Explain epigenomics at the genome scale

6.5.9 · Biology › Systems Biology & Frontiers

Epigenomics HAI KYA?

Genome-wide kyun matter karta hai: Ek liver cell aur neuron ka DNA identical hota hai, lekin unke epigenomic landscapes bilkul alag hote hain. Sirf poora pattern dekhne se hi hum cell identity, development, aur disease ko samajh sakte hain.

Epigenomic Marks ke Teen Pillars

1. DNA Methylation (5-methylcytosine, 5mC)

   • KAHAN: CpG dinucleotides mein cytosines (~28 million sites in human genome)
   • KYA: Cytosine base par methyl group (CH₃) add hota hai
   • EFFECT: Promoters mein hone par generally genes ko silence karta hai

2. Histone Modifications (100+ types)

   • KAHAN: Histone protein tails par (8 histones per nucleosome)
   • KYA: Acetylation, methylation, phosphoryl ubiquitination
   • EFFECT: Chromatin structure ko open ya close karta hai

3. Chromatin Accessibility

   • KAHAN: Woh regions jahan DNA histones se unwind hoti hai
   • KYA: Transcription factors ke liye physical access
   • EFFECT: Active regulatory regions ko mark karta hai

Epigenome Map Kaise Karte Hain: Key Technologies

1. Whole-Genome Bisulfite Sequencing (WGBS)

THE PRINCIPLE: Unmethylated cytosines bisulfite treatment mein uracil mein convert ho jaate hain; methylated cytosines nahi hote.

DERIVATION FROM SCRATCH:

Starting point: DNA mein do cytosine states hoti hain

• Unmethylated C
• 5-methylcytosine (5mC)

Step 1: Bisulfite chemistry $\text{Bisulfite}+\text{C}\to \text{U (uracil)}$ $\text{Bisulfite}+\text{5mC}\to \text{5mC (protected)}$

Step 2: PCR amplification U ko T se replace karta hai $\text{U}\stackrel{\text{PCR}}{\to }\text{T}$

Step 3: Sequencing reads

• Original unmethylated C → T ke roop mein reads hota hai
• Original methylated 5mC → C ke roop mein reads hota hai

Step 4: Har CpG par methylation level calculate karo $\beta \text{-value}=\frac{\text{Methylated reads (C)}}{\text{Methylated reads (C) + Unmethylated reads (T)}}$

Range: 0 (completely unmethylated) se 1 (fully methylated) tak

YE KYU KAAM KARTA HAI: Chemical discrimination ek sequence signature create karti hai jo hum standard sequencing se padh sakte hain.

Solution: $\beta =\frac{80}{80+20}=\frac{80}{100}=0.80$

Interpretation: 80% cells mein ye CpG methylated hai. Ye likely ek partially silenced promoter ya mixed cell population hai.

YE STEP KYU? Ratio directly sample mein methylated DNA molecules ki proportion ko reflect karta hai.

2. ChIP-seq for Histone Marks

THE PRINCIPLE: Antibodies specific histone modifications + unke bound DNA ko capture karti hain; sequencing genome-wide locations reveal karta hai.

DERIVATION:

Step 1: Proteins ko DNA se cross-link karo (formaldehyde) $\text{Histone-DNA complex}\stackrel{{\text{CH}}_2\text{O}}{\to }\text{Covalent bonds}$

Step 2: Chromatin ko sonicate karo → 200-500 bp fragments

Step 3: Specific antibody ke saath immunoprecipitation $\text{H3K27ac antibody}+\text{All fragments}\to \text{H3K27ac-bound fragments only}$

Step 4: Cross-links reverse karo, DNA sequence karo

Step 5: Reads ko genome par map karo → enrichment peaks = modification sites

SIGNAL QUANTIFY KARNA: $\text{Enrichment}={\log }_2\left(\frac{\text{ChIP reads}}{\text{Input control reads}}\right)$

Enrichment >2 (4-fold increase) wale peaks = true modification sites.

YE KYU KAAM KARTA HAI: Antibody molecular filter ki tarah kaam karti hai—sirf woh DNA capture hota hai jo target histone modification se attached hai.

Hum kya seekhte hain:

1. Ye region ek active enhancer hai (H3K27ac active regulatory elements ko mark karta hai)
2. Gene XYZ is cell type mein likely high expression hai
3. Is region ko disturb karne se XYZ expression reduce hoga

YE INFERENCE KYU? H3K27ac, active enhancers par recruit hone wale p300/CBP acetyltransferases dwara catalyzed hota hai. Spatial proximity regulatory control suggest karta hai.

3. ATAC-seq for Chromatin Accessibility

THE PRINCIPLE: Hyperactive Tn5 transposase sirf open chromatin mein sequencing adapters insert karta hai.

MECHANISM:

Step 1: Tn5 ko sequencing adapters ke saath load karo $\text{Tn5}+\text{Adapter DNA}\to \text{Tn5-adapter complex}$

Step 2: Tn5 accessible DNA ko cut karta hai aur adapters insert karta hai ("tagmentation") $\text{Open chromatin}\stackrel{\text{Tn5}}{\to }\text{Adapter-flanked fragments}$

Step 3: PCR amplify karo aur sequence karo

Step 4: Reads map karo → peaks = accessible regions

ENRICHMENT CALCULATION: $\text{Accessibility score}=\frac{\text{Reads in region}}{\text{Total reads}}\times 10^6$ (Reads per million, RPM)

YE KYU KAAM KARTA HAI: Nucleosome-free regions (jaise active promoters/enhancers) Tn5 ko access dete hain; closed chromatin physically block hoti hai.

Analysis:

• Peak presence → nucleosome-depleted region (NDR)
• Location (-200 bp) → likely promoter
• Peak height (RPM = 500) → highly accessible

Prediction: Is cell type mein gene ABC actively transcribed hai. Transcription factors is promoter se bind kar sakte hain.

YE KYU MATTER KARTA HAI? 98% active promoters ATAC-seq peaks dikhate hain. Ye ek functional signature hai.

Epigenomic Landscape: Integration

The Histone Code: Combinations Define Function

Alag histone marks distinct chromatin states create karte hain:

CHROMATIN STATE KI DERIVATION:

Agar hum ek locus par marks M₁, M₂, .., Mₙ observe karte hain, toh chromatin state S hai: $S=f(M_1,M_2,\dots ,M_n)$

Example: Promoter with H3K4me3 + H3K27ac + accessible chromatin: $S=f(\text{H3K4me3},\text{H3K27ac},\text{ATAC})=\text{Active Promoter}$

Promoter with H3K4me3 + H3K27me3 (dono!): $S=f(\text{H3K4me3},\text{H3K27me3})=\text{Bivalent/Poised}$

BIVALENT DOMAINS KYU? Stem cells developmental genes ko poised rakhte hain—differentiation ke dauran quickly activate YA silence hone ke liye ready.

Genome-Wide Analysis: The Numbers

Human genome epigenomics at scale:

$\text{Total CpG sites}\approx 28\times 10^6$ $\text{Regulatory elements (ENCODE)}\approx 4\times 10^5\text{ to }10^6$ $\text{Cell-type-specific enhancers}\approx 5\times 10^4\text{ to }10^5\text{ per cell type}$

COVERAGE CALCULATION:

30× coverage par WGBS ke liye: $\text{Reads needed}=3\times 10^9\text{ bp}\times 30=9\times 10^{10}\text{ reads}$ $\text{Data size}\approx 9\text{ TB raw data}$

ITNA DATA KYU? 28 million CpGs mein se har ek ko methylation accurately measure karne ke liye 30 reads chahiye. Isliye epigenomics ko computational infrastructure ki zaroorat hoti hai.

Findings:

1. DNA hypermethylation tumor suppressor promoters par

   • MLH1 methylation: Normal β = 0.05 → Cancer β = 0.85
   • Effect: DNA repair gene silence hota hai → microsatellite instability

2. Histone mark changes

   • Differentiation genes ke enhancers par H3K27ac ka loss
   • Unhi enhancers par H3K27me3 ka gain
   • Effect: Cells apni differentiated identity kho dete hain

3. Chromatin accessibility

   • Oncogene enhancers par naye ATAC peaks (MYC, KRAS)
   • Effect: Oncogenes hyperactive ho jaate hain

DIFFERENTIAL METHYLATION QUANTIFY KARNA: $\Delta \beta ={\beta }_{\text{cancer}}-{\beta }_{\text{normal}}=0.85-0.05=0.80$

Koi bhi Δβ > 0.25 significantly hypermethylated maana jaata hai.

YE PATTERN KYU? Cancer cells ko tumor suppressors silence karne AUR growth genes activate karne ki zaroorat hoti hai. Epigenomic reprogramming dono kaam DNA mutate kiye bina karta hai—isliye ye ek therapeutic target hai (epigenetic drugs ise reverse kar sakte hain).

YE SAHI KYUN LAGTA HAI: Zyaadatar examples promoter methylation se genes silence hote dikhate hain (jaise tumor suppressors mein).

Haqeeqat ye hai:

• Gene bodies mein methylation actually active transcription ke saath correlate karti hai
• Enhancers ki methylation context ke hisaab se activity increase YA decrease kar sakti hai
• Sirf promoter CpG island methylation reliably silence karti hai

STEEL-MANNING: Ye idea sabse common pattern ko capture karta hai (promoter silencing), jo X-inactivation, imprinting, aur tumor suppression ke liye crucial hai. Lekin biology methylation ko ek context-dependent mark ki tarah use karti hai.

THE FIX: Hamesha specify karo KI methylation KAHAN hoti hai: "Promoter hypermethylation genes silence karta hai; gene body methylation active transcription mark karta hai."

YE TEMPTING KYU HAI: Peaks 80% time gene expression ke saath correlate karte hain.

The trap: Correlation causality nahi hoti. Peak ho sakta hai:

• Transcription ka ek consequence ho (active transcription chromatin ko open rakhta hai)
• Ek inactive enhancer ho (accessible hai lekin activators se bound nahi)
• 3D chromatin looping ke zariye ek door gene ko regulate kar raha ho

THE FIX: ATAC-seq ko combine karo:

• Active marks ke liye ChIP-seq (H3K27ac) → confirm karta hai ki enhancer active hai, sirf open nahi
• Hi-C ya ChIA-PET → physical enhancer-promoter contacts map karta hai
• CRISPR deletion → test karta hai ki gene expression ke liye enhancer required hai ya nahi

Epigenomics poori library ka ek MAP banana jaisa hai jo dikhata hai:

• Kaunse sections locked hain (promoters par methylation)
• Kaunsi books mein bookmarks aur sticky notes hain (histone modifications)
• Kaunse aisles mein wide-open doors hain (accessible chromatin)

Ek brain cell aur ek liver cell dono ke paas same library card hai (same DNA), lekin bilkul alag maps! Brain cell mein "neuron books" section wide open hai aur tons of bookmarks hain, jabki liver cell mein wahi books locked hain.

Scientists special chemicals use karte hain (jaise DNA methylation ke liye bisulfite) taaki genome mein millions of spots par ek saath sabhi modifications ko "tag" kiya ja sake. Ye aise hai jaise ek highlighter use karo jo sirf UV light mein bookmarks par glow kare—phir hum poori library ko upar se photograph karte hain taaki pattern dekha ja sake.

Jab kuch galat ho jaata hai (jaise cancer), toh ye aise hota hai jaise kisi ne randomly important books lock kar di hain aur woh books unlock kar di hain jo sikhati hain "bina ruke kaise grow karein." In changes ko map karke, doctors figure out kar sakte hain ki kaunse locks ko special drugs se dobara khola jaana chahiye.

ChIP-seq memory: Capture Histones with Immuno-Precipitation, then sequence

WGBS memory: Whole Genome Bisulfite Sequencing—bisulfite Bare Cytosines ko Thymines mein badalta hai

Connections

• DNA Structure and Replication - epigenetic marks bases/histones ko modify karte hain sequence change kiye bina
• Gene Expression Regulation - epigenomics genome scale par regulatory code reveal karta hai
• Chromatin Structure - histone modifications aur accessibility chromatin states define karti hain
• Cell Differentiation - epigenomic reprogramming cell fate decisions drive karta hai
• Cancer Biology - epigenomic dysregulation cancer ka ek hallmark hai
• CRISPR and Genome Editing - epigenome editing (dCas9-DNMT, dCas9-TET) targeted modification enable karta hai
• Stem Cells - bivalent domains developmental genes ko poised rakhte hain
• Evolution and Comparative Genomics - epigenomic variation phenotypic diversity mein contribute karta hai

#flashcards/biology

Epigenomics kya hai aur ye epigenetics se kaise alag hai? :: Epigenomics sabhi chromosomes mein ek saath sabhi epigenetic modifications (DNA methylation, histone modifications, chromatin accessibility) ka genome-wide study hai. Epigenetics single genes ka study karti hai; epigenomics cell identity aur function samajhne ke liye 3 billion base pairs mein complete regulatory landscape examine karta hai.

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