Gene Expression Modifications: Transgenic Strategies

Transgenic strategies in mice involve:

• Random insertion of gene into mouse genome
• Targeted insertion or deletion of gene through homologous recombination with mouse gene

Key terms:

• Knock-out = removing a gene, taking it out.
• Knock-in = inserting a gene.

Random insertion leads to constitutive expression. Targeted insertion leads to conditional expression.

RNA Interference: MicroRNA

RNA interference is a process whereby small non-coding RNA molecules target mRNAs to inhibit gene expression.

MicroRNA (miRNA) is naturally produced by cells as hairpin structures. Loose nucleotide pairing allows broad targeting of related mRNAs. When miRNA binds to mRNA, it blocks translation of mRNA and sometimes facilitates its degradation.

Abnormal expression of miRNAs contributes to certain malignancies, for example, by silencing an mRNA from a tumor suppressor gene.

RNA Interference: Small Interfering RNA (siRNA)

Small interfering RNA (siRNA) is usually derived from an exogenous double-stranded RNA (dsRNA) source, such as a virus. Once inside a cell, siRNA requires complete nucleotide pairing, leading to highly specific mRNA targeting. This results in mRNA cleavage prior to translation.

siRNA can be produced by transcription or chemically synthesized for gene “knockdown” experiments.

Genetic Terms: Codominance and Variable Expressivity

Codominance

Definition: Both alleles contribute to the phenotype of the heterozygote.

Example: Blood groups A, B, AB; alpha-1-antitrypsin deficiency; HLA groups.

Variable expressivity

Definition: Patients with the same genotype have varying phenotypes.

Example: Two patients with neurofibromatosis type 1 (NF1) may have varying disease severity.

Genetic Terms: Incomplete Penetrance and Pleiotropy

Incomplete penetrance

Definition: Not all individuals with a disease show the disease.

Calculation: % penetrance × probability of inheriting genotype = risk of expressing phenotype.

Example: BRCA1 gene mutations do not always result in breast or ovarian cancer.

Pleiotropy

Definition: One gene contributes to multiple phenotypic effects.

Example: Untreated phenylketonuria (PKU) manifests with light skin, intellectual disability, and musty body odor.

Genetic Terms: Anticipation and Loss of Heterozygosity

Anticipation

Definition: Increased severity or earlier onset of disease in succeeding generations.

Example: Trinucleotide repeat diseases (e.g., Huntington disease).

Loss of heterozygosity

Definition: If a patient inherits or develops a mutation in a tumor suppressor gene, the wild type allele must be deleted/mutated/eliminated before cancer develops. This is not true of oncogenes.

Example: Retinoblastoma and the “two-hit hypothesis,” Lynch syndrome (HNPCC), Li-Fraumeni syndrome.

Genetic Terms: Epistasis and Aneuploidy

Epistasis

Definition: The allele of one gene affects the phenotypic expression of alleles in another gene.

Example: Albinism, alopecia.

Aneuploidy

Definition: An abnormal number of chromosomes, due to chromosomal nondisjunction during mitosis or meiosis.

Example: Down syndrome, Turner syndrome, oncogenesis.

Topics Covered:

• Transgenic Strategies in Mice
• MicroRNA and Gene Regulation
• Small Interfering RNA (siRNA)
• Codominance and Variable Expressivity
• Incomplete Penetrance and Pleiotropy
• Anticipation and Loss of Heterozygosity
• Epistasis and Aneuploidy