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Heritable disorders


What is a heritable disorder?  How is it inherited?

The majority of genetic disorders are inherited from one or both parents. Genetic disorders occur because of a defect in our DNA, in one or both copies of a particular gene (we all carry one paternal and one maternal copy for each one of our genes). A DNA defect in one copy of the gene, either paternal or maternal is rarely expressed physically, i.e. there are no symptoms of a disease, because the presence of a second, intact copy is usually sufficient to ensure a normal function for this gene. 

 

Therefore, most genetic disorders are entirely harmless in an individual, known as the carrier, who carries one defective and one normal copy of the gene. However, if two defective genes are passed on by two parent-carriers, then they will cause the disease in the child. This form of genetic inheritance is known as recessive: the majority of genetic traits falls under this category, for example cystic fibrosis, a- and b- thalassaemia, spinal muscular atrophy and thousands of others.

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Typical inheritance pattern of a recessive disorder, in this example cystic fibrosis.

 
A smaller proportion of genetic traits are dominant – this means that the inheritance of one defective gene from either parent suffices to cause the disease. Examples of these are Huntington’s chorea, dwarfism and others.

 

Finally another group of genetic traits are inherited through the sex chromosome X and, therefore genetic defects are unmasked at a higher frequency in boys (who only carry one copy of chromosome X), than girls (who carry two copies of chromosome X).  These are known as X-linked disorders, and examples of these include colour-blindness, Duchenne muscular dystrophy, haemophilia and others.

Tests
In this section we present some of the tests available at Genomedica. If a particular test is not listed here please contact the laboratory, as we have many links with external laboratories for more specialized tests. We also collaborate closely with GENDIA, Holland (www.gendia.net) who act as a referral lab for many of the rarer genetic disorders.

 

All the tests listed below are also available prenatally.

  1. Cystic fibrosis
  2. a- and b-thalassaemia
  3. Sickle cell anaemia
  4. Duchenne muscular dystrophy
  5. Spinal muscular atrophy
  6. Achondroplasia
  7. Congenital deafness
  8. FRAX
  9. Familial breast cancer genes BRCA1 and BRCA2

1. Cystic fibrosis
Cystic fibrosis is the most common autosomal genetic disorder in Caucasians, affecting approximately 1 in 2,500 live births, although the estimated frequency varies from region to region. In Greece, cystic fibrosis constitutes the second most common genetic disorder, after β-thalassaemia and related haemoglobinopathies, with up to 5% of the population being carriers.

The disease affects primarily the lungs and the digestive system and can cause male infertility (
http://hcd2.bupa.co.uk/fact_sheets/html/Cystic_fibrosis.html) for more information on cystic fibrosis).
The disease is caused by a defect in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Several mutations of the gene are known, with ΔF508 being the most common. Carriers of CFTR mutations do not express any symptoms of the disease - only individuals who have inherited two copies of the mutant gene are affected by cystic fibrosis.  Couples, where both partners are carriers of a mutation, stand a 1 in 4 risk of having a child with cystic ibrosis.


We provide the following screening tests:

  • ΔF508, the most common mutation found in the European population, including Greece, which comprises 53.4% of all mutations.
  • CF29: which includes 29 of the most common mutations, covering 74% of all the known mutation in cystic fibrosis.
  • Full sequencing of the gene which covers 99% of all known mutations.


The results are available in 7 days for ΔF508 and CF29, or 30 days for the 99% screen (pdf please see list of tests ). This test is also available prenatally.


2. α- and β-thalassaemia
Both alpha and beta thalassaemias are genetic diseases that affect the amount of haemoglobin that is produced by the body. (http://www.hbregistry.org.uk/information/thalassaemia.html for more information on haemoglobinopathies).

Their frequency is highest in the Eastern Mediterranean region, southern Italy, Middle East, North Africa and parts of India, southern China and SE Asia. β-thalassaemia is an characterized by reduced synthesis of the β-globin chain due to mutations in the β-globin gene.

As a consequence, α-globin chains are in excess and form tetramers that are insoluble and precipitate within red blood cells, leading to their premature destruction in the bone marrow and marked trapping in the spleen. Individuals affected with β –thalassaemia are reliant on frequent blood transfusions and iron chelation treatment  throughout their lifetime. Lifespan is expected to be markedly shortened. Carriers of α- or β –thalassaemia are generally asymptomatic. Couples where both partners are carriers of the mutation have a 1 in 4 risk of having an affected child.


We screen concurrently for alpha and beta thalassaemia mutations, as well as sickle cell anaemia. The results are available in 5 days and are also available prenatally (pdf see our lists of tests ).


3. Sickle cell anaemia
Sickle cell anaemia or drepanocytosis is a genetic disease caused by a mutation in the beta chain of haemoglobin, which results in the production of abnormal red blood cells, with a characteristic sickle shape. Sickle-cells tend to form clumps inhibiting proper blood flow in the vessels and organs, eventually resulting in organ damage and serious infections.


The highest incidence of sickle cell anaemia occurs in populations of sub-Saharan Africa, although it can also be seen at a lesser frequency in the Eastern Mediterranean region. It is also common in Afro-Carribean communities in Europe, the USA and the Caribbean.


We screen for the HbS mutation in our laboratory. The results of the screen are available in 5 days (pdf please consult our lists of tests ).

4. Duchenne muscular dystrophy/Becker muscular dystrophy
Duchenne muscular dystrophy (DMD) and its milder form, Becker muscular dystrophy, are both caused by a genetic defect in the dystrophin gene on chromosome Xp21, that impedes the production of a muscle protein known as dystrophin, an important structural component of muscle tissue. Lack of dystrophin causes gradual muscle weakening and wastage.DMD is an X-linked disorder and therefore much more prevalent in boys than girls. For more information go to http://www.muscular-dystrophy.org/about_muscular_dystrophy/conditions/97_duchenne_muscular_dystrophy


At Genomedica, we screen for DMD using MLPA methodology for partial deletions or more rarely duplications of the dystrophin gene. Results are available in 10-15 days – pdf please consult our list of tests.

5. Spinal muscular atrophy (SMA)
SMA is a group of autosomal recessive neuromuscular disorders characterized by degeneration of the anterior horn cells of the spinal cord, leading to symmetrical muscle weakness and atrophy. It has a highly variable phenotype, ranging in severity from being fatal at infancy to just experiencing muscle weakness but an otherwise normal adult life. SMA is usually subdivided into three clinical groups, SMA I, II and III depending on the time of onset and the severity of the disease.

 

For more information please refer to http://www.nlm.nih.gov/medlineplus/spinalmuscularatrophy.html
The genes responsible for SMA are SMN1 and SMN2 – two defective copies of SMN1 result in the expression of the SMA phenotype, while SMN2 determines the severity of the disease.  
Results are available in 7-15 days. pdf Please refer to our complete list of tests .

6. Congenital deafness
Hearing loss is the most common birth defect, affecting approx. 1 in 500 newborn. Its causes are very heterogeneous and include both genetic and non-genetic ones. Congenital deafness, i.e. hearing loss that is present at birth, can be one of the features in a number of genetic syndromes, such as Treacher-Collins, Usher or Charcot-Marie-Tooth, or it can occur as an isolated, non-syndromic condition. It is estimated that congenital deafness is caused by genetic factors in at least 2/3 of cases in the western world.

 

One of the major causes of autosomal recessive, non-syndromic hearing loss are mutations in the GJB2 gene. The GJB2 gene encodes the gap junction protein connexin-26 (CX26). It is estimated that over than 200 different mutations have been identified in the gene. The 35delG mutation accounts for about 70% of GJB2 mutations in many populations, with carrier frequencies up to 1 in 28 in the Mediterranean region.


At Genomedica we screen for mutations of the GJB2 gene. A number of tests are available:

  • Screening for the mutation 35delG, the most common mutation of the connexin-26 gene, which accounts for 70% of all mutations. Results are available in 5 days.
  • Sequencing of the GJB2 gene (99% of all mutations). Results are available in 30 days.
  • Screening of 6 genes responsible for congenital deafness by array-CGH. Results available in 30 days

pdf Please refer to our complete list of tests.

7. Achondroplasia/hypochondroplasia
Achondroplasia or dwarfism is an autosomal dominant genetic disorder that results in shortened limbs. Approximately 75% of new cases of achondroplasia are detected as novel mutations in the FGFR3 gene (fibroblast growth factor receptor gene), resulting in abnormal cartilage formation. A different mutation in the FGFR3 gene causes the milder form of achondroplasia, known as hypochondroplasia, a condition also characterized by short limbs (micromelia), short stature and disproportionately large head compared to the rest of the body.


We screen for the mutations 1138G>C and 1138G>A in the FGFR3 gene. Results are available in 5 days. pdf Please consult our complete list of tests.

8. Fragile X syndrome
Fragile X or Martin-Bell syndrome is a genetic disorder that is most frequently seen in males, as it is associated with a complex X-linked form of inheritance. The main features are mental retardation, an elongated face with poor muscle tone and macro-orchidism (large testes) in men. Females tend to show a milder form of the syndrome. Fragile X is caused by a mutation of the FMR1 gene on chromosome X, resulting in an expansion of the CGG trinucleotide repeat. Normal individuals have 5-55 copies of the CGG repeat, while affected individuals have >230. The increased copy number of the trinucleotide repeats causes methylation and non-expression of the FMR1 gene.
We screen for mutations of the FMR1 gene by MLPA methodology. Results are available in 10-15 days. pdf Please consult our complete list of tests .

9. Familial breast cancer BRCA1 and BRCA2
Mutations in the genes BRCA1 and BRCA2 were the first mutations identified to have an association with a higher risk of developing breast cancer. Although familial breast cancer accounts for only 5-10% of all breast cancers, women with BRCA1 or BRCA2 mutations have a 50-85% higher risk of developing breast cancer in their lifetime. For more information on familial breast cancer genes please refer t
o http://www.cancerhelp.org.uk/type/breast-cancer/about/risks/breast-cancer-genes. At Genomedica we screen for mutations of the BRCA1 and BRCA2 genes by performing sequencing of the entire gene and by MLPA.

 
   
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