Understanding Genetic Influences on a Trait



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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





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