the genetics of heterochromatin in metazoa

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The genetics of heterochromatin in metazoa

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Hermann Joseph Muller

1946 Nobel Prize in Medicine:

"for the discovery of the production of mutations by means of X-ray irradiation"

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White wild-type

White mutant

The true meaning of "red eye reduction":

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Gene behavior can change depending on where on the chromosome the gene lies.

= “position effect” (bar is the most commonly used example)

“Position effect variegation” (PEV): cell-to-cell variability of expression of a gene that has been relocated to a new position in the genome.

Epigenetic phenomenon:Stable change in expression without change in sequence!

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Enhancer of PEV

Suppressor of PEV

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2 genes:

Su(var)2-5

Su(var)3-9

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HP1(Sarah Elgin)

(heterochromatin protein 1)

Identified in a BIOCHEMICAL scheme to discover proteins that are associated with heterochromatin.

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HP1 = Su(var)2-5

Conserved in humans and in mice(both in terms of sequence and intranuclearlocation!).

biochemistry genetics

Why does HP1 go to places that HP1 goes to?

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“Biochemical epistasis”(T. Jenuwein)

Overexpression of mouse Su(var)3-9 leads to a MASSIVE redistribution of HP1 in the

nucleus of mouse cells.

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Who would have thunk it?

NCBI: Su(var)3-9 contains a domain (the SET domain) that is somewhat similar to, ahem, RUBISCO methyltransferase.

Su(var)3-9 is a HISTONE methyltransferase.

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Histone methylation

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Calling David Duchovny and Gillian Anderson

• Su(var)3-9 was given this name because it was the 9th gene isolated on the 3rd chromosome in a screen for Su(var)s.

• It methylates lysine 9 in histone H3.

This was discovered 18 years after it was named.

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And finally

• HP1 preferentially BINDS histone H3 methylated on lysine 9.

• That’s why Su(var)3-9 determines localization of HP1 to heterochromatin (it methylates histones in heterochromatin).

• At least in fission yeast, and perhaps in worms, this has to do with RNAi.

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HP1 HP1

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HP1 HP1 HP1 HP1 HP1 HP1HP1 HP1= = =

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Remembrance of things past:chromatin as an epigenetic vehicle

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Homology(orthologs of heterochomatin proteins in fission yeast, insects, and humans)

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Analogy

Fission yeast, flies, mammals. Budding yeast.

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Nature, October 10, 2002

The polycomb group protein EZH2 is involved in progression of prostate cancer

Varambally et al.

Prostate cancer is a leading cause of cancer-related death in males and is second only to lung cancer. Although effective surgical and radiation treatments exist for clinically localized prostate cancer, metastatic prostate cancer remains essentially incurable. Here we show, through gene expression profiling, that the polycomb group protein enhancer of zeste homolog 2 (EZH2) is overexpressed in hormone-refractory, metastatic prostate cancer. … Dysregulated expression of EZH2 may be involved in the progression of prostate cancer, as well as being a marker that distinguishes indolent prostate cancer from those at risk of lethal progression.

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From egg to embryo

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Homeotic mutations (W. Bateson)

“… Not that there has merely been a change, but that something has been changed into the likeness of something else.”

Genetics

Allele

Heterozygous

Homozygous

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wt antennapedia

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The segmentation hierarchy

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“Do you have any idea who I think I am?!!”

1. Segment identity is determined by transcription factors.

2. They act on target genes only transiently. Then they go away, and the activity of their targets is maintained by large complexes: Polycomb represses genes, and Trithorax activates them.

3. Nobody knew how Polycomb and Trithorax do this.

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How Polycomb and Trithorax work

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extra sex combs

enhancer of zeste

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E(z) does it

Posted September 13, 2002 – CELL immediate early publication

Czermin, B., Melfi, R., McCabe, D., Seitz, V., Imhof, A., and Pirrotta, V. Drosophila Enhancer of Zeste/ESC complexes have a histone H3 methyltransferase activity that marks chromosomal Polycomb sites. Cell. Published online September 13, 2002. 10.1016/S0092867402009753

Müller, J., Hart, C.M., Francis, N.J., Vargas, M.L., Sengupta, A., Wild, B., Miller, E.L., O'Connor, M.B., Kingston, R.E., and Simon, J.A. Histone methyltransferase activity of a Drosophila Polycomb group repressor complex. Cell. Published online September 13, 2002. 10.1016/S0092867402009765

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“Influential ideas are always simple. Since natural phenomena need not be simple, we master them, if at all, by formulating simple ideas and exploring their limitations.”

Al Hershey

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Regulation of genes occurs via the interaction of trans-acting factors (proteins) with cis-acting sequences near the genes themselves.

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stimulus

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Bicoid is the anterior morphogen

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What democracy, I mean, gene regulation, is really like

• Trans-acting factors do not distribute in the nucleus based on the primary sequence of the genome: some factors fail to bind most genes that have sequences waiting for them, and other factors bind a large number of genes that do NOT have sequences for them

• Even when a factor binds next to a gene, many times, nothing happens; the same factor bound to two different genes can exert diametrically opposite effects

• Most genes in the human genome are under considerable regulatory influence from entities other than “simple” trans-acting factors; these entities include noncoding RNA and modified histones

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Boyer and Young Cell Sept. 23, 2005

Fischle, Wang, Allis COCB 2003

David Allis: “the histone code”

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1963-2000 2000 - …

Henry et al. (11/1/2003) Genes Dev. 17: 2648.

Genetic information

Lac operator

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Genetic information

Genetic information

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