Number of mutations searched for genetic counselling of parents

Question

Hello,
Once the child has been diagnosed with CF, could you tell me which kind of test is used and how many mutations are sought in the parents blood sample withdrew to check the mutations in a genetic counselling?

Answer

Hello,
The type of test used and the mutations searched for in the parents depend on the CFTR mutations found in the CF diagnosed child. The techniques used for investigating the parents are those that have identified the mutations in the child.
If the child’s 2 mutations are among the 30 most common mutations identified by the screening kit used in France, the same kit can be used to search for mutations in the parents.
Mutations that are not part of the 30 most common mutations are rare or exceptional. The identification of those mutations then requires:
-firstly, a gradual exploration of the coding regions of the gene which starts with the mutation richest regions and uses a scanning technique called High Resolution Melting (HRM) or a systematic sequencing immediately: the aim in this case is searching specifically the mutation found in the child;
- then (if the previous exploration was not successful), looking for large rearrangements by a so-called technical Comparative Genomic Hybridization (CGH) or a so called MPLA (Multiplex Ligation-Dependent Probe Amplification technique: 2% mutations).
These complex techniques are mastered by a few reference laboratories for molecular genetics of cystic fibrosis. They identify 99.5% of the 1,900 currently listed mutations in the CFTR gene (which means that 0.5% of the mutations are currently not identifiable).
The analysis of both parents' blood samples has two aims:
-Check the heterozygous (healthy carrier) status of the 2 reported parents, in order to accurate the genetic counselling about the risk for this couple to transmit the disease to subsequent pregnancies;
- and also, sometimes, to clarify the child’s genotype: "homozygous" or "compound heterozygous". Actually, when a single mutation is found in the CF diagnosed child, it can be difficult to know if the child is "homozygous" for this mutation (the same mutation transmitted by each of both parents) or if he/she is "compound heterozygous" (the mutation transmitted by each of both parents is different). Indeed, in a "compound heterozygous" CF child, the CFTR mutation of one parent can sometimes interfere with the identification of the mutation of the other parent, especially in case of a mutation with a particular "rearrangement" or important "deletion".
Hope that answers your question.
Dr. Gilles Rault (with the kind contribution of Professor Claude Férec)

02.07.2013