In a 23‑year‑old with hundreds of colonic polyps and no family history of polyps in the past two generations, could this represent familial adenomatous polyposis (FAP) or another hereditary polyposis syndrome, and how might it occur without a known familial pattern?

Yes, This Is Almost Certainly Familial Adenomatous Polyposis (FAP)

A 23-year-old with hundreds of colonic polyps has FAP until proven otherwise, and approximately 25-30% of FAP cases occur as de novo mutations without any family history 1. The absence of affected relatives in the past two generations does not exclude this diagnosis.

Why This Occurs Without Family History

FAP is caused by germline mutations in the APC gene and follows autosomal dominant inheritance 2. However, spontaneous (de novo) mutations account for 25-30% of all FAP cases 1. This means:

• The mutation arose for the first time in this patient
• Neither parent carries the mutation
• No family history would be present
• The patient can still pass it to their children with 50% probability

One case report documented a severe FAP presentation in a 2-year-old with a de novo pathogenic APC variant (c.4132 C>T), emphasizing that "molecular screening should extend to very young patients even in the absence of familial history" 3.

Immediate Diagnostic Steps

Order germline genetic testing for APC gene mutations immediately 2. The presence of hundreds of polyps in a young adult is pathognomonic for FAP, and genetic confirmation will:

• Establish the diagnosis definitively
• Guide surgical timing and approach
• Enable cascade testing of family members (particularly this patient's children)
• Inform surveillance for extracolonic manifestations

Additional Testing to Consider

• MUTYH gene testing if APC testing is negative—biallelic MUTYH mutations cause MUTYH-associated polyposis (MAP) with autosomal recessive inheritance and a similar but less severe phenotype 2
• High-quality colonoscopy with pan-colonic dye spray to accurately define polyp burden 4
• Upper endoscopy to evaluate for duodenal polyps (second leading cancer site in FAP) 5

Critical Management Points

Surgical Intervention

Prophylactic proctocolectomy is required in almost all FAP patients since untreated disease progresses to colorectal cancer in nearly 100% of cases 2, 1. The timing depends on polyp burden, dysplasia grade, and patient factors, but should not be delayed once diagnosis is confirmed.

Surveillance Requirements

• Duodenal surveillance is mandatory—duodenal cancer is the second leading cause of death in FAP patients 5
• Screen for desmoid tumors, which carry the highest extracolonic mortality risk 5
• Evaluate for thyroid nodules and other extracolonic manifestations 5

Family Implications

This patient's children are at 50% risk and require:

• Genetic testing in early teens (or earlier if symptomatic) 6
• Annual colonoscopy starting around age 10-15 years if they carry the mutation 2

Common Pitfalls to Avoid

1. Do not dismiss FAP due to negative family history—de novo mutations are common 1
2. Do not delay genetic testing—it guides all subsequent management decisions
3. Do not focus solely on the colon—extracolonic manifestations require systematic surveillance 5
4. Do not assume siblings are unaffected—if this is a de novo mutation, siblings are not at increased risk, but if inherited from a mosaic parent, they may be affected

The case reports emphasize that "clinicians should take note of family history but be prepared to consider FAP in young patients even without familial disease" 6, and that "clinical suspicion for FAP should be kept in mind, especially in young patients without family history" 7.