In recent years, the demand for prenatal diagnosis of inherited genetic diseases has dramatically increased for several reasons:
1. Sampling techniques have become safer.
2. The number of diseases that can be diagnosed prenatally has increased due to the continuous identification of new disease-genes.
3. Analytical technologies have been developed allowing rapid and cost-effective diagnosis of the small amounts of DNA that can be obtained from prenatal sampling.
4. The improved knowledge among medical professionals of the opportunities offered by prenatal diagnosis has contributed to the increase in requests.
Moreover, alongside more widespread and well-known genetic diseases, such as cystic fibrosis or thalassemias, for which prenatal diagnosis has been routinely performed for several decades, there is now an increase in requests for prenatal diagnosis for rare diseases, such as those of amino acid metabolism, organic aciduria, and β-oxidation diseases, which are identified through the extensive screening.
Prenatal diagnosis, therefore, has become a valuable aid for the reproductive choices of many couples, provided that they have a range of complete, precise, and objective information that makes it possible to make informed choices. Therefore, the prenatal diagnosis must be accompanied by multidisciplinary pre-test and post-test counseling offered by professionals willing to give couples all the time they need. At the same time, it should be remembered that prenatal diagnosis is a medical act and, as such, should be used only when there are appropriate indications, without wanting to persist in “medicalizing” excessively the pregnancy that, in most cases, does not indicate the prenatal diagnosis. Also, because there are thousands of hereditary genetic diseases, only a small percentage can be diagnosed prenatally. Therefore, in many cases, couples who already have a child with severe genetic disease use prenatal diagnosis for hereditary genetic diseases, and usually the couples with the greatest desire to have a second child. In these couples, there is a high risk (already assessed in the couple through molecular analysis and multidisciplinary counseling) that a second affected child will be born.
From the methodological point of view, in almost all couples who perform prenatal diagnosis for hereditary genetic diseases, the mutation or mutations in the family are already known and are sought on DNA extracted from chorionic villi or amniocytes (possibly cultured). Before searching for pathogenic variants, however, it is necessary to analyze some polymorphisms located on different chromosomes to (1) confirm paternity from the fetal DNA sample since only parental mutations should be sought on the fetal DNA sample; (2) exclude that the fetal DNA sample is contaminated with maternal DNA; in cases of relevant contamination (Fig. 1), in which the fetal DNA sample shows small amounts of an allele of maternal origin, a new sample must be collected. However, this is very rare because before analysis, a macroscopic and microscopic cleaning of the fetal sample is carried out; (3) to define the sex of the unborn child. In the case of recessive X-linked diseases, such as hemophilia A or B, in the case of a female unborn child (who may be healthy or a healthy carrier), the search for mutations in the fetal DNA can be avoided.
Fig1. Contamination analysis
Also, for hereditary genetic diseases, the analysis of fetal DNA in maternal blood offers new opportunities. It is impossible to replace fetal DNA analysis with invasive analysis because even if technologies are used to concentrate fetal DNA, a quantity of maternal DNA is still present in the maternal blood sample. Therefore, when a positive signal is obtained for the maternal mutation, it is impossible to ascribe it with certainty to the fetal sample. However, fetal DNA analysis may also offer some contributions to the prenatal diagnosis of inherited genetic diseases:
• In recessive X-linked diseases, Y-chromosome polymorphisms can be tested on fetal DNA; if they are absent, the fetus is female, and invasive prenatal diagnosis can be avoided.
• In cases of autosomal recessive diseases in which the maternal and paternal mutations are different, only the paternal mutation can be tested in fetal blood; if it is absent, invasive prenatal diagnosis can be avoided because the fetus has only a 50% risk of carrying the maternal mutation.
