Diagnosis of Genetic Diseases (With Diagram)

In this article we will discuss about the diagnosis of genetic diseases.

The most common and most successful test methods for prenatal diagnosis is the examination of amnion cells. Amniotic fluid is extracted by way of a suprapubic amniocente­sis. This is done by the penetration of the amnion sac above the symphysis with a hollow needle. The process is almost painless.

The timing of the test is usually determined by the necessity; after all, the earlier the test, the ear­lier the condition of the embryo will be known. Some geneticists suggest that the twelfth and twentieth week of pregnancy is the best time. But some others are agreed that the period between the 14th and 16th week is the best time.

The amount of fluid which is to be ex­tracted from the amnion generally lies between 5 ml. to 20 ml. Although, the amount of fluid extraction is actually dependent upon the avail­ability of the amniotic fluid and the stage of pregnancy. The test involves no danger for the prospective mother and the risk of inducing a miscarriage seems to be less than 1%.

After extraction of the fluid, it is centrifuged and the precipitated cells are cultured. The culturing of cells takes time, sometimes up to a few weeks. Apart from the testing for chromosomal anomalies by using the cultured amniotic cells, supernatant amniotic fluid is bio­chemically assayed to determine the biochemi­cal defect of the embryo, e.g., the determina­tion of fetoprotein (specially α₁ – fetoprotein) level in amniotic fluid. We are able to diagnose anencephaly and spina bifida. α₁ – fetoprotein usually present in small traces in the amniotic fluid if the foetus is normal, but it shows a mas­sive increase if the foetus is defected by anen­cephaly or spina bifida.

The amniocentesis allows us to diagnose different defects of the foetus prenatally which are originated due to chromosomal abnormalities, neural tube defect and biochemical disorders. By the use of amniocentesis we can also determine the sex of the foetus. The brief schematic representa­tion of amniocentesis is as follows (Fig. 19.1).

During recent years sampling the amniot­ic fluid by amniocentesis is done in conjunc­tion with solography for prenatal diagnosis of the genetic diseases. In some cases foetal blood sample is also taken to determine the diagnosis of certain diseases like thalassemia.

Remark:

Before going ahead with the test men­tioned above, patients must be informed about the possibilities and the dangers of the test methods. They should know that the first test may be unsuccessful and amniocentesis may have to be repeated.

It must also be made clear that a negative test result does not guar­antee a healthy child except for the one spe­cific defect that has been excluded. Again, it is important to recognise that, in the presence of unsuspected twins, incomplete or misleading results may be obtained.

The fol­lowing table gives some information regarding the genetic diseases for which pre­natal diagnosis has already been successfully carried out:

A. Disorders of Lipid Metabolism:

1. Fabry disease

2. Gaucher disease

3. Gangliosidosis (generalized type and juvenile type)

4. Krabbe disease

5. Refsuim disease

6. Wolman disease

7. Hyperlipoproteinemia

B. Disorders of Carbohydrate Metabo­lism:

1. Galactosemia

2. Glycogen storage disease

3. G-6-P-D deficiency

4. Fucosidosis

C. Amino Acid and Related Disorders:

1. Cystinosis

2. Congenital Hyperammonemia

3. Histidinemia

4. Hurler’s disease

5. Hunter’s disease

6. I-cell disease

7. Adrenogenital syndrome

8. Lesch Nyhan syndrome

9. Xeroderma pigmentosum

10. Sickle cell anemia

11. Myotonic muscular dystrophy

12. Thalassemia

13. Anencephaly, spina bifida.