Mental retardation and developmental delay is a type of severe inability in the mental performance of adaptive behaviors of the individual, which imposes a heavy burden on the family and health systems of the society, and according to the statistics of the World Health Organization, its prevalence in developed societies is 1% to 3% and in developing societies it is more than This is the amount. Since the prenatal diagnosis of this group of diseases in order to prevent their recurrence in the family depends on the diagnosis of their genetic causes, the services of a comprehensive step-by-step approach, which includes the following examinations, are offered to patients in this center:
- karyotype
- Review of fragile X syndrome
- Investigating Subtelomeric Rearrangements (Subtelomeric Rearrangements)
- Investigating Microdeletion/Microduplication Syndromes
Karyotype:
In this test, the chromosomal status of blood cells is checked in terms of number and structure. This test is performed on a peripheral blood sample.
Fragile X syndrome:
One of the tests that has been used since the 80s to identify Fragile X is examining the fragility of chromosomes by cytogenetic method. Of course, this test is not responsive in all cases and cannot identify all patients. Molecular tests or advanced cytogenetic techniques such as FISH are complementary to the examination of this disease.
In the molecular method, carrying out pre-mutation and full mutation carriers are identified by performing PCR test using specific primer and checking the number of CGG trinucleotide repeats in FMR1 gene. In cases where the number of CGG trinucleotide repeats is very high, the PCR test cannot identify the defective gene. In such cases, Southern blotting or Expended Long PCR can be used.
In addition, the technique of checking gene continuity is also used to identify the carriers. In this technique, pathogenic gene carriers are identified using molecular markers. Among these markers, FRAXC2, FRAXA, FRAXC1 and DXS297 STRs can be mentioned.
Prenatal diagnosis of the disease is also carried out in families at risk. In these cases, before the genetic examination of the embryo, its gender is determined. Considering the dependence on X and at the same time the disease is recessive, there is no need for another genetic examination if the fetus is a girl. But in the boy, genetic testing is done.
Assessment of Subtelomeric Rearrangements
10% to 15% of the causes of mental retardation are caused by deletion or duplication in the subtelomeric regions of chromosomes. Due to technical limitations, karyotype cannot detect this group of chromosomal abnormalities, and in people with mental retardation or developmental delay who have a normal karyotype, this group of abnormalities should also be investigated. Modern molecular cytogenetic methods have made it possible to check the rearrangements of subtelomeric regions of all chromosomes in one test with less time and cost compared to FISH technique. In addition, considering that a group of chromosomal abnormalities of the subtelomeric regions are hereditary and the risk of their recurrence in subsequent pregnancies is also high, these techniques have provided the possibility of prenatal diagnosis of these abnormalities on placental villi or amniotic fluid samples.
Assessment of Microdeletion/Microduplication Syndromes
Since the standard chromosomal examination (karyotype) is not able to detect syndromes of microchromosomal deletions, and on the other hand, modern molecular cytogenetic techniques are able to examine the DNA copy number of a large number of genomic regions in one test, it is possible to examine this group of chromosomal abnormalities at the same time. as follows) in affected people, it plays an effective role in diagnosing this group of diseases. In addition, considering that it is not possible to distinguish most of these syndromes based on clinical and paraclinical symptoms alone, screening a significant number of the most common syndromes in one test is helpful in diagnosing the causes of this group of diseases with much less time and cost. The syndromes that can be checked with this method are:
- 1p36 Deletion Syndrome
- Cri Du Chat Syndrome, 5p 15
- MECP2/Xq28 Duplication
- 2p16 Microdeletion
- Digeorage Syndrome 22q11
- Rubinstein-Taybi Syndrome
- 3q29 Microdeletion
- Digiorage Region 2, 10p15
- Smith-Magenis Syndrome
- 9q22.3 Microdeletion
- Langer-Giedion Syndrome, 8q
- Sotos Syndrome 5q35.3
- 15q24 Deletion Syndrome
- Miller-Dieker Syndrome, 17p
- WAGR Syndrome
- 17q21 Microdeletion
- NF1 Microdeletion Syndrome
- Williams syndrome
- 22q13/phelan-McDermid
- Prader- Will/ Angelman
- Wolf-Hirschhorn4p 16.3
It should be noted that in a group of the above abnormalities, which are hereditary, the risk of recurrence in subsequent pregnancies is high, and therefore prenatal diagnosis of these diseases on placental villi or amniotic fluid samples is recommended to prevent the recurrence of the disease in the family. All the mentioned cases, including post-birth and prenatal diagnosis of the mentioned anomalies, can be done in this center.