Arxiu de la categoria: GENETICS

Pharmacogenetics: a drug for each person

Sometimes, some people say that the medications prescribed by doctors are not good. Can this be true? Not all drugs work for the same population. Keep reading and discover the secrets of pharmacogenetics.

INTRODUCTION

The same that happens with nutrients, happens with drugs. Another objective of personalized medicine is to make us see that not all medicines are for everyone. However, it does not come again because around 1900, the Canadian physician William Osler recognized that there was an intrinsic and specific variability of everyone, so that each one reacts differently to a drug. This is how, years later, we would define pharmacogenetics.

It is important to point out that it is not the same as pharmacogenomics, which studies the molecular and genetic bases of diseases to develop new treatment routes.

First, we need to start at the beginning: what is a drug? Well, a drug is any physicochemical substance that interacts with the body and modifies it, to try to cure, prevent or diagnose a disease. It is important to know that drugs regulate functions that our cells do, but they are not capable of creating new functions.

Apart from knowing if a drug is good or not for a person, you also have to take into account the amount that should be administered. And we still do not know the origin of all diseases, that is, we do not know most of the real molecular and genetic causes of diseases.

The classification of diseases is based mainly on symptoms and signs and not on molecular causes. Sometimes, the same group of pathologies is grouped, but among them there is a very different molecular basis. This means that the therapeutic efficacy is limited and low. Faced with drugs, we can manifest a response, a partial response, that produces no effect or that the effect is toxic (Figure 1).

efectivitat i toxicitat
Figure 1. Drug toxicity. Different colours show possible responses (green: drug not toxic and beneficial; blue: drug not toxic and not beneficial; red: drug toxic but not beneficial; yellow: drug toxic but beneficial) (Source: Mireia Ramos, All You Need is Biology)

DRUGS IN OUR BODY

Drugs usually make the same journey through our body. When we take a drug, usually through the digestive tract, it is absorbed by our body and goes to the bloodstream. The blood distributes it to the target tissues where it must take effect. In this case we talk about active drug (Figure 2). But this is not always the case, but sometimes it needs to be activated. That’s when we talk about a prodrug, which needs to stop in the liver before it reaches the bloodstream.

Most of the time, the drug we ingest is active and does not need to visit the liver.

active and prodrug
Figure 2. Difference between prodrug and active drug (Source: Agent of Chemistry – Roger Tam)

Once the drug has already gone to the target tissue and has interacted with target cells, drug waste is produced. These wastes continue to circulate in the blood to the liver, which metabolizes them to be expelled through one of the two routes of expulsion: (i) bile and excretion together with the excrement or (ii) purification of the blood by the kidneys and the urine.

THE IMPORTANCE OF PHARMACOGENETICS

A clear example of how according to the polymorphisms of the population there will be different response variability we find in the transporter genes. P glycoprotein is a protein located in the cell membrane, which acts as a pump for the expulsion of xenobiotics to the outside of the cell, that is, all chemical compounds that are not part of the composition of living organisms.

Humans present a polymorphism that has been very studied. Depending on the polymorphism that everyone possesses, the transporter protein will have normal, intermediate or low activity.

In a normal situation, the transporter protein produces a high excretion of the drug. In this case, the person is a carrier of the CC allele (two cytokines). But if you only have one cytosine, combined with one thymine (both are pyrimidine bases), the expression of the gene is not as good, and the expulsion activity is lower, giving an intermediate situation. In contrast, if a person has two thymines (TT), the expression of the P glycoprotein in the cell membrane will be low. This will suppose a smaller activity of the responsible gene and, consequently, greater absorption in blood since the drug is not excreted. This polymorphism, the TT polymorphism, is dangerous for the patient, since it passes a lot of drug to the blood, being toxic for the patient. Therefore, if the patient is TT the dose will have to be lower.

This example shows us that knowing the genome of each individual and how their genetic code acts based on it, we can know if the administration of a drug to an individual will be appropriate or not. And based on this, we can prescribe another medication that is better suited to this person’s genetics.

 APPLICATIONS OF THE PHARMACOGENETICS

The applications of these disciplines of precision medicine are many. Among them are optimizing the dose, choosing the right drug, giving a prognosis of the patient, diagnosing them, applying gene therapy, monitoring the progress of a person, developing new drugs and predicting possible adverse responses.

The advances that have taken place in genomics, the design of drugs, therapies and diagnostics for different pathologies, have advanced markedly in recent years, and have given way to the birth of a medicine more adapted to the characteristics of each patient. We are, therefore, on the threshold of a new way of understanding diseases and medicine.

And this occurs at a time when you want to leave behind the world of patients who, in the face of illness or discomfort, are treated and diagnosed in the same way. By routine, they are prescribed the same medications and doses. For this reason, the need has arisen for a scientific alternative that, based on the genetic code, offers to treat the patient individually.

REFERENCES

  • Goldstein, DB et al. (2003) Pharmacogenetics goes genomic. Nature Review Genetics 4:937-947
  • Roden, DM et al. (2002) The genetic basis of variability in drug responses. Nature Reviews Drug Discovery 1:37-44
  • Wang, L (2010) Pharmacogenomics: a system approach. Syst Biol Med 2:3-22
  • Ramos, M. et al. (2017) El código genético, el secreto de la vida. RBA Libros
  • Main picture: Duke Center for Applied Genomics & Precision Medicine

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Insulin: a point in favour for transgenics

Despite the arguments and positions against transgenics, it is undeniable that insulin is a great transgenic success. It is essential in some types of diabetes; and since it was discovered, the life expectancy of diabetics has increased more than 45 years. Therefore, let’s know in detail.

REMINDER OF GENETIC ENGINEERING

Genetic engineering allows to clone, that is, to multiply DNA fragments and produce the proteins for which these genes encode in organisms different from the one of origin. That is, if in an organism there is an alteration or mutation of a gene that prevents the genetic code from translating it into proteins, with the techniques of recombinant DNA a gene is obtained without the mutation in another organism. Thus, it is possible to obtain proteins of interest in organisms different from the original from which the gene was extracted, improve crops and animals, produce drugs and obtain proteins that use different industries in their manufacturing processes. In other words, through genetic engineering, the famous transgenics are obtained.

They offer many possibilities in the industrial use of microorganisms with applications ranging from the recombinant production of therapeutic drugs and vaccines to food and agricultural products. But, in addition, they have a promising role in medicine and in the cure of diseases.

And is that the result of obtaining a recombinant DNA, from it, will be made a new protein, called recombinant protein. An example of this is the case of insulin.

WHAT IS INSULIN?

Insulin is a hormone produced in the pancreas and with an important role in the metabolic process. Insulin comes from the Latin insulae, which means island. Its name is due to the fact that inside the pancreas, insulin is produced in the islets of Langerhans. The pancreas is related to the general functioning of the organism. It is located in the abdomen and is surrounded by organs such as the liver, spleen, stomach, small intestine and gallbladder.

Thanks to it we use the energy of the food that enters our body. And this happens because it allows glucose to enter our body. This is how it provides us with the necessary energy for the activities we must perform, from breathing to running (Video 1).

Video 1. Insulin, Glucose and You (Source: YouTube)

HOW DOES INSULIN WORK?

Insulin helps glucose enter the cells, like a key that opens the lock on the cell doors so that glucose, which is blood sugar, enters and is used as energy (Figure 1). If glucose cannot enter because there is no key to open the door, as with people with diabetes, blood glucose builds up. An accumulation of sugar in the blood can cause long-term complications. That’s why it’s important for diabetics to inject insulin.

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Figure 1. Picture of the funcioning of insulin in cells (Source: Encuentra tu balance)

WHY DO WE USE TRANSGENIC INSULIN?

First, the insulin obtained from animals such as dogs, pigs or cows was used. But although, above all, pork insulin was very similar to human insulin, it was not identical and contained some impurities. This fact caused rejection and, in some cases, allergies. In addition, to be obtained from the pancreas of pigs, for each pancreas only insulin was obtained for the treatment of 3 days (at more than the cost of care of the animal). The result was low performance and high costs.

But with recombinant DNA insulins, more is obtained at a lower cost. For this reason, currently, the original insulin is obtained from a human of genetic engineering, despite the fact that animal insulins are still a perfectly acceptable alternative.

Through genetic engineering, insulin has been produced from the E. coli bacterium. It was in 1978 when the sequence of the insulin was obtained and introduced inside the bacteria so that it produced insulin. This is how E. coli has gone from being a common bacterium to a factory producing insulin. Insulin is extracted from the bacteria, purified and marketed as a medicine.

The advantages of “human” insulin, obtained by genetic engineering, are the easy maintenance of bacteria, a greater quantity of production and with lower costs. More or more, the compatibility of this insulin is 100%, however there may be reactions due to other components.

On an industrial scale, the production of recombinant proteins encompasses different stages. These stages are fermentation, in which the bacteria are cultivated in nutritious culture media; the extraction to recover all the proteins inside, the purification, which separates the recombinant protein from the other bacterial proteins; and finally the formulation, where the recombinant protein is modified to achieve a stable and sterile form that can be administered therapeutically.

Each of the previous phases implies a very careful handling of the materials and a strict quality control to optimize the extraction, purity, activity and stability of the drug. This process can be simple or more complex depending on the product and the type of cell used. Although the complexity of the process would increase the final cost of the product, the value will not exceed the expense of isolating the compound from its original source to reach medicinal quantities, which is what we have shown with insulin. That is, producing human insulin has a lower cost than obtaining insulin from pigs.

Genetic engineering allows numerous potentially therapeutic proteins to be made in large quantities. Currently, there are more than 30 proteins approved for clinical use, in addition to hundreds of therapeutic protein genes that have been expressed at the laboratory level and that studies continue to demonstrate their clinical adequacy.

REFERENCES

  • Ramos, M. et al. El código genético, el secreto de la vida (2017) RBA Libros
  • Alberts, B. et al. Biología molecular de la célula (2010). Editorial Omega, 5a edición
  • Cooper, G.M., Hausman R.E. La Célula (2009). Editorial Marbán, 5a edición
  • Naukas
  • Vix
  • Main picture: UniversList

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The reality of mutations

Do you remember the ninja turtles? Leonardo, Raphael, Michelangelo and Donatello were four turtles that suffered a mutation when they were bathed with a radioactive liquid. Fortunately or unfortunately, a mutation cannot turn us into ninja turtles, but it can have other effects. Next, I tell you what mutations are.

WHAT ARE MUTATIONS?

Our body is like a great factory in which our cells are the workers. These, thanks to their internal machinery, make the factory stay afloat with the least possible problems. The constant operation of our cells (24/7), sometimes causes errors in their machinery. This generates imperfections in the genetic code, which generally go unnoticed. It is true that cells do everything possible to fix the failures produced, but sometimes they are inevitable and lead to the generation of diseases or even to the death of the cell.

Mutations are these small errors, it means, mutations are stable and inheritable changes that alter the DNA sequence. This fact introduces new genetic variants in the population, generating genetic diversity.

Generally, mutations tend to be eliminated, but occasionally some can succeed and escape the DNA repair mechanisms of our cells. However, they only remain stable and inheritable in the DNA if they affect a cell type, the germ cells.

The organisms that reproduce sexually have two types of cells: germinal and somatic. While the former transmit genetic information from parents to children, somatic cells form the body of the organism. Because the information of germ cells, which are what will give rise to gametes (sperm and oocytes) passed from generation to generation, they must be protected against different genetic changes to safeguard each individual.

Most mutations are harmful, species cannot allow the accumulation of large number of mutations in their germ cells. For this reason not all mutations are fixed in the population, and many of these variants are usually eliminated. Occasionally some may be incorporated into all individuals of the species.

The mutation rate is the frequency at which new mutations occur in a gene. Each specie has a mutation rate of its own, modulated by natural selection. This implies that each species can be confronted differently from the changes produced by the environment.

Spontaneous mutation rates are very low, in the order of 10-5-10-6 per gene and generation. In this way, mutations do not produce rapid changes in the population.

THE ROLE OF NATURAL SELECTION

Changes of nucleotides in somatic cells can give rise to variant or mutant cells, some of which, through natural selection, get more advantageous with respect to their partners and proliferate very fast, giving us as a result, in the extreme case, cancer, that is, uncontrolled cell proliferation. Some of the cells in the body begin to divide without stopping and spread to surrounding tissues, a process known as metastasis

But the best way to understand the role of natural selection of which the naturist Charles Darwin spoke is with the example of spotted moths (Biston betularia). In England there are two types of moths, those of white colour and those of black colour (Figure 1). The former used to be the most common, but between 1848 and 1898 black moths were imposed.

biston
Figure 1. Biston betularia, white and black moths (Source: TorruBlog)

This change occurred at the same time that cities became more industrial, in which coal became the main fuel for power plants. The soot of this rock dyed the sky, the soil and the buildings of the cities black. Tree trunks were also affected, where the moths were camouflaged.

The consequence of this fact was that white moths could not hide from their predators, whereas those that were black found a successful exit camouflaging well on the tinted trunks. With the change of colour of their hiding place they had more opportunities to survive and reproduce (Video 1).

Video 1. Industrial melanism, white and black moth (Source: YouTube)

This is a clear example of how changes in the environment influence the variability of gene frequencies, which vary in response to new factors in the environment.

TYPES OF MUTATIONS

There is no single type of mutation, but there are several types of mutation that can affect the DNA sequence and, rebound, the genetic code. However, not all mutations have the same effect.

There are many and different types of mutations, which are classified by mutational levels. These levels are based on the amount of hereditary material affected by the mutation and go up in rank according to the number of genes involved. If the mutation affects only one gene we speak of gene mutation, whereas if it affects a chromosomal segment that includes several genes we refer to chromosomal mutation. When the mutation affects the genome, affecting whole chromosomes by excess or by defect, we speak of genomic mutation.

An example of a point mutation is found in cystic fibrosis, a hereditary genetic disease that produces an alteration in the secretion of mucus, affecting the respiratory and digestive systems. A point mutation affects the gene that codes for the CFTR protein. The affected people receive from both parents the defective gene, which, having no copy of the good gene, the protein will not be functional. The result is that the secretions produced by the human body are thicker than usual, producing an accumulation in the respiratory tract.

REFERENCES

  • Ramos, M. et al. El código genético, el secreto de la vida (2017) RBA Libros
  • Alberts, B. et al. Biología molecular de la célula (2010). Editorial Omega, 5a edición
  • Cooper, G.M., Hausman R.E. La Célula (2009). Editorial Marbán, 5a edición
  • Bioinformática UAB
  • Webs UCM
  • Main picture: Cine Premiere

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Rare diseases: fight against oblivion

We are ending the month of February, and this means that the Rare Disease Day is approaching. Marfan syndrome, Williams syndrome, DiGeorge syndrome, Crohn’s disease, Fanconi anaemia, mucopolysaccharidosis, among many others make up the list of these diseases. Why are they called minority diseases or rare diseases?

WHAT ARE MINORITY DISEASES?

A minority disease is that which affects less than 1 in 2,000 people. Although individually they are rare, there are many diseases of this type (6,000-7,000), so there are many affected patients.

Although the definition of minority disease is what I have just said, in the pharmaceutical industry it is that disease in which it is not profitable to develop a drug due to the low number of patients, the limited information available, the poor diagnosis, the lack of clinical studies and the difficult location of patients. It is for this reason that the families themselves create their own foundations to obtain financing for the investigation of these diseases.

A few years ago these diseases were socially forgotten, but, fortunately, they are now socially transcendental and recognized.

As I said, there are around 7,000 minority diseases described and every year between 150 and 250 new ones are described, thanks to new technologies.

A large number of these diseases affects children, that is, they manifest themselves at an early age. It is necessary to know that most have a genetic basis, caused by mutations in specific genes such as cystic fibrosis or several muscular dystrophies. But there are also related to environmental factors, such as some types of anaemia due to lack of vitamins or due to medications. This is the case of malignant mesothelioma, a breast cancer, in which more than 90% of cases are due to asbestos exposure. However, there are still many without knowing their origin or data on their prevalence.

MINORITY DISEASES IN NUMBERS

The fact that these diseases affect few people and the ignorance of their symptoms by the public and professionals, it is estimated that the time that elapses between the appearances of the first symptoms until diagnosis is 5 years. In 1 of every 5 cases, more than 10 years may pass until the correct diagnosis is obtained. This means not receiving support or treatment or receiving inadequate treatment and worsening the disease.

Not all hospitals have the means to treat those affected, for this reason it is estimated that practically half of sufferers have had to travel and travel in the last 2 years out of their province because of their illness, either in look for a diagnosis or treatment.

Minor diseases represent a significant economic cost. The cost of diagnosis and treatment accounts for around 20% of the annual income of each affected family. This means an average of more than 350€ per family per month, a figure very representative of the high cost involved in the care of rare diseases. The expenses to cover in the majority of cases are related to the acquisition of medicines and other health products, medical treatment, technical aids and orthopaedics, adapted transport, personal assistance and adaptation to housing.

TREATMENT FOR MINORITY DISEASES

Only 1-2% of minority diseases currently have some type of treatment, therefore, much remains to be investigated.

There are 4 basic types of treatment for rare genetic diseases:

PHARMACOLOGICAL THERAPIES

It consists in the modification of a normal or pathological biochemical reaction by an external chemical agent.

The development of a drug is a very expensive process and difficult to quantify. Currently many millions have to be invested for a new drug to reach the patient.

But what is a medication? A medicine is a small organic molecule, which typically has to be:

  • Specific to solve a molecular problem (ex: prevent an abnormal interaction between two proteins)
  • Very active and very tuned for your target
  • Very little toxic
  • Distribute well throughout the body and reach the target tissue
  • Cheap to produce or, at least, that can be synthesized in industrial quantities
  • Stable
  • New (patentable)
  • It has to be commercialized

GENE THERAPY

Attempt to correct defective genes responsible for diseases in the somatic (non-sexual) line, either by:

  • Loss of function: incorporate the normal gene (ex: phenylketonuria)
  • Function gain: eliminate the responsible mutation, eliminating the protein (ex: Huntington)

Limitations:

  • Only the reversible characteristics of a genetic disease can be corrected
  • The size of the DNA to be incorporated in the patient’s genome
  • Immune response against the viral vector (retroviruses, adenoviruses, adenoassociates)
  • Inactivation of an essential gene that can cause a problem greater than the disease
  • Directionally to appropriate target cells

CELLULAR THERAPY

Describes the process of introducing new cells into an affected tissue, with or without previous gene therapy. It is necessary to introduce many cells because the treatment is effective and, sometimes, these cells can go to unwanted tissues or have some types of abnormal growth.

SURGERY

For example in congenital heart defects.

RARE DISEASE DAY

For rare diseases to cease to be, Rare Disease Day is celebrated on the last day of February, with the aim of raising awareness and awareness among the public about rare diseases; as well as showing the impact on patients’ lives and reinforcing their importance as a priority in public health.

It was established in 2008 because, according to the European Organization for Rare Diseases (EURORDIS), the treatment of many rare diseases is insufficient, as well as in social networks to support people with minority diseases and their families. In addition, while there were already many days devoted to people suffering from individual diseases (such as AIDS, cancer, etc.) before there was not a day to represent people suffering from minority diseases. It was chosen on 29th February because it is a “rare” day. But it is celebrated on the last day of February in years that are not leap years.

Then I leave the promotional video for the Rare Disease Day 2015:

Video 1. Rare Disease Day 2015 Official Video (Source: YouTube)

REFERENCES

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From lab to big screen (I)

A little more than a month for the great gala of the cinema, the Oscars, I present some films related to genetics. There is a variety of feature films, especially sci-fi. For this reason, this is the first of several articles about cinema. In this article I will focus on films based on genetic diseases.

WONDER (2017)

Director: Stephen Chbosky

Cast: Jacob Tremblay, Julia Roberts, Owen Wilson

Genre: Drama

Synapsis: A 10 years-old boy born with a facial deformity is destined to fit in at a new school, and to make everyone understand he is just another ordinary kid, and that beauty is not skin deep.

Relation with genetics: Auggie suffers Treacher Collins syndrome, a condition that affects the development of bones and other tissues of the face. This condition affects an estimated 1 in 50,000 people. In most cases, it is due to a genetic mutation of chromosome 5. Specifically, in the gene TCOF1, involved in the development of bones and other tissues of the face.

Video 1. Wonder trailer (Source: YouTube)

JULIA’S EYES (2010)

Director: Guillem Morales

Cast: Belén Rueda, Lluís Homar, Julia Gutiérrez Caba

Genre: Terror

Synopsis: It tells the story of a woman slowly going blind, the death of her twin sister tries to uncover the mysterious.

Relation with genetics: Both Julia and her sister suffer from retinitis pigmentosa. This disease causes the progressive loss of vision, affecting the retina, which is the layer of light-sensitive tissue in the back of the eye.

The first symptoms tend to be the loss of night vision and difficult to guide in low light. Later, the disease produces the appearance of blind spots in the lateral vision. With the passage of time, these blind spots come together producing a tunnel vision (Figure 1). Finally, this leads to blindness.

retinitis_pigmentosapic
Figure 1. Comparison between normal view and view wtih retinitis pigmentosa (Source: EyeHealthWeb)

The inheritance pattern can be autosomal dominant, recessive or linked to the X chromosome. In the first case, a single copy of the altered gene in each cell is sufficient to cause the disease. Most people with autosomal dominant retinitis pigmentosa have an affected father and other family members with the disorder.

Video 2. Julia’s eyes trailer (Source: YouTube)

MY SISTER’S KEEPER (2009)

Director: Nick Cassavetes

Cast: Cameron Díaz, Abigail Breslin, Alec Baldwin

Genre: Drama

Synopsis: Sara and Brian Fitzgerald’s life with their young son and their two-year-old daughter, Kate, is forever altered when they learn that Kate has leukaemia. The parents’ only hope is to conceive another child, Anna, specifically intended to save Kate’s life. Kate and Anna share a bond closer than most sisters: though Kate is older her life depends on Anna. Until Anna, now 11, says “no. Seeking medical emancipation, she hires her own lawyer, initiating a court case that divides the family and that could leave Kate’s rapidly failing body in the hands of fate.

Relation with genetics: Leukemias are the first type of cancer in which genetic alterations were described, such as translocations, which are the most frequent (more than 50% of cases). In addition, these have high prognosis and diagnostic value. There are many types of leukemias, therefore it is a diverse group of blood cancers, which affect blood cells and bone marrow. It is the most frequent type of cancer in children; however, it affects more adults than children.

A first classification is based on the lineage: lymphoid (blood-forming cells) or myeloid (cells of the bone marrow). At the same time, you are (lymphoid or myeloid) also classified according to the clinical presentation: acute (symptoms in short period of time and severe symptoms) or chronic (the time is longer).

In adults, acute myeloid leukemia (AML) and chronic lymphocytic leukemia (CLL) are more frequent, while in children it is acute lymphoblastic leukemia (ALL).

Video 3. My sister’s keeper trailer (Source: YouTube)

LORENZO’S OIL (1992)

Director: George Miller

Cast: Nick Nolte, Susan Sarandon, Peter Ustinov

Genre: Drama

Synopsis: The story concerns 5-years-old Lorenzo, suffering mightily from an apparently incurable and degenerative brain illness called ALD. His parents, an economist and a linguist, refuse to accept the received wisdom that there is no hope, and set about learning biochemistry to pursue a cure on their own. The film becomes an intriguing scientific mystery mixed with a story of pain, grief, and the strain on the two adults.

Relation with genetics: Lorenzo suffers from adrenoleukodystrophy (ADL) or also known as Schilder’s disease. It is a disease that mainly affects boys, since it has a pattern of inheritance linked to the X chromosome. It is in this chromosome where the ABCD1 gene is located, involved in the transport of very long chain fatty acids in peroxisomes (organelles involved in the metabolism of fatty acids).

It mainly affects the nervous system and the adrenal glands, which are small glands located in the upper part of each kidney. In this disorder, myelin deteriorates, the coating that isolates the nerves in the brain and spinal cord, reducing the ability of the nerves to transmit information to the brain. In addition, damage to the outer layer of the adrenal glands cause a shortage of certain hormones, resulting in weakness, weight loss, changes in the skin, vomiting and coma.

Video 4. Lorenzo’s oil trailer (Source: YouTube)

REFERENCES

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Cracking the genetic code

In the same way that Alan Turing decoded Enigma, the encryption machine used by the German army in World War II, several scientists managed to decipher the genetic code. The solution to this framework has allowed us to understand how cells work and make genetic manipulation possible.

INTRODUCTION

A code is a system of replacing the words in a message with other words or symbols, so that nobody can understand it unless they know the system. For example the genetic code.

Although it seems to be a lie, all living beings (except for some bacteria) biologically work in the same way. And it is that Jacques Monod already said, everything that is verified as true for E. coli must also be true for elephants.

From the cells of the blue whale, the largest animal on the planet, to the cells of a hummingbird, passing through humans, are the same. This is thanks to the genetic code, which allows the information of each gene to be transmitted to the proteins, the executors of this information.

This flow of information was named by Francis Crick, in 1958, as the central dogma of molecular biology (Figure 1). In it he claimed that information flows from DNA to RNA, and then from RNA to proteins. This is how genetic information is transmitted and expressed unidirectionally. However, later modifications were added. Crick claimed that only DNA can be duplicated and transcribed to RNA. However, it has been seen that the replication of its RNA also occurs in viruses and that it can perform a reverse transcription to generate DNA again.

main-qimg-eee77f2b58be05c964ce0c04756f2cfb
Figure 1. Central dogma of molecular biology. Red arrows: Francis Crick’s way. Grey arrows: later modifications (Source: Quora)

THREE LANGUAGES OF CELLS

Inside the cells three different languages ​​are spoken, but they can be related through the genetic code.

The one we already know is the language of deoxyribonucleic acid (DNA), wound in a double chain and composed of 4 letters that correspond to the nitrogenous bases: adenine (A), thymine (T), cytosine (C) and guanine (G).

Another language very similar to the latter is that of RNA. It differs from DNA mainly in three aspects: (i) it is composed of a single chain instead of being double-stranded, (ii) its sugars are ribose instead of deoxyribose (hence the name of ribonucleic acid) and (iii) it contains the base uracil (U) instead of T. Neither the change of sugar nor the substitution of U by T alters the pairing with base A, so that RNA synthesis can be performed directly on a DNA template.

The last language that remains for us to know is that of proteins, formed by 20 amino acids. The amino acids constitute each and every one of the proteins of any living organism. The order of the amino acids that form the chain of the protein determines its function (Figure 2).

aminoacids
Figure 2. Table of 20 amino acids (Source: Compound Interest)

THE GENETIC CODE

As we have been saying, the genetic code is the rules that follow the nucleotide sequence of a gene, through the RNA intermediary, to be translated into an amino acid sequence of a protein. There are several types of RNA, but the one that interests us is the messenger RNA (mRNA), essential in the transcription process.
The cells decode the RNA by reading its nucleotides in groups of three (Figure 3). Since mRNA is a polymer of four different nucleotides, there are 64 possible combinations of three nucleotides (43). This brings us to one of its characteristics: it is degenerate. This means that there are several triplets for the same amino acid (synonymous codons). For example, proline is coded by the triplets CCU, CCC, CCA and CCG.

genetic_code_med
Figure 3. The genetic code with the table of 20 amino acids (Source: BioNinja)

The genetic code is not ambiguous since each triplet has its own meaning. All triplets make sense, either encode a particular amino acid or indicate read completion. Most amino acids are encoded by at least two codons. Methionine and tryptophan are the only amino acids that are codified only by a codon. But each codon codes only for an amino acid or stop sign. In addition, it is unidirectional, all triplets are read in the 5′-3′ direction.
The AUG codon serves as the start codon at which translation begins. There is only one start codon that codes for the amino acid methionine, while there are three stop codons (UAA, UAG and UGA). These codons cause the polypeptide to be released from the ribosome, where the translation occurs.
The position of the start codon determines the point where translation of the mRNA and its reading frame will begin. This last point is important because the same nucleotide sequence can encode completely different polypeptides depending on the frame in which it is read (Figure 4). However, only one of the three reading patterns of a mRNA encodes the correct protein. The displacement in the reading frame causes the message no longer to make sense.

Marco de Lectura
Figure 4. Possible frameshifts (Source: marcoregalia.com)

 

As we said at the beginning, one of the main characteristics of the genetic code is that it is universal, since almost all living beings use it (with the exception of some bacteria). This is important because a genetic code shared by such diverse organisms provides important evidence of a common origin of life on Earth. The species of the Earth of today probably evolved from an ancestral organism in which the genetic code was already present. Because it is essential for cellular function, it should tend to remain unchanged in the species through the generations. This type of evolutionary process can explain the remarkable similarity of the genetic code in present organisms.

Although the human being itself continues to be an enigma for science, the revolution of the deciphering of the genetic code has allowed us to delve into the functioning of our body, specifically that of our cells, and cross borders to genetic manipulation.

 

REFERENCES

  • Alberts, B. et al. Biología molecular de la célula (2010). Editorial Omega, 5a edición
  • Cooper, G.M., Hausman R.E. La Célula (2009). Editorial Marbán, 5a edición
  • Gotta Love Cells
  • BioNinja
  • Main picture: eldiario.es

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Prions: special proteins

Do you remember mad cow disease? Some years ago it caused media hype because this illness, which affected animals, affected people too. Then, it was discovered that prions were the cause. So, I will discuss what prions are and the diseases that they produce.

WHAT ARE PRIONS?

Prions are proteins, but with different characteristics. Proteins are molecules formed by amino acids, which are bound by peptide bonds. All proteins are composed by carbon, hydrogen, oxygen and nitrogen. They are localized in all cells of the body and they participate in all biological processes that are produced. While DNA carries genetic information of the cell, proteins execute the work led by this information.

Proteins are the most varied macromolecules. In each cell there are miles of different proteins, with an extended range of functions. Between them: to be structural components of cells and tissues, to act in the transport and storage of little molecules, to transmit information between cells and to proportionate a defence in front of an infection. However, the main function is to act as enzymes, which catalyse most of chemical reactions in biological systems.

Prions are proteins with pathogenic and infectious characteristics (Video 1). They are not virus nor alive organisms; they are proteins without nucleic acid, it means, without DNA. They are localized in the surface of the central nervous system, especially in neurons; although they are also located in other body tissues of adult animals. Significant levels have been detected in the heart and skeletal muscle, and to a lesser extent in other organs except the liver and pancreas.

Video 1. What are prions? (Source: YouTube)

THE CELLULAR PRION PROTEIN

There is a change in the configuration of the cellular prion protein PrPc (Figure 1) in the diseases caused by prions. This protein has a protector role for cells and helps them respond in front of lack oxygen. The consequence of prions on this protein is the alteration of its functionality, producing a protein PrPSc with altered configuration. However, both configurations have the same sequence of amino acids. The secret of the different behaviour is the wrong folding, it means, the wrong conformation.

prpc prpsc
Figure 1. Left: normal protein (PrPc). Right: protein with altered configuration (PrPSc)(Source: Searching for the Mind with Jon Lieff, M. D.)

PRION DISEASES

Prion diseases are neurodegenerative processes produced by abnormal metabolism of a prion protein. These affect humans and animals and have a fatal clinical evolution, with the death as final.

There are various prion diseases, however, symptoms and clinical features are shared (Table 1). Some of these clinical features are dementia, ataxia (discoordination in the movement of body), insomnia, paraplegia and abnormal behaviors. The brain acquires a spongiform aspect, it means, an aspect like a sponge. This is due to accumulation of prion proteins in neurons, where amyloid plaques are formed.

Amyloid plaques are caused by accumulation of amyloid peptide, an essential protein for cellular function of the body. This accumulation in the brain can generate toxicity for nervous cells.

Until today there is any treatment to cure, improve or control symptoms and signs of these diseases.

Tabla 1. Prion diseases and its clinical features (Source: Rubio, T. & Verdecia, M. Enfermedades priónicas. MEDISAN 2009; 13(1))

DISEASE SYMPTOMS AGE DURATION
Creutzfeldt-Jakob

Dementia

Ataxia

< 60 years

1 month – 10 years

(average 1 year)

Kuru

Ataxia

Dementia

40 years (29-60) 3 month – 1 year
Fatal familial insomnia

Insomnia

No autonomy

Ataxia

Dementia

45 years (35-55) 1 year

CREUTZFELDT-JAKOB SYNDROME

During 18th century, European farmers described a neurodegenerative disease that affected sheep and goats, called scrapie. The affected animals will compulsively scrape off their fleeces against rocks, trees, or fences. Furthermore, its brain looked like a sponge. So, this is the birth of the word spongiform.

However, until 20th century, in 1920, neurologists Creutzfeldt and Jakob described the first cases of spongiform encephalopathy in humans (Figure 2) and called the disease with their names.

creutzfeldt-jakob-disease-cjd.jpg
Figure 2. Comparison of two brains: a brain affected by Creutzfeldt-Jakob disease (left) and a healthy brain (right) (Source: Health & Medical Information)

In this disease, there is a loss of memory, lack coordination and damage of mental abilities. The balance problems are common and, sometimes, are manifested in the beginning. Many patients lose autonomy and are unable to take care of themselves in later stages of the disease.

Due to prion nature of the disease, any symptom is possible and it depends on the area of the brain that is being affected.

KURU

It is a rare disease, localized in New Guinea. The main risk factor to suffer the disease is the intake of brain human tissue, which can contain infectious particles.

It is the reason that it is associated with people who practice a form of cannibalism, in which the brains of dead people are eaten as part of a funeral ritual. Although this practice ended in 1960, cases of kuru have been reported years later.

FATAL FAMILIAL INSOMNIA

It is a familial and inherited disease, which people affected suffer progressive insomnia. The human brain needs to sleep and rest, so permanent insomnia (there is not treatment with drugs) causes the death of patients.

Insomnia is due to a permanent and irreversible alteration of the sleep-wake cycle, which is characterized by the inability of the patient to develop REM and non-REM sleep.

REFERENCES

  • Alberts, B. et al. (2016). Biología molecular de la célula. Barcelona: Omega.
  • Rubio, T. & Verdecia, M. Enfermedades priónicas. MEDISAN 2009; 13(1)
  • Wemheuer, W. M. et al. Similarities between Forms of Sheep Scrapie and Creutzfeldt-Jakob Disease Are Encoded by Distinct Prion Types. Am J Pathol. 2009; 175(6): 2566–2573
  • Manual MSD
  • Early Clinical Trial
  • MedlinePlus
  • Main picture: Canal44

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