Understanding Genetic Influences on a Trait

Understanding Genetic Influences on a Trait

• Understanding Genetic Influences on a Trait

• Accounting for Variation in an Observed Trait
• Why have animal models?
• Response to Selection Study

• Types of Mouse Models of Human Disorders

• Transgenic Models
• Mutagenic Models

You are reading a Times article with a friend and you come across the

• You are reading a Times article with a friend and you come across the

• following statement:

• “The study hypothesized that some of these susceptibility factors may be

• allelic variants of genes that govern embryonic serotonin neuron

• development and that these alleles may contribute to behavioral disorders

• by adversely increasing or decreasing serotonin system activity”.

• The article is about finding ways to create mouse models for human DCG’s.

• Your classmate has not yet taken Dr. Carey’s Behavioral Genetics course but

• fortunately you have. Describe to him/her what an allelic variant is and at

• how it might affect a behavioral disorder. Lastly, your friend looks at you as if

• you were crazy and says, “How does a mouse study, help US?” Based on what

• you have taken away from this course, convince your friend about the

• relevance and validity of mouse models. (20 Points)

The extent to which variation on a trait in a

• The extent to which variation on a trait in a

• given population at a given point in time is

• attributable to genetic variation between

• individuals.

• VP = VG + VE

• h2 = VG/VP

• Synteny: The co-localization of genes on

• chromosomes of related species.

• Homolog: The situation where nucleic

• acid or protein sequences are similar

• because they have a common

• evolutionary origin. Often used loosely

• to indicate that sequences are very

• similar.

• Ortholog – gene sequences are similar

• between species.

• Paralog – gene sequences are similar

• within a species.

Pros

• Pros

• Test many hypothesizes

• No human genetics ethical dilemma

• Shortens length of study

• (Avg. gestational period is 19 days)

• (Females become fertile in 3 weeks)

• (Approximately 40 days for turnover; giving approximately 365/40 generations annually)

Response to 30 generations

• Response to 30 generations

• of selection for Open-Field

• Activity in Lab Mice (DeFries et al. 1978)

Humans

• Humans

• Includes anxiety and neuroticism
• Think about what happens to you when you are anxious?

Humans

• Humans

• Includes anxiety and neuroticism
• Think about what happens to you when you are anxious?
• (Sweaty, elevated heart rate, preoccupied thoughts, urge to defecate, involuntary movement, Immobile)

The Basics

• The Basics

• Typically transgenic mice are used to show how the over-expression of a gene product affects physiology, behavior, etc.

• Costs ≈ $3000 per mouse

• Relatively straightforward procedure.

• Transgenic mouse can be made in 3-6 months.

Targeted Transgenesis in Mice

• Targeted Transgenesis in Mice

• - Methodology

• - Example

• - Issues

• Targeted Mutagenesis in Mice

• - Types of mutagenic mice

• - Methodology - Example

• - Issues

Alzheimer’s Transgenic Mice

• Alzheimer’s Transgenic Mice

Issues

• Issues

• Integration of transgene is random.
• It may disrupt the function of another gene
• May integrate into a part of the genome where gene expression is suppressed.
• May integrate into a part of the genome under the control of a locus control region.
• Number of copies cannot be controlled.

Issues

• Issues

• Localization of expression cannot be controlled completely.
• Over time, the transgene is frequently “shut off”.
• These issues can be overcome by generating several transgenic lines using the same construct and comparing the data across lines.

The Basics

• The Basics

• Targeted mutant mice – Alteration of gene of interest through homologous recombination

• The gene of interest can be eliminated (knock-out or null mutant) or altered (knock-in).

• Loss of gene function or alteration of function is typically evaluated in KO and KI mice

The gene of interest can be “conditionally” deleted or altered.

• The gene of interest can be “conditionally” deleted or altered.

• Very labor intensive and technically challenging methods are involved.

• Typical time to generate a null-mutant mouse is 1-3 years.

Knock-out: Remove gene or exon of gene that renders the gene product non-functional.

• Knock-out: Remove gene or exon of gene that renders the gene product non-functional.

• Knock-in: Replace the natural gene or exon with an altered gene or exon that alters the function of the gene product.

• Congenic*: Introgression of a locus of interest from another strain; selecting for a given marker in the donor strain.

Conditional KO: Gene is modified so that it is deleted only under specific conditions.

• Conditional KO: Gene is modified so that it is deleted only under specific conditions.

• Inducible/Tissue specific expression: Gene expression is regulated by a drug supplement (typically a chemical not endogenous to the mouse).

Targeted Transgenesis in Mice

• Targeted Transgenesis in Mice

• - Methodology

• - Example

• - Issues

• Targeted Mutagenesis in Mice

• - Types of mutagenic mice

• - Knock-out/ Knock-in/ Conditional

• Knock-out/ Inducible expression

• - Methodology - Example

• - Issues

• Definitiion

• Chimera - an animal that has two or more different populations of genetically distinct cells that originated in different zygotes

Gene targeting in the mouse

• Gene targeting in the mouse

Problems with KO’s and KI’s

• Problems with KO’s and KI’s

• Targeting vector components (such as the marker used) may affect the expression of genes close to the mutated gene.

• Potential developmental effects, such as:

• Altered development
• Compensatory mechanisms
• Knock-out is lethal

Problems with KO’s and KI’s

• Problems with KO’s and KI’s

• Genetic background: No behavioral trait is determined by a single gene.

• It is expensive and labor intensive.

• Mice take a long time to create.

Conditional targeted mutation: Use of Cre recombinase to delete genes in a temporal or spatial

• Conditional targeted mutation: Use of Cre recombinase to delete genes in a temporal or spatial

• manner.

• Cre recombinase is an enzyme that when in contact with LOXP sites flanking a region of DNA, it can alter the DNA between them.

Inducible Transgenic Mice

• Inducible Transgenic Mice

• The inserted gene is activated at a desired developmental stage.

• An novel transcription factor designed to bind to the gene and cause expression in the presence of an activator compound is used.

Problems of conditional KO’s and inducible transgenics

• Problems of conditional KO’s and inducible transgenics

• Limited availability of tissue-specific mice to knock-out or induce expression in a tissue specific fashion.

• Conditional knockout may not delete the gene in all of the desired cells.

• Gene induction/repression may not be efficiently regulated.

NEIL 1 KO mouse model of

• NEIL 1 KO mouse model of

• Metabolic Syndrome (Vartanian et al, 2006)

Voluntary Ethanol Consumption in Chrna6 and Chrnb3 Knockout Mice (Vidable,. L, et al. 2008; Ehringer Lab)

• Voluntary Ethanol Consumption in Chrna6 and Chrnb3 Knockout Mice (Vidable,. L, et al. 2008; Ehringer Lab)

Voluntary Ethanol Consumption in Chrna6 and Chrnb3 Knockout Mice(Ehringer Lab, Summer ‘08)

• Voluntary Ethanol Consumption in Chrna6 and Chrnb3 Knockout Mice(Ehringer Lab, Summer ‘08)

Examples of transgenic mice:

• Examples of transgenic mice:

• Inducible Breast Cancer

• Inducible Memory

• Molecular defects of Alz.

• No Immune System