can be used as a fingerprint in the identification or characterization of individuals. These markers have become an increasingly helpful tool in genetic research and applications to biotechnology. The basic premise behind molecular markers is that there is natural genetic variation in individuals, and many genetic sequences are polymorphic, meaning they differ among individuals. Molecular markers seek to exploit this variation to identify individuals, traits, or genes on the basis of genetic differences.

Working with the human genome, Botstein proposed the use of DNA fragments as genetic markers for monitoring segregation. The first molecular markers to be used were fragments produced by digestion of DNA with restriction enzymes. The variation in fragment size obtained from different individuals after the digestion created the class of markers called restriction fragment length polymorphism (RFLP).

One of the quickest ways to discover the location of a gene is through reverse genetics; that is, starting from the trait of interest, one would identify the protein involved. By knowing that genes code proteins, one can try to locate the actual gene. If the sequence of the amino acids of the protein is known, the genetic code can be used to establish the sequence of corresponding nucleotides, which is at least a part of the gene. From this sequence, a complementary sequence of nucleotides can be built and used as a probe. These synthetic sequences, a single DNA strand, can be used to detect genes within the billions of nucleotide bases. The probes can be radioactively labeled to facilitate identification. After the probes hybridize with the corresponding genes in the chromosomes, it is possible to identify their location by the detection of radioactivity, revealed on X-ray film. Each probe with the complementary gene can be observed on the film as a dark spot or band. Increasingly, fluorescent dyes are used instead of radioactive probes.

DNA probes are used for mapping genes in the chromosomes and for genetic tests, as in the case of the diagnosis of breast cancer. These probes are also used in the characterization of individuals at a molecular level, a process called DNA fingerprinting.

Gene Therapy

Gene therapy has become an increasingly important topic in science-related news. The basic concept of gene therapy is to introduce a gene with the capacity to cure or prevent the progression of a disease. Gene therapy introduces a normal, functional copy of a gene into a cell in which that gene is defective. Cells, tissue, or even whole individuals (when germ-line cell therapy becomes available) modified by gene therapy are considered to be transgenic or genetically modified. Gene therapy could eventually target the correction of genetic defects, eliminate cancerous cells, prevent cardiovascular diseases, block neurological disorders, and even eliminate infectious pathogens. However, gene therapy should be distinguished from the use of genomics to discover new drugs and diagnosis techniques, although the two are related in some respects. The two main types of gene therapy are somatic cell gene therapy and reproductive or germ-line gene therapy. This chapter also discusses therapeutic cloning, which involves stem cell manipulation for tissue and organ production.

Germ-line cell therapy involves the introduction of corrective genes into reproductive cells (sperm and eggs) or zygotes, with the objective of creating a beneficial genetic change that is transmitted to the offspring. When genes are introduced in a reproductive cell, descendant cells can inherit the genes.

Gene therapy of somatic cells, those not directly related to reproduction, results in changes that are not transmitted to offspring. An example of gene therapy in somatic cells is the introduction of genes in an organ or tissue to induce the production of an enzyme. This alteration does not affect the individual's genetic makeup as a whole and it is not transmitted to its descendants. With somatic cell gene therapy, a disabled organ is better able to function normally. This technology has many applications to human health. One variant of somatic cell gene therapy is DNA vaccines, which allow cells of the immune system to fight certain diseases in a method similar to conventional vaccines.

Stem cell therapy involves the use of pluripotent cells, or cells that can differentiate into any other cell type. Stem cells are found in developing embryos and in some tissues of adult individuals. This therapy is similar to a conventional transplant, with the objective of regenerating or repairing a damaged organ or tissue. The procedure has a reduced probability of rejection because it uses the individual's own cells. For instance, stem cells differentiated into nerve cells could be used by patients suffering from paralysis, with the goal of helping them recovering movement; or in cases of heart stroke, muscle cells might be used to rejuvenate the cardiac muscles. Furthermore, the future may bring the growth of stem cells from an individual's body to produce certain tissues or organs in vitro. Stem cell research could eventually blend gene therapy with genetic engineering to create healthy stem cells that can be used to generate healthy organs and tissue.

A fundamental requirement for gene therapy is the correct identification of genes coding for diseases. This can be accomplished at a spectacular speed with the information from the Human Genome Project. Scientific magazines have been announcing, with great frequency, the discovery of genes responsible for several medical conditions, from Alzheimer's disease to baldness. The knowledge of the genes involved in these traits allows unequivocal diagnosis of the disease in the patient, an essential step before treatment can be initiated for the genetic disease. Biotechnology is contributing to the development of the needed genetic tests for detection of defective genes.

The most complex phase in gene therapy is the development of mechanisms to deliver the therapeutic genes to the target organ in an accurate, controlled, and effective way. That step has been developing more slowly and is currently the most limiting factor for gene therapy.

Experimental Design

An experiment is a study designed to compare benefits of an intervention with standard treatments, or no treatment, such as a new drug therapy or prevention program, or to show cause and effect (see ). This type of study is performed prospectively. Subjects are selected from a study population, assigned to the various study groups, and monitored over time to determine the outcomes that occur and are produced by the new drug therapy, treatment, or intervention.

Experimental designs have numerous advantages compared with other epidemiological methods. Randomization, when used, tends to balance confounding variables across the various study groups, especially variables that might be associated with changes in the disease state or the outcome of the intervention under study. Detailed information and data are collected at the beginning of an experimental study to develop a baseline; this same type of information also is collected at specified follow-up periods throughout the study. The investigators have control over variables such as the dose or degree of intervention. The blinding process reduces distortion in assessment. And, of great value, and not possible with other methods, is the testing of hypotheses. Most important, this design is the only real test of cause–effect relationships.

The disadvantages of experimental design involve subject participation criteria that may limit generalizability of findings. Restrictive criteria for inclusion or exclusion of subjects may produce a very homogeneous study population that restricts application of the results to patients with other characteristics. Clinical trials, especially those focused on chronic diseases, may require years of follow-up and prolonged observation to determine treatment outcomes. The result is often higher costs, increased likelihood that patients will be lost to follow-up, and delayed treatment recommendations. Large sample sizes are typically required to demonstrate differences among study groups, especially if there is wide variability in responses to treatment. Increasing the size of the study population also raises the cost of the trial and may make it difficult to locate a sufficiently large pool of eligible patients. Ethical concerns also arise in clinical trials, and subjects may not comply with the treatment and assignment.

Clinical trials can be divided into three types: (1) therapeutic trials in which therapeutic agents or procedures are given to patients in an attempt to cure the disease, relieve symptoms, or prolong survival; (2) intervention trials in which the investigator intervenes before the disease has developed in individuals with certain characteristics that increase their risk of developing the disease; and (3) prevention trials in which an attempt is made to determine the efficacy of a preventive agent or procedure among people who do not have the disease but may be at greater risk for developing it. The randomized, controlled clinical trial is the most widely accepted approach for comparing the benefits of treatments. The basic design of a randomized, controlled clinical trial is outlined in . Community intervention trials have the same basic design; the difference is that groups of people are assigned to the various study groups.

Observational Epidemiology

Epidemiological study designs also can be described according to a number of different aspects. One aspect is whether the study is observational or experimental. Investigators in observational studies may plan and identify variables to be measured, but human intervention is not a part of the process. Experimental studies, in contrast, involve intervention in ongoing processes to study any resulting change or difference. Epidemiological studies are also descriptive or analytical in nature. Descriptive studies attempt to uncover and portray the occurrence of the condition or problem, whereas analytical studies determine the causes of the condition or problem.

Observational epidemiology provides information about disease patterns or drug use problems by various characteristics of person, place, and time. This approach is used by public health professionals for efficient allocation of resources and to target populations for education, prevention, and treatment programs. It also is used by epidemiologists to generate hypotheses regarding the causes of disease or drug use problems. Some researchers do not consider experimental studies to be true epidemiological studies in the traditional sense because they follow clinical or planned research designs. Descriptive studies provide insight, data, and information about the course or patterns of disease or drug use problems in a population or group. Analytical studies are used to test cause–effect relationships, and they usually rely on the generation of new data.

There are five general types of epidemiological study design: (1) case reports and case series, (2) cross-sectional studies, (3) case-control studies, (4) cohort studies, and (5) experiments. A case report is a descriptive study of a single patient, and a case series is a collection of case reports. A cross-sectional study is a prevalence study—a basic descriptive study that examines relationships between a disease or drug use problem and other characteristics of people in a population at one point in time. A case-control study compares people who have the disease or problem (cases) to those who do not (controls) with respect to characteristics of interest (i.e., potential causes). A cohort study is an incidence study that measures characteristics or attributes in a population free of a disease or drug use problem and relates them to subsequent development of the disease in that population as it is followed over time. This type of study is also referred to as a longitudinal study. Experimental studies are clinical trials and intervention studies designed to compare outcomes between two or more treatment or intervention groups.

These different aspects and relationships of epidemiological study designs are summarized and portrayed in . At the basic level, investigators can exert control over the process by assigning subjects to study groups, such as in experiments, or they do not exert control and only observe what is occurring naturally. Experimental studies are further divided into studies that assess effects or outcomes in individual subjects or studies that assess effects in communities or large groups of people. Observational studies are divided into descriptive or analytical types, with the latter type being further defined by the focus of sampling for the exposure or disease.

Morbidity and Mortality

Morbidity and mortality are two very important aspects of medical surveillance. They are primary indicators of the healthiness or sickness of a population.

Any disturbance in the function or structure of a human body is considered to be a disease. Disease, illness, injury, disorder, and sickness all are categorized under a single term, morbidity. Morbidity is the extent of disease, illness, injury, or disability in a defined population. It is either a deviation from a state of health and well-being or the presence of a specific symptom or condition. Morbidity is usually expressed in terms of prevalence, attack rates, or incidence rates. In essence, morbidity refers to the rate of disease in a population—in other words, the number of ill people present in a certain population that is healthy but at risk for developing the disease.

Mortality means death, or it describes death and related issues. The mortality rate is the rapidity with which people in a given population die of a particular condition. Three things generally cause death: degeneration of vital organs and related conditions; specific disease states; and massive trauma or physical harm caused by environmental or social conditions, such as accidents, disasters, and homicides.

Numerical information about death is a basic component of vital statistics and epidemiology. In many countries, laws require the registration of vital events, including births, marriages, divorces, and deaths. Mortality rates are the foundation for vital statistics. Deaths are certified by a physician or a coroner; they must be recorded and reported to local health departments or state offices of vital statistics. On a death certificate, the cause of death is stated, or an underlying cause of death can be noted.

Mortality statistics are reported from the information recorded on death certificates. Public health agencies and other organizations (e.g., insurance companies) produce and revise tables of mortality that are published on a regular basis. These tables of mortality provide actual numbers of deaths as well as death rates by age, sex, and cause of death. Special mortality tables can also present other variables in the context of vital statistics. Examples of different types of mortality rates include annual death rate, infant mortality rate, fetal death rate, abortion rate, maternal mortality rate, and case fatality rate.

Medical Surveillance

One of the most basic functions of epidemiology is detecting the occurrence of health problems or exposures in a target population. This process of detection, called medical surveillance, is conducted to identify changes in the distribution of diseases, thereby permitting their prevention or control within the population. The term surveillance means "to watch over." Medical surveillance traditionally involved monitoring the spread of infectious diseases through a population. Today, however, surveillance programs are applied to a variety of health problems and conditions. Medical surveillance involves the following key features:

• Continuous data collection and evaluation
• An identified target population
• A standard definition of the outcome under study
• Timely collection and dissemination of information
• Application of the data to disease control and prevention
  

Surveillance activities provide data about the distribution of a disease by person, place, and time. These three classic variables are the most important in epidemiology, because patterns of occurrence indicated by these variables can help identify possible causes of a disease. A great variety of information is collected during surveillance, including demographic information about affected and unaffected individuals, their behaviors, and the geographic location of health problems.

Many diseases, such as cancer, heart conditions, sexually transmitted diseases, and drug addiction, are studied through medical surveillance. The goals of medical surveillance activities include the following:

• Identifying patterns of disease occurrence
• Detecting disease outbreaks or epidemics
• Developing ideas about possible causes
• Identifying cases for further investigation
• Planning health services to fulfill specific needs
  

The term population-based means that the target group under study or surveillance is the general population, usually in terms of geographic residence.

Fast Food - What We Hate !!

Scottish girls take second place in the female rankings, with almost 33 per cent overweight. English girls are fourth, with 29.3 per cent too heavy for their height.

The heaviest girls are in Portugal (34.3 per cent), while the slimmest are in Latvia and Lithuania (3.5 per cent overweight).

Among the boys, Scotland was again second, with almost 35 per cent too heavy for their height. Only Spanish boys are heavier.

English boys are in sixth place at 29 per cent - compared to the lean lads of Lithuania, where only 8 per cent are overweight.

Obesity experts said the results could be partly explained by a couch potato lifestyle, in which TV dinners have replaced family meals and computer games are preferred to outdoor play.

Dr Tim Lobstein, of the International Association for the Study of Obesity, said: "There is a big industry selling us more TV to watch, more computer games to play, more DVDs to sit and watch.

"There is a big industry promoting screen watching which is a sedentary behaviour and you just get fatter while you do it."

The figures, which were compiled by the IASO from government and scientific studies, come as British doctors warn they are treating children as young as two for obesity.

Drastic stomach surgery, including gastric banding, is being carried out on children as a last resort.

Type 2 diabetes, which was once a problem for overweight middle aged adults, is being diagnosed in teenagers, and chubby children are being equipped with masks to ensure they do not suffocate in their sleep.

Dr Steve Ryan, medical director of Alder Hey children's hospital in Liverpool, said: "There are more and more of these children and significant numbers are obese from two or three years old.

"When I was a paediatrician starting out in 1991, there were very few children overweight but that has changed and we are starting to see complications resulting from this.

"It is here and we are having to deal with it."

But while British youngsters totter near the top of the heavyweight league, children in other countries are starting to lose weight.

New data shows rates of childhood obesity are stabilising in France and falling in Switzerland.

Restrictions on the advertising of junk food to children, the banning of vending machines in schools and national healthy eating drives may all have played a role in the change, the European Congress on Obesity heard yesterday.

Obesity experts welcomed the figures but warned the situation is still bleak - particularly in the UK, where rates of child obesity have quadrupled since 1984.

Dr Lobstein, director of the IASO's childhood obesity programme, said: "It is encouraging that there may be some signs this tidal wave of obesity is easing but it is not really subsiding, it is only stopping at a high level.

"The tide has come in but it is not going out. And in Britain, it is still coming in and is rising. The old picture of a jolly fat person couldn't be further from the truth.

"Although some fat people might indeed be jolly, the majority don't enjoy their condition and wish they were slim."

Dr Ian Campbell, medical director of the charity Weight Concern, said childhood obesity could only be tackled by parents, schools and government working together.

Safe, accessible exercise facilities and nourishing and affordable meals should be a priority, he said.

• Cash is the most effective way to get children to lose weight, according to a study.

More than 100 families were given either a low carbohydrate diet, a low energy diet or support in the form of a weekly motivational letter.

The final option was giving children a cash bonus each time they improved their body mass index. The congress heard that cash was the best motivator for children.

For adults it was a combination of all four methods.