Rare diseases are characterized by their low prevalence, which is defined as the number of people in a specific group who suffer from a certain disease at a specific time.
While there is no single definition for the term “rare disease,” they are all based on the frequency with which they occur, and in some cases include the severity of symptoms or availability of treatment.
- In the United States, rare diseases are defined as any disease or condition affecting fewer than 200,000 people in the United States, which is equivalent to a prevalence of 0.2% in the United States.
- In Japan, a rare disease is defined as a disease affecting fewer than 50,000 people, which is equivalent to a prevalence of less than 0.04%.
- The European Union defines a rare disease as one with a prevalence of less than 0.05%, i.e. diseases that affect less than 0.05% of the population and are life-threatening or chronically debilitating.
However, despite their low prevalence, according to the Orphanet database, there are more than 7,000 catalogued rare diseases, which together affect around 10% of the population, making them a public health problem.
These diseases are also known as “orphan diseases”, due to the lack of interest in research or clinical trials.
Diagnosis of rare diseases
Rare diseases are often chronic and debilitating conditions, present a great phenotypic heterogeneity and are mostly unknown to healthcare professionals, which makes diagnosis complex and lengthy (between 5 and 10 years on average). In fact, figures indicate that 40% of those affected by rare diseases don’t have a diagnosis, which has a great impact on the quality of life of sufferers, since the lack of knowledge of the population can lead them to face stigma and psychological problems due to the scarcity of understanding and support. For this reason, research into rare diseases is essential, and research programs such as the UDN (Undiagnosed Diseases Network) have been developed in an attempt to respond to the high number of undiagnosed rare disease queries.
Genetics and rare diseases
About 80% of these diseases have a Mendelian inheritance pattern, i.e. they are the result of mutations in a single gene. This type of inheritance is classified as dominant, recessive or sex chromosome linked. If you want to learn more about the different types of inheritance, you can do so in our blog on hereditary diseases. Therefore, in the fight against rare diseases, genetics plays an essential role, as it helps to identify the underlying causes of the diseases and to develop specific treatments.
Within rare diseases we can also distinguish between those that develop in adulthood and the so-called congenital defects, which are those pathologies that occur during embryonic development. Most cases of rare diseases appear in pediatric age, due to the high frequency of a genetic origin and congenital anomalies, however, the prevalence is higher in adults, due to the high mortality of some severe childhood diseases.
As an example of a rare disease, we will talk about type 1 nephrotic syndrome, also known as Finnish type nephrotic syndrome. This syndrome starts prenatally, and its most important clinical signs and symptoms are as follows:
- Proteinuria. This is an excessive excretion of protein in the urine, which in prenatal stages is detected in amniotic fluid and, to a lesser extent, in maternal blood.
- Hypoalbuminemia. Albumin is the most abundant protein in the blood, whose function is the transport of lipid molecules and the maintenance of oncotic pressure, essential for the correct distribution of body fluids between the intravascular and extravascular compartments. Due to the increased excretion of protein in the urine, this syndrome is associated with a pathological decrease in blood albumin.
- Edema. Another consequence of this syndrome is fluid retention, which may lead to edema in different parts of the body.
- Hyperlipidemia. Due to massive protein loss and albumin loss, the liver produces more proteins, including low-density lipoproteins (LDL) and very low-density lipoproteins (VLDL), which are the main transporters of cholesterol and triglycerides in the body. These lipoproteins are released into the bloodstream and cause increased levels of cholesterol and triglycerides in the blood, which is known as hyperlipidemia.
This disease is caused by mutations in the NPHS1 gene, a gene located on the long arm of chromosome 19, which codes for the nephrin protein. This protein is expressed exclusively in podocytes, which are the epithelial cells that form the visceral layer of Bowman’s capsule (glomerular capsule) in the kidney, whose function is essential in glomerular filtration. To date, more than 200 mutations have been described in the NPHS1 gene.
The inheritance of this rare disease is autosomal recessive, i.e. two mutated alleles are necessary for the disease to develop. When a person has only one mutated allele of the gene, they may be completely healthy, but is known as a carrier, as it can be transmitted to offspring. In this fact lies the importance of knowing the carrier status, both for this and other autosomal recessive diseases.
For family planning purposes, when considering having biological children, it can be very helpful to know if you or your partner are carriers of a rare disease and the probability that your offspring will suffer from this disease. This is especially useful if any member of your family suffers from this type of disease.
24Genetics and rare diseases
At 24Genetics we offer you our DNA health test with more than 200 diseases, including a large number of rare hereditary diseases. In addition, you will find complex diseases and various mutations that increase susceptibility to cancer. Click here to access an example of our genetic health report and check all the personalized information you can get.
Bibliography
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Orphanet: Congenital nephrotic syndrome, Finnish type. Retrieved February 20, 2023, from https://www.orpha.net/consor/cgi-bin/OC_Exp.php?Lng=ES&Expert=839
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