For hereditary monogenic diseases, in addition to the analysis aimed at identifying affected patients (preferably at an early stage or asymptomatic), it is possible to carry out investigations aimed at finding healthy carriers, with two distinct modalities: cascade analysis and carrier screening.
Cascade Analysis
The first case is represented by the cascade analysis. Typically, an affected patient is identified, disease-causing mutations are identified, and then the search for mutations is extended to blood relatives based on the type of Mendelian transmission. For example (Fig. 1), in an autosomal recessive disease, such as cystic fibrosis, we study the patient’s siblings (each of whom has an a priori 2:3 risk of being a carrier), then the parents’ siblings (each of whom has an a priori 1:2 risk of being a carrier) and, in case of positivity, the children, and so on with a “cascade” mechanism. When a carrier is identified, it is essential to perform molecular analysis in the partner who has an a priori risk of 1:25 of being a carrier, as all subjects of the general population (in this case, not only the mutations of the family are analyzed, but an extended molecular analysis is performed). A targeted multi-disciplinary genetic counseling must accompany each step of the molecular analysis.
Fig1. Cystic fibrosis. (Copyright EDISES 2021. Reproduced with permission)
Carrier Screening
For some diseases that are particularly frequent in specific geographical areas or ethnic groups with a strong tendency to intermarriage, carrier screening is possible even if there is no case in the family. For example, among Ashkenazi Jews, carrier screening (usually premarital) is performed for some more frequent diseases, such as Tay-Sachs disease and cystic fibro sis; in Italy, pregnancy guidelines suggest screening for thalassemia (in this case, screening is not based on mutation detection but hemoglobin analysis by chromatography or electrophoresis). For more than 10 years in some Italian regions (including Veneto), carrier screening (also in this case, in the premarital period) for cystic fibrosis has been started based on the analysis of panels of mutations most common in both partners and on the possibility of prenatal diagnosis if both partners are carriers. Preliminary data seem to indicate a reduction in disease incidence in screened areas. However, examples of carrier screening are few so far, both because the costs are still high and especially because screening should be accompanied by proper multidisciplinary counseling for couples, and, in large numbers, this is difficult to achieve.
