Differential Gene Expression: The Gene’s Role in Development

Lange

BIOL 370 – Developmental Biology

Topic #2

Cloning - the process of producing similar populations of genetically identical individuals that can occur in nature when organisms such as bacteria, insects or plants reproduce asexually.

Cloning (using modern biotechnology) also refers to scientific processes used to create copies of DNA fragments (molecular cloning), cells (cell cloning), or organisms (organismal cloning) in the lab or in a clinical setting.

Cloned mammals have been created using nuclei from adult somatic cells

The basic process of producing “Dolly”… the first cloned mammal.

Murray BarrCanadian physician and medical researcher who discovered the Barr Body

in 1948. He also played a key role in understanding the genetics behind calicoism in cats.

Mary LyonIn 1961 she proposed the concept of X-inactivation: one of the two X chromosomes

inside a female mammal shuts off under normal circumstances. Yet in some circumstances different X chromosomes will shut off, resulting in the calico

appearance. She observed this in the coat color patterns in mice.

The kitten “CC”

In image (a) we see the kitten named “CC” who is a clone of the material cat in image (b) named “Rainbow”. The kitten appears different and behaves differently from the mother due to a non-randomized X-inactivation, but they are genetically identical.

What

Timing of X-inactivation:

In studies using mouse cells to study the process of X-inactivation:

•it has been determined that an early, imprinted inactivation of the paternally-derived X chromosome in two-cell or four-cell stages of development occurs.

•However, in the early blastocyst, this initial, imprinted X-inactivation is reversed in cells of the inner cell mass, and in these cells both X chromosomes become active again.

•Each of these cells then independently and randomly inactivates one copy of the X chromosome. This inactivation event is irreversible during the lifetime of the cell, so all the descendants of a cell which inactivated a particular X chromosome will also inactivate that same chromosome.

• X-inactivation is an epigenetic change that results in a different phenotype and is not a change at the genotypic level.

•For an individual cell or lineage the inactivation is therefore skewed or 'non-random', and this can give rise to mild symptoms in female 'carriers' of X-linked genetic disorders.

It is hypothesized there is an autosomally produced 'blocking factor' that binds to an X chromosome after fertilization and prevents its inactivation. Potentially, the blocking factor may affect the selection of which X chromosome at an early cell stage or perhaps at a later stage. The later the stage of effect, the more likely the display of calicoism.

So, what is the way in which cells, which are identical genetically, will differentially grow and develop into specific cell types, tissues, organs, etc.?

•Differential gene expression – this can arise at a variety of levels including: gene transcription, nRNA processing, and mRNA translation.

•Histone protein differences - histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes.

• Methylated histones often inhibit gene expression• Acetylated histones often activate gene expression

The genetic elements regulating tissue-specific transcription can be identified by fusing reporter genes to suspected enhancer regions of the genes expressed in particular cell types

A reporter gene is a gene that researchers attach to a regulatory sequence of another gene of interest.

Enhancer region modularity

Silencers. Analysis of β-galactosidase staining patterns in 11.5-day embryonic mice

Here we see the NRSE (neural restrictive silencer element) sequence of a gene for the development of the nervous system.

The lack of the NRSE sequence in (b) results in the mouse displaying excessive B-galactosidase (the product produced by the lacZ gene sequence.

Methylation of globin genes in human embryonic blood cells

Methylation typically results in inhibition of a gene.

DNA methylation can block transcription by preventing transcription factors from binding to the enhancer region

X chromosome inactivation in mammals

Prader-Willi Syndrome and Angleman’s Syndrome

Two genetic disorders that affect development and are caused by issues associated with chromosome 15.

Prader-Willi Syndrome (PWS) - a rare, genetic disorder in which seven genes on Chromosome 15 are deleted or unexpressed on the paternal chromosome. It was first described in 1956 and characteristic of PWS include:

•low muscle tone, •short stature, •incomplete sexual development, •cognitive disabilities, •a chronic feeling of hunger that can lead to excessive eating and life-threatening obesity.

Angelman Syndrome (AS) a neuro-genetic disorder characterized by:

•severe intellectual and developmental disability,•sleep disturbance, •seizures, •jerky movements (especially hand-flapping), •frequent laughter or smiling, and usually a happy demeanor.

AS is a classic example of genomic imprinting in that it is caused by deletion or inactivation of genes on the maternally inherited chromosome 15 while the paternal copy, which may be of normal sequence, is methylated and therefore silenced.

Inheritance patterns for Prader-Willi and Angelman syndromes (Part 1)

Note that the region can be deleted or methylated into inactivation.

Inheritance patterns for Prader-Willi and Angelman syndromes (Part 2)

The Dscam gene of Drosophila can produce 38,016 different types of proteins by alternative nRNA splicing

This Dscam gene in the fruit fly is homologous to a DNA sequence for nervous system development on human chromosome 21.

In cases where this sequence is disrupted in humans embryologically, the offspring will develop a form of Downs Syndrome.

Dscam protein is specifically required to keep dendrites from the same neuron from adhering to each other

Muscle hypertrophy through mispliced RNA

A television program on TLC focused on a child who has a believed to be similar mutation causing muscle hypertrophy. The television show was called “The World’s Strongest Toddler”

Degradation of casein mRNA in the presence and absence of prolactin

The lack of hormone resulted in rapid decay of mRNA, while the presence of the hormone exerted the oposite effect. How might this relate to developmental expression of milk in females?

The lymphoid precursor cell can generate B cells or T cells

B cells will form antibodies, whereas T cells primairly attack directly.

End.