One potential strategy for gene therapy to correct the...

One potential strategy for gene therapy to correct the effects of dominant gain-of-function mutations is to express small interfering RNAs (siRNAs) that will cause degradation of mRNA produced by the dominant mutant allele. The siRNAs can be delivered to patient cells by a synthetic gene encoding a hairpin $R N A$ that will be processed by Dicer and used to target the RISC complex to the mutant allele mRNA (review Figs. 17.32 and 17.33 ). a. A major problem with this gene therapy strategy is designing an siRNA that will prevent the expression of the mutant allele specifically and not the normal allele. Explain why this is a problem. b. Specificity for the mutant allele is a particular problem in designing an siRNA therapy gene for Huntington disease. Explain this issue and suggest a possible solution that would allow you to use RNAi. c. Another potential strategy to correct the mutant Huntington disease allele is to cut the DNA in the middle of the repeat tract. Exonucleases present in the nucleus would then degrade many of the repeats prior to repair by NHEJ. Could you use CRISPR/Cas9 technology to correct Huntington disease mutations with this approach? (Hint: See Problems 28 and $29 .$

One potential strategy for gene therapy to correct the effects of dominant gain-of-function mutations is to express small interfering RNAs (siRNAs) that will cause degradation of mRNA produced by the dominant mutant allele. The siRNAs can be delivered to patient cells by a synthetic gene encoding a hairpin $R N A$ that will be processed by Dicer and used to target the RISC complex to the mutant allele mRNA (review Figs. 17.32 and 17.33 ).
a. A major problem with this gene therapy strategy is designing an siRNA that will prevent the expression of the mutant allele specifically and not the normal allele. Explain why this is a problem.
b. Specificity for the mutant allele is a particular problem in designing an siRNA therapy gene for Huntington disease. Explain this issue and suggest a possible solution that would allow you to use RNAi.
c. Another potential strategy to correct the mutant Huntington disease allele is to cut the DNA in the middle of the repeat tract. Exonucleases present in the nucleus would then degrade many of the repeats prior to repair by NHEJ. Could you use CRISPR/Cas9 technology to correct Huntington disease mutations with this approach? (Hint: See Problems 28 and $29 .$

Key Concepts

RNA Interference (RNAi) Mechanism

RNA interference is a natural process in which small RNA molecules, such as siRNAs, mediate the sequence-specific degradation of target mRNAs. The mechanism involves processing long double-stranded or hairpin RNAs by the enzyme Dicer into siRNAs, which are then incorporated into the RNA-induced silencing complex (RISC) to guide the complex to complementary RNA sequences, leading to their degradation and decreased expression of the target gene.

Allele-specific Gene Silencing

Allele-specific gene silencing refers to the selective targeting of a mutant allele while sparing the normal allele. This strategy is crucial when the detrimental effects are due to a mutant protein that exerts a dominant, gain-of-function effect. However, designing siRNAs that can distinguish between nearly identical sequences, such as those differing by only a single nucleotide mutation, poses significant challenges in order to achieve the required specificity without affecting the normal gene.

Dominant Gain-of-Function Mutations

Dominant gain-of-function mutations involve genetic changes where the mutant allele produces either an abnormal protein or an altered amount of protein that interferes with normal cellular functions, often exerting a toxic effect. Therapeutic strategies targeting these alleles must be precise, as indiscriminate gene silencing could reduce the expression of both the harmful and the functional protein, thereby potentially worsening the condition.

CRISPR/Cas9 Genome Editing

CRISPR/Cas9 is a genome editing technology that uses a guide RNA to direct the Cas9 nuclease to a specific DNA sequence, where it introduces a double-strand break. This break can then be repaired by the cell’s endogenous repair mechanisms, such as non-homologous end joining (NHEJ), which can result in the removal or alteration of the target sequence. Leveraging CRISPR/Cas9 offers an approach to potentially correct or mitigate the effects of mutant alleles by directly modifying the genome.

Non-Homologous End Joining (NHEJ) DNA Repair

Non-homologous end joining is a cellular mechanism for repairing double-strand breaks in DNA. NHEJ directly ligates the ends of the broken DNA, often resulting in insertions or deletions. This repair process can be exploited in genome editing applications, where inducing breaks at specific sites—such as within pathogenic repeat tracts—can lead to deletions that remove harmful mutations or repeat expansions.

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