Index
The blueprint for the human body is ‘written’ in DNA, and the ‘words’ are called genes.
An abnormality causes genetic syndromes in the genome, such as an extra piece or copy of a chromosome or a mistake in the DNA.
An error in DNA can change the way a gene works. This can lead to a pattern of abnormalities called a genetic syndrome.
These syndromes and the impact of their associated abnormalities can vary greatly and often require additional medical attention at the delivery time.
A syndrome is a disease or disorder with more than one identifying characteristic or symptom.
Each genetic syndrome will have many typical characteristics, depending on which aspects of development are affected by the abnormal genes or chromosomes.
A child can be born with obvious body deformities, abnormal organ function (e.g., heart, brain, intestine, or kidney), or neurological problems.
Genetic syndromes are defined based on a genetic and biological condition already present before birth.
However, many genetic syndromes take effect only once the baby is born and begins to feed and grow.
These babies may look and act completely normal at birth but develop problems later in life.
Although some genetic diseases occur at birth or in early childhood, such as cystic fibrosis or Duchenne muscular dystrophy.
Other syndromes become visible only after several years with no apparent symptoms.
This is the case of Huntington’s disease, whose first symptoms appear around 40.
Before that, the person is a carrier of the disease but has no signs or symptoms: it is called an asymptomatic person.
DNA, the heart of genetic diseases
The human body comprises billions of “cells,” each with a nucleus. This core contains all of our genetic information.
Chromosomes consist of DNA carrying our genes. It is a type of detailed plan of our body and instructions that allow its correct functioning.
By reading this “plan,” the body produces everything it needs to grow, develop, and defend itself from external aggression.
However, these instructions sometimes contain abnormalities that may be responsible for genetic diseases:
Innate or acquired genetic syndrome
A genetic disease is not necessarily inherited.
Some of them are innate (because they are inherited from the parents); this is the case of autosomal dominant, recessive autonomic, or X chromosome-related diseases.
This type of disease is transmittable from one generation to another.
However, other syndromes are acquired and may be related to a genetic abnormality that occurred accidentally during the formation of gametes (sperm, oocytes), and this mutation is transmissible to offspring.
They can also occur during life and affect only a few cells within the body, like cancer.
In this case, the mutation is not communicable and has not been inherited.
Causes of genetic syndromes
Each cell in the human body contains 23 pairs of chromosomes. One set is inherited from the father’s partner, and one set from the mother.
Each chromosome comprises many genes, approximately 2,000 on each chromosome, for a total of 50,000 genes in each cell.
These genes make all the proteins in the body, which promote development and growth and perform all body functions.
When one or more of these genes or chromosomes is missing or mutated, or if there are extra chromosomes, the proteins may not be produced, incorrectly produced, or overproduced.
These situations can cause abnormal growth and development and result in a genetic syndrome.
Sometimes these abnormal genes or chromosomes are passed down from a parent, and sometimes the changes happen spontaneously for no reason.
Genetic syndromes are diverse, and to know the risk of the genetic disorder in pregnancy and its recurrence in future generations, it is necessary to know the mechanism of its inheritance.
Risk factors for a baby being born with a defect due to a genetic abnormality may increase when the following antecedents are present:
- You already have a child with a genetic syndrome.
- There is a family history with a genetic syndrome.
- One of the parents has a chromosomal abnormality.
- The fetus has abnormalities observable on ultrasound.
Some disorders are more common in certain ethnic groups, such as:
- Anemias such as sickle cell disease and thalassemia are more common in people from the Mediterranean, the Middle East, and Africa, including African Americans, the Caribbean, Hispanics, and Asians.
- Genetic Diseases of Ashkenazi Jews are a group of diseases most commonly seen in people of Eastern European Jewish descent.
- Tay-Sachs disease is more common in French Canadians.
Until now, genetic syndromes have been classified according to the type of abnormality in:
- Chromosomal abnormalities.
- Single gene defects.
- Multifactorial problems.
- Teratogenic problems.
Chromosomal abnormalities
Chromosomal abnormalities can be numerical; they are those in which there is a net gain (duplication) or loss (deletion) of genetic material, or structural, where there is breakage and subsequent reunion of the chromosome arms.
In structural abnormalities, rearrangement occurs, including:
- Inversions are caused by a two-break event and end-to-end inversion of the intermediate chromosome segment.
- Translocations result from the exchange of chromosomal segments between two or more chromosomes.
- Insertions occur when a chromosome segment moves and inserts into a new region of the same chromosome or moves to another chromosome.
Single gene defects
Autosomal diseases affect chromosomes and can be:
Dominant
In an autosomal dominant disease, the person inherits one regular copy of the gene from one parent and one abnormal copy of the same gene from the other parent.
If the father has the disorder, the baby has a 50% chance of inheriting it.
The abnormal copy of the gene is dominant compared to the other: it is the one that is expressed.
This means that the disease affects any carrier of the mutated gene, even if the second copy of the gene is healthy.
Recessive
In the case of an autosomal recessive disease. An abnormality is only expressed when both parents have abnormal genes.
If both parents are carriers, the baby has a 25 percent chance of having the disorder.
If the person inherits a single defective copy and the other is normal, the usual copy will compensate for the defect of the abnormal copy; in most cases: these people are called ‘healthy carriers’ and do not develop the syndrome, but their sons inherit it.
Diseases related to the X chromosome
The X chromosome contains several genes essential for growth and development.
In men, the Y chromosome is much smaller and contains few genes. If one of the genes on the X chromosome is mutated, there is no ordinary copy of the gene to make up for its defect.
In the case of women, if a gene located on one of the X chromosomes is mutated, the normal gene on the other X chromosome can compensate for its defect.
In this case, the woman is a healthy carrier of X-linked disease. In some cases, women have mild signs of the disease.
The same disease can be expressed in different ways, with different symptoms, even among members of the same family – this is called the variable expressiveness of a disease.
Also, the symptoms of a disease can change during life.
Multifactorial problems
Some congenital disabilities do not follow a single gene or pattern of chromosomal abnormality. They can be due to various problems or the combined effect of genes and the environment.
It is difficult to predict the inheritance of abnormalities caused by multiple factors.
Teratogenic problems
Certain substances are known to cause abnormalities in babies.
Many congenital disabilities occur when the fetus is exposed to teratogens (substances that cause abnormalities) during the first trimester of pregnancy when organs are formed.
Some known teratogens include the following:
- Some medications
- The alcohol.
- Exposure to high levels of radiation.
- Certain infections (such as rubella).
Manifestations (symptoms) of genetic syndromes
There are a large number and variety of genetic disorders and syndromes diagnosed in children.
Some genetic disorders range from the addition or loss of an entire chromosome in each cell to the loss of part of a chromosome to the microdeletion of several contiguous genes within a chromosome.
These deletions can cause mild to very severe developmental disabilities.
Syndromes can present characteristic symptom patterns such as facial features, limb length, organ abnormalities, abnormal muscle tone, and hypermobile joints.
Examples of these syndromes are Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), Trisomy 13 (Patau syndrome), and Turner syndrome.
Genetic disorders that result from a single gene abnormality can include cystic fibrosis, Prader-Willi syndrome, phenylketonuria, Taylor Sachs disease, fragile X syndrome, Williams syndrome, and diabetes.
Children with a genetic syndrome often have related health problems, including cardiovascular abnormalities, obesity, high respiratory infections, thyroid deficiencies, visual impairments, sensory processing disorders, or gastrointestinal problems.
Children are likely at risk for developmental delays in many areas of function, which may include: planning motor skills, language and cognitive skills, self-care skills (including feeding difficulties), social engagement, and intercourse—companions, among others.
Genetic disease is not due exclusively to genetic changes in the cell’s nucleus.
Mitochondria (cell organelles located in the cytoplasm) have their genetic inheritance, including several genes.
Its mutation generally causes functional limitations of the organs that consume energy, such as the central nervous system, skeletal and cardiac muscles, liver, or kidneys.
An example is Leber’s optic neuropathy, which causes loss of vision.
Diagnosis
Genetic (or inherited) diseases are due to changes (mutations) in chromosomes or DNA (a molecule that carries genetic information).
Numerous structural abnormalities supporting genetic inheritance in the cell’s nucleus belong to chromosome mutations.
We usually have 46 chromosomes or 23 pairs. Each copy of a pair comes from one from the mother, the other from the father.
Twenty-two pairs of chromosomes are similar in males and females. The sex chromosomes, or gonosomes, are the 23rd pair.
In the diagnosis, it is necessary to take a careful family history of both parents to see if any other problem can be found.
Currently, there is a method of detecting carriers for certain common diseases, looking for a copy of a faulty gene even in healthy people.
Using carrier detection, some syndromes can be diagnosed, such as:
- Cystic fibrosis is a disease that primarily affects the lungs, but it can also cause problems with digestion. It can be mild to very severe.
- Spinal muscular atrophy causes muscle weakness that worsens over time.
- Fragile X syndrome can cause intellectual disability (in the past, this was called mental retardation). Additionally, women who are carriers are also at risk for specific problems.
Usually, the first step is a blood sample from the mother. If you are found to be a carrier of one of these syndromes, the next step is to see if the father is also a carrier of the same syndrome.
For Fragile X syndrome, no blood needs to be drawn from the father.
By analyzing the DNA present in a pregnant woman’s blood, non-invasive prenatal diagnostic techniques now make it possible to detect with great certainty if the embryo has an abnormality related to the number of chromosomes as early as the 10th week of pregnancy.
Current chromosomal diagnoses have been decisively complemented by molecular genetic techniques that allow the highly reliable identification of structural abnormalities that could not, or only with difficulty, be diagnosed by light microscopy.
Treatment
Studying the impact of genetic variation on cognitive, social, emotional, and behavioral development can help to understand and identify early markers of developmental risk and the factors that mediate this risk.
Although for many genetic syndromes today, there is no cure to eradicate them, there are treatments that help improve the quality of life and improve some symptoms.
