Chapter 1. Preimplantation Genetic Diagnosis

Instructor's Notes

The case study “Preimplantation Genetic Diagnosis” involves the detection of disease genes in the setting of a fertility clinic, following a family that had previously undergone genetic diagnosis and counseling. After the introduction, the study examines the genetic basis of disease and the concept of recessive and dominant mutations (Section 6.1: Genetic Analyses of Mutants to Identify and Study Genes, p. 224). During the course of the study, students will use materials from the textbook and additional web links to cover the following concepts:

• Pedigree analysis and how to determine outcomes
• Cystic fibrosis as a disease, and the most common mutations in the CFTR gene (Section 6.4, p. 254; Section 11.3, p. 494)
• Splicing of genes during transcription to generate mRNA
• PCR and DNA sequencing as diagnostic tools and how to interpret results from experiments using them (Section 6.2, pp. 239, 244);
• Utility of animal models to help characterize genes
• Tissue-specific approaches used in animal models (Cre-lox system, Section 6.5, p. 264)
• Principles of gene editing to correct DNA mutations (Section 6.5, p. 266)

Students are awarded points for answering questions throughout the activity, for correctly interpreting the pedigree, calculating genetic probabilities, interpreting the data, understanding the molecular basis of cystic fibrosis, suggesting follow-up experiments, and so forth. Each question is worth 1 point. Multiple choice questions are automatically graded; students are given one opportunity to correct an incorrect answer before moving on. Short-answer questions are accepted provisionally; instructors must evaluate the answers and determine whether they are acceptable. The final score is recorded in the gradebook.

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"Very good, Emily," Evan congratulated. "You certainly have the genetics background for this type of work."

The most common mutation, the F508del mutation, causes the protein to fold improperly and be degraded before it reaches the plasma membrane.

Evan gave Emily more details about the CFTR gene and its protein product. "The CFTR gene is 230,000 base pairs long, consists of 27 exons, and creates a protein that is 1480 amino acids long. This figure depicts the CFTR gene and its mRNA, showing which amino acids encode the protein domains: two transmembrane segments (T1 and T2), two ATP-binding domains (A1 and A2), and the regulatory domain (R).

"Arrows point to areas where we have exon-specific primers that we use to detect the most common mutations: Exon 4–specific primers for the R117H mutation; exon 11–specific primers for the F508del mutation; exon 12–specific primers for the G542X, G551D, and G553X mutations; exon 23–specific primers for the W1282X mutation; and exon 24–specific primers for the N1303K mutation. We also have other primer sets developed so that we can amplify any of the exons from genomic DNA."

"The nephew (G) in family 40513 was found to have the F508del mutation in one allele, which his father (D) also carries. However, neither our standard CF panel, nor the extended CF panel, was able to identify a mutation in the allele he inherited from his mother. Because we don’t know what CFTR mutation this family carries, we can’t easily determine whether the father (B) is a carrier."

Emily spoke up, "You must be able to identify the mutation via sequencing, right?"

Evan nodded, "Yes, we could sequence the entire CFTR gene, but that would take some time and even then, we might not find the mutation if it’s in an upstream or downstream regulatory region. Our team began working to identify the mutation right away, and I’ll show you what they found a little later. This family wanted to have another child soon, so they opted to take advantage of our IVF and preimplantation diagnosis services."

"Oh!" Emily exclaimed, "Because cystic fibrosis is recessive, all you have to do is find an embryo that doesn’t have the mother’s mutation!"

"That’s right," Evan said. "If the embryo doesn’t have the maternal mutation, then it most likely inherited the unaffected allele from its mother, and it will either be a carrier, like the father, or completely unaffected."