Module 12: Real-World Clinical Case Studies: Pharmacogenomics (CYP2D6), Rare Diseases & Pathogens

1. Clinical Case 1: Pharmacogenomics of the Complex CYP2D6 Locus

Cytochrome P450 2D6 (CYP2D6) metabolizes approximately $25\%$ of commonly prescribed drugs, including anti-depressants, beta-blockers, tamoxifen, and codeine. Because of a neighboring, $94\%$ identical pseudogene (CYP2D7), the locus undergoes extensive non-homologous recombination, producing gene deletions (*5), duplications (*1xN, *2xN), and hybrid fusion genes (*36-*10, *13, *4):

Short-read sequencing cannot phase these copy-number alterations across the pseudogene. Long reads spanning the entire $28\text{ kb}$ CYP2D6/2D7 locus provide unambiguous star-allele calling using specialized tools like Aldy and Cypiripi.

2. Clinical Case 2: Resolving Pathogenic Tandem Repeat Expansions

Over 50 neurological and neuromuscular disorders are caused by microsatellite repeat expansions that cannot be sized by short-read NGS:

• Amyotrophic Lateral Sclerosis / Frontotemporal Dementia (ALS/FTD): Hexanucleotide $(GGGGCC)_n$ expansion in the non-coding promoter of C9orf72. Healthy individuals carry $< 30$ repeats; affected patients carry hundreds to thousands of repeats ($> 5\text{ kb}$). Long reads directly traverse the repeat, reporting exact repeat count and simultaneous promoter hypermethylation ($5\text{mC}$).
• Fragile X Syndrome: Trimeric $(CGG)_n$ expansion in 5' UTR of FMR1 ($> 200$ repeats causes gene silencing via hypermethylation).
• Huntington Disease: Exonic $(CAG)_n$ expansion in HTT ($> 36$ repeats leads to polyglutamine toxicity).

3. Clinical Case 3: Real-Time Pathogen & Antimicrobial Resistance (AMR) Tracking

Oxford Nanopore MinION sequencers enable real-time field surveillance of bacterial and viral outbreaks:

# Real-time basecalling and AMR plasmid profiling
dorado basecaller dna_r10.4.2_e8.2_400bps_fast@v4.3.0 ./live_run/ \
    --min-qscore 8 | \
    samtools fastq - | \
    minimap2 -ax map-ont /databases/card_amr_database.fa - | \
    samtools view -b -F 4 - | \
    samtools sort -o amr_hits.bam -

samtools index amr_hits.bam
# Extract confirmed beta-lactamase and carbapenemase resistance genes
samtools idxstats amr_hits.bam | awk '$3 > 0 {print $1, $3 " supporting reads"}'

4. Clinical Long-Read Structural Diagnostic Report Architecture

A validated clinical long-read report requires five mandatory components:

1. Assay Details: Technology (ONT PromethION R10.4.2 / PacBio Revio), chemistry, basecaller version (Dorado v4.3.0 SUP), and mean Q-score ($\ge \text{Q}18$).
2. Primary Diagnostic Finding: Structural variant notation (e.g. chr9:g.27573480_27575200del (1,720 bp)), zygosity, supporting reads (DV), total depth, and VAF.
3. Gene Impact & Inheritance: Disrupted exons, transcript ID, haploinsufficiency classification, and inheritance pattern.
4. ACMG/ClinGen Classification: Pathogenic / Likely Pathogenic classification supported by evidence codes.
5. Therapeutic / Management Implications: Approved targeted drugs, contraindicated therapies, or familial cascade testing recommendations.