ABCT 101 · L4 · Genomics & Bioinformatics

📚 Lecture 4 · Genomics & Bioinformatics Prof. Quanhua Mu · 23 Sep 2026

Genomics asks three questions — read, understand, edit. Every platform, file format and tool belongs to exactly one of them.

101 · L4 Genomics & Bioinformatics — From Reads to Gene Therapy

Master Logic · 主線邏輯

Master Logic · 主線邏輯基因組學三問 —— Read / Understand / Edit。本講每個平台、檔案格式與工具都恰好屬於其中一問;記住三問,工具清單就變成地圖。
Genomics asks three questions — read, understand, edit. Every platform, file format and tool in this lecture belongs to exactly one of them; keep the three questions in mind and the tool zoo becomes a map.

Lecture 4 · Genomics & Bioinformatics (Prof. Quanhua Mu)

The fourth lecture of ABCT5101, delivered by Prof. Quanhua Mu (ABCT, quanhua.mu@polyu.edu.hk). This is the lecture that maps most directly onto sequencing work as it is actually done — which platforms, which file formats, which software, and what the data can and cannot tell you.

The lecture opens by asking what bioinformatics can answer. The five questions on the slide are worth remembering because each one maps to a technique introduced later in the deck:

Question askedTechnique it maps to
Where did human beings originate?Phylogenetics, comparative genomics
Why do monkeys have long tails while humans do not?Comparative genomics, gene annotation
Why do some people get sickle cell anemia?Germline variants, single-gene inheritance
How can we diagnose Down syndrome?Copy number variation, non-invasive screening
What are the drivers of chronic myeloid leukemia, and how can we treat it?Gene fusions, targeted therapy

Genomics = three questions: read, understand, edit

Every genomics tool answers one of three questions: read / understand / edit.

The lecture defines genomics as the study of genomes, organised around exactly three questions. Everything in the rest of the session sits under one of these three:

  1. What is the sequence of the genome? — to read
  2. What does the genome sequence mean? — to understand
  3. How do we modify the genome sequence? — to edit
Structure of the lecture Read (part 1) covers sequencing technology, FASTQ, assembly and alignment. Understand (part 2) covers annotation, variants, CNV, fusions and their clinical meaning. Edit (part 3) covers CRISPR and approved gene therapies. If you can place a fact into one of these three, you can place it into the lecture.

The Human Genome Project and the cost collapse

HGP: 3 Gb, $3B, 13 years; now a genome costs a few hundred dollars — the cost collapsed faster than Moore′s law.

FactFigure
Scale of the genome3 billion base pairs (3 Gb)
Cost of the Human Genome Project3 billion US dollars
Where the result was publishedNature, 2001

The lecture then puts up a single slide titled "Sharp decrease of sequencing cost" — and that slide is the whole economic story of this field. The $3 billion first genome is the reference point; everything since is a fall from it.

Real-world hook The $3B / 3 Gb pairing is the single most quoted number pair in genomics, and it is also a business fact: a $3 billion entry cost is a government project; a $1,000 entry cost is a clinical product. Every business model in this industry — NIPT, tumour panels, population screening — exists because of the fall between those two numbers.

Sequencing technologies — and how to choose

Choose by read length vs accuracy: Illumina short/accurate; PacBio/Nanopore long/noisier.

TechnologyPosition in the lecture
Sanger sequencingThe original method; still the accuracy reference
Next generation sequencing (NGS)Short-read, massively parallel — the workhorse
PacBio long-read sequencingLong reads
Nanopore long-read sequencingLong reads
BGI DNB-SeqNamed on the "features of different sequencing technologies" slide
BGI Cyclone-seqNamed on the same slide

The lecture does not stop at listing platforms. It sets a discussion exercise: "Choose the most proper sequencing platform(s) — discuss with your neighbour classmate." And then it supplies the answer that matters in practice:

Exam focus "In reality, one sequencing project may combine multiple sequencing techniques." The exam-level point is not that one platform is best — it is that platform choice is driven by the question, and real projects mix technologies.

The FASTQ file and the quality check

FASTQ: 4 lines per read; line 4 must equal line 2 in length; Q30 = 99.9%.

Whatever platform produced the data, the raw output has to be read. The lecture walks through the FASTQ format field by field — four fields per record:

FieldContent
1Begins with a @ character, followed by a sequence identifier and an optional description
2The raw sequence letters
3Begins with a + character, optionally followed by any description
4Encodes the quality values for the sequence in Field 2, and must contain the same number of symbols as letters in the sequence

The tool named for the first look at this file is FastQC — described on the slide simply as "a simple and useful quality check tool".

Two workflows: de novo vs re-sequencing

De novo assembly (no reference) vs re-sequencing (map to a reference).

The lecture contrasts the two standard pipelines. They share a beginning and diverge at the middle:

De novo genome sequencingRe-sequencing against a reference
StepsSequencing → reads → QC and cleaning → genome assembly → genome sequence → gene annotation → further analysisSequencing → reads → QC and cleaning → mapping → variant detection → gene prediction → further analysis
Typical use on the slideSequencing the genome of a new speciesSequencing the genome of a human tumour

De novo assembly is framed by two concepts the slides name: K-mer and De Bruijn graph-based assembly. The assembler table is worth memorising because read length is the selection criterion:

AssemblerRead typeBest suited to
SOAPdenovoNGS (short reads)Multiple species
SPADESNGS (short reads)Mostly prokaryotes
MEGAHITNGS (short reads)Mostly metagenomes
FlyeTGS (long reads)Long-read assembly
canuTGS (long reads)Long-read assembly

Reference genomes and the file formats around them

References (hg19/hg38) and the format chain: FASTQ → SAM/BAM → VCF.

ItemWhat the slide says
FASTAFiles usually end in .fasta, .fa, .fna, .faa — e.g. hg19.fa
Human genome assembliesThe lecture distinguishes old (and rarely used) versions from current versions
Where to get themUCSC (hgdownload.soe.ucsc.edu, hg38 bigZips) and NCBI (GCF_000001405.26)
Other speciesUCSC downloads page and NCBI datasets

Comparing genomes: phylogenetics, SNP and GWAS

Comparing genomes: phylogenetics (relatedness), SNP (single-letter differences), GWAS (trait associations).

  • Phylogenetics — comparing genomes across species. The lecture's worked example is the phylogeny of SARS-CoV-2, which is also a reminder that a phylogeny is built from sequence, not from phenotype.
  • SNP and genomic diversity — single nucleotide polymorphisms as the substrate of human variation (cited to Science, 2008).
  • GWAS — genome-wide association studies, comparing a case cohort against a control cohort. The slide points to the EBI GWAS catalog as the reference database.

From genome to gene: annotation

Annotation turns coordinates into genes: SnpEff, ANNOVAR.

The lecture makes a historical point that is easy to miss: annotating genes in model organisms started long before NGS existed. Researchers used genetic mapping to locate genes in organisms such as the fruit fly. Genes discovered in that era were mostly named after their function, usually by whoever found them — and some have funny names. The slide's editorial note: gene names are important, and if you discover a new gene you get to propose a name.

  • Browsing genes — the lecture uses IGV (Integrative Genomics Viewer, igv.org) to view alignment and genome data, and GeneCards for gene records (the worked examples are PD-1 and PD-L1).
  • Annotating a new genome — the method is similarity search: find the most similar gene sequence in an existing database. The tool is BLAST; for large-scale work you run BLAST locally, or use DIAMOND.
  • Published annotations — NCBI datasets and the Ensembl FTP server (ftp.ensembl.org/pub/current_gtf/). Human annotation is distributed as GTF and GFF3 from Ensembl and GENCODE.
Exam focus — the three numbers Human genome: ~3 Gb. Protein-coding genes: ~20,000. The coding genome: ~50 Mb. Note what that implies — the coding portion is roughly 1.6% of the genome, which is exactly why non-coding variation is the hard problem later in the lecture.

The lecture's light relief on this topic is a recent paper on "the lost tail of the human" (Nature, 2024) — the molecular answer to the monkey-tail question it opened with.

How much of the genome you actually sequence: WGS, WES, targeted

Breadth vs depth: WGS widest/shallowest; panel narrowest/deepest/cheapest.

ApproachWhat is captured
Whole genome sequencing (WGS)Everything — introns, exons and the space between
Whole exome sequencing (WES)The exons only
Targeted sequencingA defined set of regions, isolated by target enrichment before sequencing

This is the commercial core of the deck: the choice between WGS, WES and a targeted panel is simultaneously a scientific decision, a cost decision and a product decision.

Alignment and variant calling

Align (BWA/minimap2) → call variants (GATK germline; Mutect2 somatic).

StepTools / formats named
DNA alignmentBWA, BOWTIE2, MINIMAP2
Viewing alignment resultsFile formats SAM and BAM

Variants are then classified by origin — germline, de novo and somatic — and the lecture uses the inheritance of sickle cell disease as the worked example of a germline single-gene disorder.

Types of genomic variation named on the slide: mutations, insertions, SNVs, deletions, tandem repeats and aneuploidy.

Detection methods are split into germline and somatic, and then into four methodological families: heuristic methods, statistical methods, deep learning methods and graph models.

What variants actually do: coding vs non-coding

Coding mutations are interpretable; non-coding mostly unknown (TERT promoter the exception).

Coding mutationsNon-coding mutations
EffectPredictable from the reading frame — missense changes, and indels which are often frameshiftingMost non-coding mutations' functions remain unknown, with very few exceptions
Named exception—TERT promoter mutation, found in many cancer types
Annotation toolsSnpEff, ANNOVAR—
The asymmetry to remember Coding variation is largely solvable — a frameshift is a frameshift. Non-coding variation is largely unsolved. That gap is why the ~50 Mb of coding genome gets most of the attention while representing ~1.6% of the sequence.

Two further concepts are introduced here:

  • Tumour mutation burden (TMB) — the number of mutations in a tumour, cited to Nature 2013 and NEJM 2017. This is a biomarker, and therefore a clinical decision point.
  • The two canonical cancer genes — and the lecture's contrast is the cleanest way to remember loss-of-function vs gain-of-function:
    GeneClassMutation type
    TP53Tumour suppressing geneLoss of function
    KRASOncogeneGain of function

Risk alleles, tandem repeats, and the EGFR T790M case

Tandem repeats: HTT <27 normal, 40+ disease; T790M is a resistance mutation.

  • Risk allele frequencies and effect sizes — the lecture plots the relationship, which is the reason common variants usually have small effects and rare variants can have large ones.
  • Case study: germline EGFR T790M and lung cancer risk (cited to Science, 2026) — a germline variant normally associated with resistance, appearing here as a susceptibility factor.
  • Tandem repeat expansions — worked example HTT (Huntington's disease): <27 repeats = normal, 40+ repeats = disease (cited to Lancet, 2007). Note the shape of this: the same locus, the same mechanism, and the phenotype decided purely by repeat count.

Copy number and structural variation

CNV by read depth: log2 ratio 0 = same, +1 = double, −1 = half.

Copy number variation (CNV) is where the lecture connects genomics to diagnostics. The key technical definition on the slide is the measurement itself:

Definition to hold onto Log2 ratio = the normalized read depth in the case sample over the control sample. CNV calling is, at bottom, a read-depth computation — which is why it works on any sequencing platform and why normalisation is where it goes wrong.
Citation on the slideWhy it is there
Lo et al., Lancet 1997; BMJ 2011The foundational work on cell-free DNA in maternal plasma — the line of work behind non-invasive prenatal testing, which is the answer to the lecture's own opening question about diagnosing Down syndrome
TCGA, 2013Large-scale cancer copy number landscape
Nature Genetics, 2022Later CNV analysis

Structural variations are treated as a separate class (cited to Chromosome Research, 2022).

Gene fusions — the clearest drug-targeting success story

Fusions come from translocation; BCR-ABL1 → imatinib is the cleanest drug-target story.

The lecture gives translocation / gene fusion its own multi-slide block, and the reason is commercial: this is where a genomic finding becomes a drug.

  • The illustration is Imatinib (Gleevec®) — a fusion-driven cancer treated by inhibiting the fusion product.
  • Fusions are classified by tissue context: haematological, soft tissue and epithelial cancers — and within epithelial cancers the slide marks the range as rare to common (cited to Genome Medicine, 2015).
  • For targeting, the lecture points to OncoKB as the actionable-gene reference.
Detection approachNamed methods
Sequencing-basedINTEGRATE, STAR-fusion, ARRIBA, CICERO — and "many others"
Non-sequencingPCR-Sanger sequencing, nucleotide hybridisation
Real-world hook Gleevec is the case every biopharma person should be able to tell from memory: a genomic rearrangement defines a patient subgroup, a molecule is designed against the product of that rearrangement, and the result turns a fatal leukaemia into a managed condition. It is the template that every "targeted therapy" pitch since has been copied from — and the reason companion diagnostics exist as a business.

What this lab contributed

This lab: MET exon 14 skipping (Cell 2018); MGMT fusion (Nat Commun 2020).

The lecture closes the variant section with two slides headed "Our works", citing Cell 2018 and Nature Communications 2020 — the MET exon 14 skipping finding in secondary glioblastoma and the MGMT gene fusion driving temozolomide resistance respectively. Whether or not this appears in an exam, it tells you something useful: the lecturer works on exactly the kind of problem this lecture describes.

Genome editing and gene therapy

Untargeted vs targeted editing; gene therapy $0.85M–$4.25M — one-time cures.

The final block answers the third genomics question — how do we modify the genome.

ConceptMeaning
Untargeted mutagenesisRandom damage — no control over where
Targeted genome editingCutting or changing a chosen site
CRISPR-based genome editingThe mechanism behind targeted editing
Designing the guidessgRNA design — the slide points to the Broad Institute's CRISPick portal

Then the lecture lands on the commercial endpoint: approved gene therapies, with prices. This table is worth knowing in full, because it is the clearest available picture of what a one-time genomic cure costs.

ProductManufacturerPrice (USD)DiseaseGene
ADSTILADRINFerring—Bladder cancerIFNα2b
BEQVEZPfizer$3.5 millionHaemophilia BF9
CASGEVYVertex$2.2 millionSickle cell diseaseBCL11A
ELEVIDYSSarepta Therapeutics$3.2 millionDuchenne muscular dystrophyDMD
HEMGENIXCSL Behring$3.5 millionHaemophilia BF9
LENMELDYOrchard Therapeutics$4.25 millionMetachromatic leukodystrophy (MLD)ARSA
LUXTURNASpark Therapeutics$0.85 millionRetinal dystrophyRPE65
LYFGENIAbluebird bio$3.1 millionSickle cell diseaseHBB
ROCTAVIANBioMarin$2.9 millionHaemophilia AF8
SKYSONAbluebird bio$3 millionCerebral adrenoleukodystrophy (CALD)ABCD1
VYJUVEKKrystal Biotech—Dystrophic epidermolysis bullosa (DEB)COL7A1
ZYNTEGLObluebird bio$2.8 millionβ-thalassaemiaHBB
ZOLGENSMANovartis$2.1 millionSpinal muscular atrophy (SMA)SMN1
Read the price column as a list of business models The range runs from $0.85M to $4.25M for a single treatment, and roughly half the list sits near $3M. Note also that bluebird bio appears three times and F9 appears twice (BEQVEZ and HEMGENIX, both haemophilia B, both ~$3.5M) — a reminder that the same gene can be attacked by competing products, and that a one-time cure has to be priced against a lifetime of factor replacement.

Exam focus — the numbers and contrasts worth memorising

ItemValue
Human genome size~3 Gb
Protein-coding genes~20,000
Coding genome~50 Mb
Human Genome Project cost$3 billion (published Nature 2001)
HTT repeats<27 normal / 40+ disease
CNV statisticLog2 ratio = normalised read depth, case over control
TP53 vs KRASTumour suppressor, loss of function  vs  oncogene, gain of function
Germline vs somatic vs de novoThe three variant origins
WGS vs WES vs targetedEverything  vs  exons only  vs  enriched regions
Short vs long read assemblersSOAPdenovo / SPADES / MEGAHIT  vs  Flye / canu
Alignment toolsBWA, BOWTIE2, MINIMAP2
If you only remember one thing from this lecture Genomics is three questions — read, understand, edit. Every tool, format and citation in the deck belongs to one of the three. When a question confuses you, first ask which of the three it is about.

Note on completeness: this session runs to 78 content slides, and a substantial number are figure or screenshot slides with no extractable text (sequence-trace images, IGV screenshots, CNV plots, pathway diagrams, the sequencing-cost curve). Every citation, tool name and number printed on the slides is reproduced above, so the missing visuals can be traced from those references.

Detail Pass — the fuller tables and figures from the slides

The fuller tables and figures from the slides — the detail layer.

This session is unusually figure-heavy: 79 slides, and a large share of the substance sits in diagrams and tables rather than in body text. The block below recovers material that only becomes visible when the slides are viewed as pages rather than read as text — including one table that is genuinely load-bearing for anyone working in sequencing.

Sequencing platforms — the full comparison table

The full platform table: read length + error type per platform.

The lecture organises every major platform into three generations and compares them on seven axes. This is the reference table of the whole lecture.

TechnologyAvg read lengthRun timeAdvantagesDrawbacksError typesYear
First generation
Sanger~1000~2 hoursLong individual read lengthFirst sequencing technology, and expensiveInsertion · deletion · substitution1977
Second generation
Roche 45470024 hoursLong read lengthHigh error rate and expensive runsInsertion · deletion2005
Illumina / Solexa2 × (36 to 100) paired-end6 days (2 days in rapid mode)Low cost and low error rateLong run timeSubstitution2006
AB SOLiD858 daysLow cost per base and low error rateVery long run timeSubstitution2007
Helicos Biosciences55Depending on read lengthLow error rateLong run timeInsertion · deletion2009
Ion Torrent4002 hoursShort run time and less expensive equipmentHigh error rateInsertion · deletion2010
Third generation
Pacific Biosciences3000 (up to 15000)20 min to 4 hoursLongest read length and fast run timeHigh error rateInsertion · deletion2010
Oxford Nanopore (MinION)9545< 6 hoursLong read; small, low-cost USB deviceHigh error rateInsertion · deletion · substitution2014
Exam focus — the pattern in this table Read the columns as a trade-off story, because the exam question will be about the trade-off, not one cell:
Read length goes 1000 (Sanger) → 55–700 (second gen) → 3000–9545 (third gen).
Run time goes 2 hours → 8 days → under 6 hours.
Error type is the field that separates the families: the Illumina/Solexa and AB SOLiD platforms make substitution errors, while every other platform listed makes insertion–deletion errors. Sanger and Oxford Nanopore make all three.
Cost is the second-generation advantage — "low cost and low error rate" is why short reads won the market.
Where BGI sits in this table The slide does not put BGI in the rows — it places two labels to the side of the table: BGI DNB-Seq beside the second-generation block, and BGI Cyclone-seq beside the third-generation block. Read the layout and the message is clear: DNB-Seq is positioned as a short-read second-generation technology and Cyclone-seq as a long-read third-generation technology. That generational placement is the point the slide is making.

Platform choice — the five practice scenarios

Platform choice by scenario: match the question to read length and accuracy.

The lecture then sets a worksheet: for each project below, state which platform(s) you would propose and why. This is the discussion exercise, and these are the five scenarios.

ProjectWhat the question is really testing
De novo sequencing and assembly of a microbial genomeSmall genome, no reference → assembly is required, and long reads resolve repeats
Sequencing a plasmidVery small, known backbone → the cheapest sufficient method wins
Re-sequencing a human genome (e.g. a cancer sample) to look for novel mutationsLarge genome, reference exists, low-frequency variants matter → depth and accuracy over read length
Rapid diagnosis of a viral infection by sequencing in the fieldSpeed and portability dominate → run time and instrument size decide it
Targeted amplicon / exome sequencingDefined regions → enrichment first, then high depth on a small footprint
The answer the slide supplies After the worksheet, the slide states the conclusion directly:
"In reality, one sequencing project may combine multiple sequencing techniques."
So the correct exam answer is never "platform X" on its own. It is which combination, and what each part is doing.

Variant detection methods — the tool matrix

The matrix: heuristic / statistical / deep-learning / graph × germline / somatic.

Variant calling is organised two ways at once: by the statistical approach, and by the variant origin. The lecture supplies the tool names for each cell.

Method familyGermline mutationsSomatic mutations
Heuristic—Varscan
Statistical methodsGATK-HaplotypeCaller, Samtools, FreeBayes, Platypus, OctopusScomatic, Mutect2, Octopus, Vardict
Deep learning methodsDeepVariant, Strelka2Strelka2
Graph modelsDragen, Pangenie—
Exam focus — two things to notice 1. The cells are not symmetric. Two tools appear on both sides (Octopus and Strelka2), but the heuristic row is empty for germline and the graph-model row is empty for somatic. Germline and somatic calling are different problems, which is why the tool lists differ.
2. The families are historical layers. Heuristic → statistical → deep learning → graph models is roughly the order the field moved. Note that DRAGEN, the graph-model entry here, is also the bioinformatics engine bundled with Illumina instruments — a reminder that "which caller" is partly decided by "which machine".

Coding mutations — the full classification

Synonymous / missense / nonsense / frameshift — know the consequence of each.

This is the slide with the worked example, and it defines four mutation outcomes from one codon. Starting from the original sequence 5'-AAG-3' / 3'-TTC-5', transcribed to mRNA 5'-AAG-3', translated to Lys:

Mutated DNAMutated mRNAProtein resultClassification
AAAAAALys (unchanged)Synonymous / silent
TAGUAGSTOPNonsense (a subset of non-synonymous)
AGGAGGArgNon-synonymous → missense
ACGACGThrNon-synonymous → missense

The slide draws the classification as a hierarchy, which is the cleanest way to remember it:

CategoryContains
Synonymous / silentThe codon changes but the amino acid does not
Non-synonymous→ Nonsense (creates a premature STOP, truncating the protein)
→ Missense (substitutes a different amino acid)
Indels are a different mechanism — hold the two apart The slide treats this on a separate panel, under the heading "Indels are often frameshifting." The illustration: a normal sequence reading as three distinct codons (ATG CAT TGC), then a single T inserted — and the note reads "Reading frame has been changed."
Substitutions change one codon. Indels change every codon downstream. That is why the consequence of a one-base insertion is usually worse than a one-base substitution.

One slide earlier, the lecture pairs the central dogma (DNA → transcription → RNA → translation → protein) with the full genetic code table, with start and stop codons marked — the reference you need to read the table above.

Genomic variation — the complete taxonomy

Complete taxonomy: SNV, indel, CNV, fusion + chromosomal-level events.

The lecture's variation slide separates two levels, and it is worth learning the separation because exam questions tend to test one level at a time.

LevelTypes
Sequence level (the slide labels these "mutations", circled together)Insertion · SNP · Deletion
/ (shown on the same sequence line)Tandem repeats — the slide's example is a run of CAG repeats
Chromosome level (seven distinct types, each illustrated)Aneuploidy · Deletion · Insertion · Duplication · Inversion · Translocation · Extrachromosomal DNA
The one that is usually forgotten Extrachromosomal DNA is the seventh chromosome-level type and the one most likely to be missed — it is shown as small circular DNA elements sitting outside the chromosomes. Note that deletion and insertion appear at both levels: the same word describes a base-level change and a chromosome-segment change. Read the axis before answering.

Gene fusions — the historical map, 1914 to 2014

1914 Boveri → 1960 Philadelphia chromosome → 2014: a century of fusions.

Rather than listing fusions, the lecture lays them on a century-long timeline and colour-codes them by tissue type — purple = haematological, green = soft tissue. The colour coding is the finding: the same mechanism recurs across unrelated tissues.

EraFusions on the slide
1914 — the conceptBoveri, mitotic aberrations
1960 — the founding observationPhiladelphia chromosome (CML)
1960s–70s — cytogeneticst(9;22) CML · t(8;21) CML · t(15;17) APL · t(8;14) Burkitt's lymphoma
1980s–90s — molecular cloningt(11;22) Ewing's sarcoma · BCR-ABL1 CML · PAX3-FKHR alveolar rhabdomyosarcoma · AML1-X AML · ATF-EWS clear cell sarcoma · FUS-ERG AML · PML-RARA APL · CHOP-FUS liposarcoma
2000s–2014 — systematic discoveryMLL-X MLL · FLI1-EWS Ewing's sarcoma · ETV6-NTRK3 Ph-like ALL · ERG-EWS Ewing's sarcoma · SYT-SSX synovial sarcoma · NPM-ALK anaplastic lymphoma · PDGFb-TEL CML · NAB2-STAT6 solitary fibrous tumour
Exam focus Three fusion names carry disproportionate weight because each is attached to a drug or a diagnostic: BCR-ABL1 (CML → Imatinib), PML-RARA (APL), and ETV6-NTRK3 (Ph-like ALL → the NTRK inhibitor story). If you memorise only three, take those.
Also note the direction of travel: 1914 the mechanism was a hypothesis about mitotic errors; by 2014 fusions were being systematically discovered and matched to drugs.

CNV in the clinic — Down syndrome as the worked example

NIPT: Lo et al. found fetal cell-free DNA in maternal plasma → Down syndrome screening.

The lecture answers its own opening question ("How can we diagnose Down syndrome?") in the CNV section, and the answer is simply a copy-number change:

  • Trisomy 21 — the karyotype shows three copies of chromosome 21 instead of two. That is the whole molecular definition.
  • The slide pairs the karyotype with a clinical-features panel: small nose, mouth and ears; almond-shaped eyes with a skin fold covering the inner corner; white spots on the coloured part of the eyes; flat, round face; slightly bent fifth finger; short, stocky build.
  • It also carries a caution worth keeping: intellectual disability and developmental delay vary by individual.
Why this example is the hinge of the whole lecture Down syndrome is the case that connects three separate parts of the deck: it is a CNV (aneuploidy), it is what non-invasive prenatal testing screens for — which is the clinical product built on the cell-free DNA work of Lo et al. cited earlier — and it is the reason read-depth measurement (the Log2 ratio) is a clinical-grade technology rather than a research curiosity. One example, three sections.

Method note: the tables above were recovered by rendering the slide pages as images and reading them, because a large share of this deck's content exists only as figures. Where a cell was legible on the slide it is reproduced as written; obvious typographical errors in the original (e.g. a manufacturer name misspelt, "expansive" for "expensive") have been corrected silently so the note is safe to revise from.

T2T-CHM13 — why the human reference genome was incomplete until 2022

The Human Genome Project draft (2001) was never actually finished: roughly 8% of the genome — centromeres, telomeres and other long repeats — could not be assembled with short reads. T2T-CHM13 (2022) is the first truly gapless, telomere-to-telomere human reference.

ReferenceYearStatus
HG19 / GRCh372009Long-standing workhorse; ~8% of the genome missing or collapsed.
HG38 / GRCh382013Improved, still contains gaps and false duplications in repeat regions.
T2T-CHM132022First complete, gapless assembly — centromeres, telomeres and segmental duplications resolved. Needed long reads (PacBio HiFi / Nanopore) to get there.

Why it matters commercially and clinically: the missing 8% is not junk — it is enriched for segmental duplications and repeat regions where structural variation lives, which is exactly where several disease-associated variants sit. Short-read resequencing could not see them.

Gene structure — the order, and what each non-coding end actually controls

Read a gene left to right as promoter → 5′ UTR → exon → intron → exon → 3′ UTR. The two UTRs are transcribed but not translated, and they do different jobs: 5′ controls how efficiently translation starts, 3′ controls how long the message lasts.

RegionTranslated?Function
PromoterNo (not even transcribed)Binding site for RNA polymerase and transcription factors; sets whether and how much transcription happens.
5′ UTRNoControls translation efficiency — the ribosome loads here and scans. Secondary structure or upstream ORFs here throttle protein output.
ExonsYesThe coding sequence that survives into mature mRNA.
IntronsNo (spliced out)Removed by the spliceosome; their boundaries are what splice-site mutations destroy.
3′ UTRNoControls mRNA stability and lifetime — AU-rich elements and miRNA binding sites here set the degradation rate, and therefore how much protein a given transcript yields.

Why this shows up in drug design: mRNA half-life is a design variable, and the 3′ UTR is one of the knobs. It also explains why mRNA level and protein level correlate poorly (the L6 number, r ≈ 0.4): two genes with identical transcript counts can produce very different protein amounts because their UTRs set different translation efficiencies and decay rates.

Three classes of variant — germline, de novo, somatic

The distinction that decides inheritance and who to test: germline is in the egg or sperm and heritable; de novo is new in the child and absent in both parents; somatic is acquired in a body tissue and not passed on at all.

ClassWhere it arisesHeritable?Typical reading
GermlineGamete; present in every cell of the childYesFamilial disease, carrier screening, predictive testing of relatives.
De novoEarly embryo or gametogenesisYes for the child, but parents test negativeExplains an affected child with no family history — the key to many severe dominant developmental disorders.
SomaticA single body cell, then its descendantsNoCancer. Present in the tumour, absent in blood — hence tumour-normal paired sequencing.

The experimental consequence: germline and de novo variants are found in blood or saliva; somatic tumour variants require tumour tissue (or ctDNA). Comparing tumour against the patient′s own germline is what separates a driver from a harmless inherited polymorphism.

Reference databases — which one answers which question

Four names carry almost all of the lookup work: NCBI for sequences and literature, UniProt for protein sequence and functional annotation, KEGG and GO for pathways and functional terms, BLAST/DIAMOND for finding similarity.

ResourceHoldsReach for it when you need
NCBIGenBank sequences, RefSeq, PubMedA nucleotide or protein sequence, or the paper behind it.
UniProtProtein sequences + curated function, domains, PTMs“What does this protein do / where is it / what modifies it?” — the proteomics-side counterpart to NCBI.
GO (Gene Ontology)Controlled vocabulary: molecular function, biological process, cellular componentTurning a gene list into functional themes (enrichment analysis).
KEGGCurated pathway and network maps“Which pathway is this gene set in?”
BLAST / DIAMONDSimilarity search (DIAMOND = faster, for big data)An unknown sequence — find its homologues and infer function.

None of these replaces the alignment and quantification tools covered earlier; they are the annotation layer you query after you have a result.

TBXT and the AluY insertion — why humans have no tail

A 2024 Nature result: an AluY element inserted into an intron of TBXT causes exon 6 to be skipped during splicing, producing a shorter TBXT isoform — and this alternative splicing is what suppressed tail formation in the primate lineage.

Why it is a good exam illustration: it is a non-coding change with a large phenotypic effect, it acts through splicing rather than through the protein sequence itself, and it shows an Alu mobile element doing real evolutionary work instead of being “junk DNA”.

It also mirrors a therapeutic idea already in the notes: if a disease is caused by exon skipping, then forcing or blocking that skip is a drug strategy — the same logic behind antisense oligonucleotides in splice-modulating therapies.

How sequencing depth is calculated (inputs from the slides, formula derived)

Where this comes from The 79-slide deck has no dedicated “depth formula” slide. The inputs are spread over four pages: p4 (human genome 3 Gb), p11 (Illumina read length 2×150), p34 (3 Gb genome / 50 Mb coding), p37 (WGS / WES / targeted coverage picture). The formula below derives the answer from those numbers; it is not a quote from the slides.

Definition and formula

Depth = the average number of times each base is read.
Depth = (number of reads × read length) ÷ target size; conversely, data needed = depth × target size.

SymbolMeaningExample
NNumber of reads (or read pairs)300 M pairs
LRead lengthPE150 → 300 bp per pair
GTarget size (genome / exome / panel)3 Gb, 50 Mb, 1 Mb

Three worked examples

ScenarioCalculationAnswer
Human WGS at 30×30 × 3 Gb = 90 Gb; 90 Gb ÷ 300 bp = 300 M~300 million PE150 pairs
WES at 100× (50 Mb exome)100 × 50 Mb = 5 Gb; ÷ 300 bp ≈ 16.7 M~16.7 million pairs
Panel at 500× (1 Mb)500 × 1 Mb = 0.5 Gb; ÷ 300 bp ≈ 1.67 M~1.67 million pairs
Quick conversion 1 Gb ÷ 300 bp ≈ 3.3 M PE150 pairs. So 30× human WGS = 90 Gb ≈ 300 M pairs; 100× WES = 5 Gb ≈ 17 M pairs.

Common traps

TrapCorrect view
Treating depth and breadth / coverage as the sameDepth is the average; breadth is the fraction of bases covered at least once (30× average still leaves some bases at 0×)
Forgetting on-target rateWES capture is typically 60–80%, so you must sequence 25–60% more to hit the nominal depth
Ignoring duplicate ratePCR duplicates consume effective depth, and the fraction rises with sequencing depth
Comparing platforms by read countConvert to total bases: 300 bp × 100 M = 30 Gb, the same as 150 bp × 200 M

Typical depths to memorise

ApplicationTypical depthWhy
Human WGS (germline)30×Balance point for heterozygous calls
WES100×Small target; needs extra depth to offset capture loss
Cancer panel500–1,000×Tumour heterogeneity, low-VAF variants
Bulk RNA-seq20–30 M reads per sampleEnough for transcript quantification
Single-cell RNA-seq20–50k reads per cellLimited transcripts per cell
三問——讀、懂、改;平台按讀長與錯誤型態選;基因融合是最乾淨的標的。