The Evolution of a Heterochromatic Domain in Drosophila: Investigating the Strange Dot Chromosome...

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The Evolution of a Heterochromatic Domain in Drosophila:

Investigating the Strange Dot Chromosome

Sarah C R Elgin

January 2012

A collaborative investigation involving:

- former members of the Elgin Lab: Lee Silver, Carl Wu, TC James, Joel Eissenberg, Lori Wallrath, Fang Lin Sun, Karmella

Haynes

- current members of the Elgin Lab: Nicole Riddle, Tingting Gu, Chris Shaffer, Wilson Leung

- modENCODE: Gary Karpen, Mitzi Kuroda, Vincenzo Pirrotta, Peter Park, and their colleagues

- Faculty and students of the Genomics Education Partnership

Goal: to understand the organization and functioning of the dot chromosome in Drosophila, an unusual heterochromatic domain. Funding: HHMI Professors Program NIH General Medical Sciences, National Human Genome Research Institute

Minimum Haploid DNA Content - the C Value Paradox

Britten and Davidson, 1969 Science 165:349

4 Allis et al: Epigenetics 2007

Larger genomes reflect high levels of repeats - retroviral and DNA transposon remnants (TEs)

Considerations for Genome Sequencing1. Satellite DNA, a sequence of tandem repeats, is very difficult to

sequence, as there are few markers to help order subclones; hence centromeric regions of the chromosomes are usually left unsequenced.

2. Other repetitious DNA, derived from transposable elements, also causes difficulties; because one finds nearly identical sequences located in different regions of the genome, mistakes can be made in assembling sequence data. High quality discrepancies can identify these.

3. Much of the repetitous DNA is packaged into heterochromatin, which maintains these regions in a compact and transcriptionally silent form.

4. However, in many higher organisms, protein-coding genes are found embedded in repetitious DNA. Check out your favorite human gene on the UCSC Browser by taking off RepeatMasker!

SCR Elgin

Coding exons1.5%

Conserved noncoding - regulatory? 3.5%

Human Genome

3 Gb~2 m/cell !

Key Questions: Is it junk or garbage? How is DNA packaged into a nucleus? How is silencing maintained – while

allowing appropriate transcription ?

Eukaryotic genomes are very large – and most of that DNA is non-coding!

TEs – retroviruses,DNA transposons

What determines phenotypes?It’s not just your DNA….

Phenotype

Development

Environment (diet)(grey bars = folate)

Epigenetics?Genotype

(Waterland and Jirtle 2003)

Chromatin structure = epigenetics !What sets and maintains tissue-specific gene expression patterns? Differences are heritable through mitosis, but independent of DNA sequence.

• DNA modification (mC)• Chromatin structure• Nuclear localization

It’s all about silencing!How is chromatin assembled?When, where and how does gene silencing occur?

Incorrect silencing can leadTo genetic disability, as seenIn Fragile X syndrome

Zoghbi and Beaudet 2007

Fragile X Foundation

Felsenfeld et al. Nature 2003, 421: 448

Chromatin formation:

First step - packaging in a nucleosome array

Second - differential packaging into heterochromatin & euchromatin

Chromatin

Lodish et.al., Molecular Cell Biology, 4th Edition

Chromosome(metaphase)

Histone protein core

Electron Micrograph of Chromatin Fibers (rat thymus nucleus)

Olins et. al., 1975 J. Cell Biol, 64:528

0.1 mm

“A eukaryotic chromosome made out of self-assembling 70A units, which could perhaps be made to crystallize, would necessitate rewriting our basic textbooks on cytology and genetics! I have never read such a naïve paper purporting to be of such fundamental significance. Definitely it should not be published anywhere!”

Anonymous review of paper submitted by C.F.L. Woodcock, 1973, showing EM pictures of nucleosome arrays.

Quoted in “Chromatin” by K.D. van Holde, 1989

The Structure of the Nucleosome Core

Rhodes, 1997 Nature 389:231, after Luger et. al., 1997 Nature 389:251

Resolution: 2.8 Å

Half of the nucleosome structure is shown

One turn of the DNA helix is visible (73 bp)

View is down the superhelix axis

Protein - DNA contact: white hooks

DNA packaging domains

• Euchromatin– Less condensed– Chromosome arms– Unique sequences;

gene rich– Replicated throughout S– Recombination during

meiosis

• Heterochromatin– Highly condensed– Centromeres and telomeres– Repetitious sequences;

gene poor– Replicated in late S– No meiotic recombination

Transcriptional activators

Hyper-acetylated histone tail

Heterochromatin Protein 1 complex

Hypo-acetylated histone tail; methylated H3/K9

Heterochromatin formation – silencing counts!

How is heterochromatin organized and packaged to

promote silencing?

The fourth chromosome appears heterochromatic but has ~80 genes:

- - do these genes have unusual characteristics?- - how has the chromosome evolved?

-- how do these genes function?

Short life cycle, easily maintained: good genetic tools

Polytene chromosomes: excellent cytology

Biochemical approaches

Simple genome, good reference sequence

PEV – reporter for gene silencing, heterochromatin

formation

Metazoan useful for behavioral, developmental and human

disease researchexpressed silenced

euchromatin heterochromatin

Fruit Flies!

Mary Lou Pardue, MIT

Using a white transgene to sample chromatin environments

mobilize P elementby crossing to stockwith transposase

inject

transposon carryingwhite gene

P[white+]

white67c23

insertion intoeuchromatin

insertion intoheterochromatin

Elgin Lab

Transposition of a P element reporter allows sampling of euchromatic and heterochromatin domains

Silenced1%

Active99%

Wallrath and Elgin, 1995

And the Y chromosome

Assessing chromatin structure- same gene, different environments

Analysis based on nuclease digestion of chromatin

The euchromatic hsp26 transgene: - DH sites: accessibility at the TSS, upstream regulatory region - irregular nucleosome array

The heterochromatic hsp26 transgene:- loss of DH sites- regular nucleosome array

Looking for heterochromatic proteins by immunofluorescent staining of the polytene chromosomes: discovery of HP1a

HP1Phase

t al 1

Heterochromatin-associated gene silencing is dependent on HP1

Eissenberg et al, 1990, PNAS 87: 9923

Mutations in

gene for HP1a

Mutations recovered by T Grigliatti as suppressors of PEV.Dosage dependent response.

HP1 interacts with both the modified histone H3K9me2/3 and the modifying enzyme

HP1: ShadowChromo

SU(VAR)3-9Histone 3methyl-Lys9

H3 K9 methyltransferase

[(SU(VAR)3-9 identified in screen by Reuter;H3 interaction first shown from work in mammals – Jenuwein, Kouzarides;

demonstrated in flies by Imhof.]

Model for spreading of heterochromatin

Establishing silencing: a multi-step process

wm4 reporter (screens by Reuter, Grigliatti, others)

Loss of euchromatin marks Gain of heterochromatin marks

Heterochromatin formation on the dot chromosome…

The fourth chromosome appears heterochromatic but has ~80 genes:

- - do these genes have unusual characteristics?- - how has the chromosome evolved?

- how do these genes function?

The Drosophila melanogaster fourth chromosome exhibits an amalgam of euchromatic and heterochromatic properties (HP1a association)

HP1Phase

t al 1

Heterochromatic properties:- late replication, lack of recombination- high repeat density (30%) - antibody staining of HP1, H3K9me2/3

But…- the fourth has ~ 80 genes in distal 1.2 Mb- these genes are transcriptionally active!

Most hsp70-white reporters exhibit variegation on insertion into the fourth chromosome

2-M1021 39C-12 2-M390 39C-52

Sun et al 2004; Riddle et al 2007

Use comparative genomics to learn more about heterochromatic domains, analyzing the dot chromosomes and a control euchromatic region of Drosophila genomes

Our GEP Research Goal:

FlyBase: http://flybase.org

Reference

Status

Completed

Annotation

Sequence Improvement

New Project

Genomics Education Partnership (GEP)

Partners are generally PUI schools; faculty join by attending one-week workshop at WU. Sharedwork organized on GEP website.

The D.melanogaster & D. virilis dot chromosomes are 25% - 30% repetitious DNA

(typical – but up to 80% in D. ananassae)

D mel D vir D mel D vir D vir D mel D vir D vir D vir

Leung et al 2010

Dot chromosome genes: introns are larger, exons show less codon bias

Heterochromatic

Euchromatic

D. melanogaster Het.D. melanogaster Euch.D. virilis Euch.

D. melanogaster DotD. virilis Dot

Intron Size

Codon Bias

Leung et al. 2010 Genetics 185:1519-1534

Almost all of the same genes are present (27/28), but rearrangements within the chromosome are common!

Initial analysis of Drosophila virilisdot chromosome fosmids

Slawson et. al., 2006 Genome Biology, 7(2):R15.

Comparison of gene order and orientation

72 genes on both the D. virilis and D. melanogaster dots.

A minimum of 33 inversions are needed to convert order and orientation!

D. virilis

D. melanogaster

Leung et al 2010

dot: D. virilis dot: D. melanogaster

CG9935CG5367

CG4038 CG11076

CG1732

CG5262

CG11077

“Wanderer” genes move between the dot chromosome and a euchromatic site in the long arms; they adopt the

properties (gene size, codon bias) of their local environment

Leung et al 2010

• Is there a homologous gene in D. melanogaster?• Is it on the dot chromosome? • Are all of the isoforms found in D melanogaster present?• How many exons? • Any unusual splice sites?• What is the order and orientation of genes compared to D. melanogaster?• Are there repetitious elements nearby?

Check out your gene on FlyBase – what is the pattern of expression inD. melanogaster? Has a function been described?

Many dot chromosome genes are expressed at a high level - how canthis occur in a heterochromatic domain?

Some things to look for while annotating dot chromosome genes….

Chromatin Immuno-precipitation - ChIP (cells or nuclei)

ChIP-chip*

ChIP-seq

1. Crosslink proteins to DNA

2. Isolate chromatin and sonicate

3. Incubate with antibody

4. Isolate AB/chromatin complexes

5. Isolate DNA from complexes

Mapping chromatin marks by ChIP-chip:Chromosome arm 3L shows a distinct shift between

heterochromatin and euchromatin

Su(var)3-9

H3K9me2

H3K9me3

Euchromatin Heterochromatin

CentromereS2 cells

Euchromatin / heterochromatin transition point from FlybasePink boxes show significant enrichment (0.1% false discovery rate)

Chromosome 4 is largely heterochromatic, but shows distinctpeaks of H3K4me2/3, indicating transcription start sites

H3K9me2

H3K9me3

H3K4me2

Centromere Telomere

A model of 9 chromatin states, based on clustering of histone modification marks, identifies large-scale

genomic domains

Heterochromatin (dark blue; H3K9me3)

TSS (red; H3K4me3 rich)

Polycomb (grey; H3K27me3 rich)

BG3 cells, chromatin states:

Pericentric heterochromatin

1 2 3 4 5 6 7 8 9

500 kb

RedVariegating

An expanded view of the fourth chromosome reveals

TSS (state 1, red) and Pc (state 6, grey) domains

interspersed within heterochromatin (states 7 & 8, blue).

Might fourth chromosome genes function early, and be silenced later? No, fourth chromosome genes show a variety ofexpression patterns, including expression in the adult

S Celniker, modENCODE

Most 4th chromosome genes lie in heterochromatic space (blue), but active genes achieve state 1 (red) at the TSS

Active fourth chromosome genes show depletion of HP1a andH3K9me3 at the TSS, but enrichment across the body of the gene

Transcription levels are similar to euchromatic genes!

TSS-relative position

RNA pol II H3K4me3 HP1 H3K9me2 H3K9me3

chromosome 4

The fourth chromosome: a repeat rich domain with “heterochromatic” genes

Future: try to determine what feature drives 4th chromosome gene expression that is absent from euchromatic genes (hsp70).

Eight new genomes

Expanded dot chromosomes?

Heterochromatin formation changes chromatin atthe nucleosome level, eliminating HS sites at the TSS of

euchromatic genes; silencing is dependent on HP1a

Fourth chromosome genes are larger, have more introns, and less codon bias than euchromatic genes

Fourth chromosome genes show high levels of HP1a and H3K9 methylation over the body of the gene,

but maintain access at the TSS.

Next steps: what makes fourth chromosome genes robust? Lets look for fourth chromosome motifs!

Question Slides

A role for POF? A different configuration for HP1a?

Are there chromosomal proteins unique to the fourth chromosome? Yes – POF (Painting of Fourth) is uniquely associated with active genes on the fourth chromosome

(See J Larsson, PLoS Genet. 11:e209 for more on POF)

Green = HP1a Red = POF

Drosophila melanogaster: 1/3 heterochromatin.Pericentric heterochromatin is under-replicated in polytene

chromosomes; the arms fuse in the chromocenter

Drawing of polytene chromosomes modified from TS Painter, 1934, J. Hered 25: 465-476.

HP1 sequence from Drosophila, mouse, human and mealy bug identifies chromo domain & chromo shadow domain

HP1 from mammals can rescue mutations in flies and yeast!

Position Effect Variegation in Drosophila:an assay for heterochromatic packaging

whiteWild Type

Inversion

HP1 is a trans-acting modifier of PEV

E(var) (3 copies HP1)

Su(var) (1 copy HP1)

Eissenberg et al

The heterochromatic hsp26 transgene: - loss of accessibility at the TSS when in heterochromatin- reversed in an HP1 mutant background

Chromosome 4 short arm is a unique heterochromatic domain

Haynes et al. 2007

Translocation away from the chromocenter results in loss of silencing – spatial organization plays a role

Define chromatin states by K-means clustering(using enrichment values for 1 kb chromatin fragments)

Select k means as starting points.

Assign each data point to

closest mean.

The centroid of each cluster is

the new mean.

Repeat the last two steps.

H3K9me3

Chromatin states are highly interspersed-a folded view of the chromosome (Bg3 cells)

Polycomb

Pericentric heterochro-matin

Chromatin states reveal cell type specific patterns (note facultative heterochromatin – light blue)

Chromosome 3L

BG3 S2

A folded view of the chromosome reveals TSS and Pc states within chromosome 4

(Bg3 cells)

Pericentric heterochromatin

4th chromosome

Chromosome 4 shows a distinct subset of Polycomb sites in a cell-type specific pattern

H3K9me3

(S2 cells)

H3K9me3

(Bg3 cells)

Polycomb

(S2 cells)

Polycomb

(Bg3 cells)

A subset of 4th genes is associated with Polycomb; these domains are permissive for reporter expression

(red eyed fly).

How are specific domains targeted for heterochomatin formation?

Repetitious elements such as 1360 are targeted by a mechanism involving the RNAi system3

hsp70-white 5’P3’P FRT FRT1360

Analysis of chromosome 4 identified 1360 as a target. P[1360, hsp70-w] silencing depends on the

reporter’s position in the genome

Haynes et al 2006 Curr Biol 16: 2222

FLP-mediated removal of 1360 results in loss of silencing

+ FLP recombinase

hsp70-white 5’P3’P FRT FRT1360 hsp70-white 5’P3’P FRT

1360 excised

FLP14 FLP16 FLP4 FLP5

Haynes et al 2006 Curr Biol 16: 2222

Mutations in RNAi components impact PEV

spn-E (hls)

Dmp68/LipdFMR1

Results from Birchler, Elgin, Schedl, others;note also esiRNA pathway, Siomi, Hannon & others

y w; 118E-10/+

y w; 39C-12/+

piwi aub hls

Mutations in RNAi components piwi, aubergine and homeless suppress PEV

y w; 118E-10/+

y w; 39C-12/+

+aubQC42

+hls∆125

B Leibovitch in Pal-Bhadra et al, 2004

A tentative model for heterochromatin targeting –HP1a – PIWI interaction (piRNA)

transposon

Aub piRNA

PIWI Heterochromatin

PIWI (binds piRNA) is an argonaute family member and nuclear protein which interacts with HP1a

Cytoplasm

Nucleus

The Evolution of a Heterochromatic Domain in Drosophila: Investigating the Strange Dot Chromosome...

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