# Clinical Cases: Transcription and RNA Processing

## Case 1: Spinal Muscular Atrophy (Splicing Defect and Therapy)

### Clinical Image
![Spinal Muscular Atrophy](case_01_image.jpg)
*Source: [Wikipedia - Spinal muscular atrophy](https://en.wikipedia.org/wiki/Spinal_muscular_atrophy) - CC BY-SA 3.0*

### Case Presentation
A 4-month-old infant is brought for evaluation of progressive weakness and poor feeding. The parents note she has become "floppy" and has difficulty lifting her head. She was born full-term and initially seemed normal but has progressively lost motor milestones. Physical examination reveals severe hypotonia, absent deep tendon reflexes, tongue fasciculations, and paradoxical breathing (abdominal breathing with chest wall retraction due to intercostal weakness). Genetic testing confirms homozygous deletion of the SMN1 gene, consistent with spinal muscular atrophy type 1 (Werdnig-Hoffmann disease). The parents ask why she has disease when she still has the SMN2 gene. The physician explains that SMN2 is nearly identical to SMN1, but a single nucleotide difference creates a weak exon 7 splice site, causing most SMN2 transcripts to skip exon 7 and produce unstable, non-functional protein. She is started on nusinersen (Spinraza), an antisense oligonucleotide administered intrathecally that binds to an intronic splicing silencer in SMN2 pre-mRNA, promoting exon 7 inclusion and restoring functional SMN protein production. Alternatively, she could receive onasemnogene abeparvovec (Zolgensma), a gene therapy delivering functional SMN1. With early treatment, her prognosis is dramatically better than the historical natural history of SMA type 1.

### Key Learning Points
- SMA results from loss of SMN1; the nearly identical SMN2 gene cannot compensate because a C-to-T transition weakens exon 7 splicing, causing it to be frequently skipped
- Nusinersen is an antisense oligonucleotide that blocks an intronic splicing silencer (ISS-N1), promoting exon 7 inclusion in SMN2 transcripts and restoring functional SMN protein
- This represents a triumph of understanding splicing biology to develop a disease-modifying therapy; gene therapy (Zolgensma) provides an alternative approach by delivering functional SMN1

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## Case 2: Alpha-Thalassemia (Splicing Mutation)

### Clinical Image
![Alpha Thalassemia Blood Smear](case_02_image.jpg)
*Source: [Wikipedia - Alpha-thalassemia](https://en.wikipedia.org/wiki/Alpha-thalassemia) - CC BY-SA 3.0*

### Case Presentation
A 25-year-old pregnant woman of Southeast Asian descent is found to have microcytic anemia (hemoglobin 9.8 g/dL, MCV 68 fL) on routine prenatal labs. Iron studies are normal, ruling out iron deficiency. Hemoglobin electrophoresis shows a normal pattern but reveals Hb H inclusions on supravital staining, consistent with hemoglobin H (HbH) disease, a form of alpha-thalassemia. Genetic testing confirms three alpha-globin gene deletions (--/-alpha), leaving only one functional alpha-globin gene. The genetic counselor explains that one of her deletional alleles (alpha-T) actually results from a splicing mutation rather than a simple deletion: a point mutation at the donor splice site of intron 1 creates an abnormal splice site that leads to aberrant mRNA processing and no functional protein. The father is tested and found to be an alpha-thalassemia carrier (two gene deletions, --/alpha-alpha). There is a 25% risk their child could inherit deletions from both parents resulting in Hb Bart's hydrops fetalis, a condition incompatible with life. Prenatal diagnosis is offered via chorionic villus sampling.

### Key Learning Points
- Splice site mutations can produce thalassemia phenotypes even without gene deletions; mutations destroying the canonical GU donor or AG acceptor sites lead to aberrant splicing and non-functional mRNA
- The consequences of splice mutations include exon skipping, intron retention, and cryptic splice site activation, all potentially producing abnormal or absent protein
- Approximately 15-50% of all disease-causing mutations affect splicing, making this one of the most common mechanisms of pathogenic variation

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## Case 3: Amanita Phalloides Poisoning (RNA Polymerase II Inhibition)

### Clinical Image
![Death Cap Mushroom](case_03_image.jpg)
*Source: [Wikipedia - Amanita phalloides](https://en.wikipedia.org/wiki/Amanita_phalloides) - CC BY-SA 3.0*

### Case Presentation
A 45-year-old man presents to the emergency department with severe abdominal pain, profuse watery diarrhea, and vomiting 10 hours after eating wild mushrooms he foraged. He initially felt fine after the meal but symptoms began 6 hours later. On examination, he appears ill with dry mucous membranes and tachycardia. Initial labs show mild elevation of AST and ALT. He is admitted for supportive care. Over the next 48 hours, his liver enzymes rise dramatically (AST 8,500, ALT 6,200), coagulopathy develops (INR 4.2), and he becomes encephalopathic. Toxicology consultation identifies this as Amanita phalloides (death cap mushroom) poisoning. The mushroom contains alpha-amanitin, a cyclic peptide that potently inhibits RNA polymerase II by binding to the enzyme and blocking translocation during elongation. Without mRNA synthesis, hepatocytes cannot produce essential proteins and undergo massive necrosis. He is treated with N-acetylcysteine, high-dose penicillin (which may compete with alpha-amanitin for hepatocyte uptake), and silibinin (milk thistle extract). Despite supportive care, he develops fulminant hepatic failure requiring emergency liver transplantation. The delayed symptom onset (6-12 hours) is characteristic and often leads to initial misdiagnosis.

### Key Learning Points
- Alpha-amanitin from Amanita phalloides specifically inhibits RNA polymerase II, blocking mRNA synthesis; this halts production of essential proteins in all cells, with hepatocytes being particularly vulnerable due to their high transcriptional activity
- RNA Pol II is responsible for transcribing all protein-coding genes; its inhibition is rapidly lethal to metabolically active cells
- The characteristic delayed onset (6-12 hours) distinguishes Amanita poisoning from other mushroom toxicities; by the time GI symptoms appear, significant hepatotoxin absorption has occurred