# Clinical Cases: DNA Structure and Replication

## Case 1: Xeroderma Pigmentosum (Nucleotide Excision Repair Deficiency)

### Clinical Image
![Xeroderma Pigmentosum](case_01_image.jpg)
*Source: [Wikipedia - Xeroderma pigmentosum](https://en.wikipedia.org/wiki/Xeroderma_pigmentosum) - CC BY-SA 3.0*

### Case Presentation
A 6-year-old girl is referred to dermatology for evaluation of severe freckling and multiple skin lesions. Her parents report that she develops severe sunburns even with brief sun exposure and has had progressive skin changes since infancy. Physical examination reveals hundreds of freckles (lentigines) concentrated on sun-exposed areas, areas of skin atrophy, and several suspicious pigmented and scaly lesions. Biopsy of one lesion confirms squamous cell carcinoma. She has already had three basal cell carcinomas removed. She also has photophobia and some developmental delay. Given her extreme sun sensitivity and early-onset skin cancers, xeroderma pigmentosum (XP) is suspected. Genetic testing confirms mutations in the XPA gene, which is essential for nucleotide excision repair (NER). Without functional NER, she cannot repair UV-induced thymine dimers and other bulky DNA adducts. The family is extensively counseled on rigorous sun protection: specialized UV-blocking clothing, sunscreen, UV-filtering film on windows, and shifting activities to nighttime when possible. She will require lifelong dermatologic surveillance with frequent skin examinations and early excision of suspicious lesions. Her lifetime skin cancer risk exceeds 10,000-fold compared to the general population.

### Key Learning Points
- Xeroderma pigmentosum results from defects in nucleotide excision repair (NER), the pathway that removes bulky, helix-distorting lesions including UV-induced thymine dimers
- Without NER, UV damage accumulates with each sun exposure, leading to mutations and dramatically elevated skin cancer risk; neurological problems also occur in some XP complementation groups
- NER involves damage recognition, dual incisions flanking the lesion (by XPF-ERCC1 and XPG endonucleases), excision of a 24-32 nucleotide segment, and gap filling by DNA polymerase

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## Case 2: Lynch Syndrome (Mismatch Repair Deficiency)

### Clinical Image
![Lynch Syndrome Colon Cancer](case_02_image.jpg)
*Source: [Wikipedia - Lynch syndrome](https://en.wikipedia.org/wiki/Lynch_syndrome) - CC BY-SA 3.0*

### Case Presentation
A 38-year-old woman presents with rectal bleeding and change in bowel habits. Colonoscopy reveals a mass in the ascending colon. Biopsy confirms adenocarcinoma. Given her young age, detailed family history is obtained, revealing her father died of colon cancer at age 45, her paternal aunt had endometrial cancer at 48, and a paternal uncle had urothelial carcinoma at 55. This pattern strongly suggests Lynch syndrome. Immunohistochemistry on her tumor shows loss of MSH2 and MSH6 protein expression. Microsatellite instability (MSI) testing confirms MSI-high status. Germline genetic testing identifies a pathogenic variant in MSH2, confirming Lynch syndrome. She undergoes surgical resection with extended colectomy. She is counseled about her elevated lifetime risks: approximately 80% for colorectal cancer, 40-60% for endometrial cancer, and increased risks for ovarian, gastric, urinary tract, and other cancers. She will undergo colonoscopy every 1-2 years and is offered prophylactic hysterectomy and bilateral salpingo-oophorectomy after completing childbearing. Her siblings and children are offered cascade genetic testing.

### Key Learning Points
- Lynch syndrome results from germline mutations in mismatch repair (MMR) genes (MLH1, MSH2, MSH6, PMS2), causing inability to correct replication errors, particularly at microsatellite sequences
- MMR deficiency leads to microsatellite instability (MSI), a hypermutator phenotype; tumors accumulate mutations rapidly, driving cancer development at young ages
- Immunohistochemistry for MMR proteins and MSI testing are used for screening; germline testing confirms diagnosis and enables cascade testing of at-risk relatives

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## Case 3: Dyskeratosis Congenita (Telomere Disorder)

### Clinical Image
![Dyskeratosis Congenita](case_03_image.jpg)
*Source: [Wikipedia - Dyskeratosis congenita](https://en.wikipedia.org/wiki/Dyskeratosis_congenita) - CC BY-SA 3.0*

### Case Presentation
A 15-year-old boy is referred for evaluation of pancytopenia. He has a history of nail dystrophy since early childhood and white patches in his mouth (oral leukoplakia). His skin shows abnormal reticular pigmentation on the neck and upper chest. Complete blood count reveals hemoglobin 9.2 g/dL, WBC 2,800/mm3, and platelets 62,000/mm3. Bone marrow biopsy shows hypocellularity consistent with aplastic anemia. Given the mucocutaneous triad (nail dystrophy, oral leukoplakia, abnormal skin pigmentation) combined with bone marrow failure, dyskeratosis congenita (DC) is suspected. Telomere length analysis by flow-FISH shows severely shortened telomeres (below the 1st percentile for age). Genetic testing identifies a mutation in TERT, encoding the catalytic component of telomerase. Without adequate telomerase activity, telomeres shorten prematurely with each cell division. Stem cells, which must divide many times, are particularly affected, explaining the bone marrow failure. He is also at increased risk for pulmonary fibrosis and solid tumors. He is evaluated for hematopoietic stem cell transplantation and counseled about the need for modified conditioning regimens given his underlying DNA repair defects.

### Key Learning Points
- Telomeres are repetitive TTAGGG sequences that protect chromosome ends; telomerase, a reverse transcriptase with an RNA template, maintains telomere length in germ cells and stem cells
- Dyskeratosis congenita results from mutations affecting telomerase (TERT, TERC) or telomere maintenance proteins (DKC1, TINF2), causing premature telomere shortening
- Stem cells with limited replicative capacity lead to bone marrow failure, pulmonary fibrosis, and liver disease; the mucocutaneous triad (nail dystrophy, oral leukoplakia, skin pigmentation) is the classic presentation