Juvenile Polyposis-HHT Overlap Syndrome

Introduction to the Condition

The American College of Medical Genetics and Genomics recommends that juvenile polyposis-HHT overlap syndrome is a combined genetic disorder caused by SMAD4 mutations, occurring in 1-2% of HHT cases, with a significant cancer risk and life-threatening arteriovenous malformations in multiple organs 1, 2

The syndrome results exclusively from mutations in the SMAD4 gene, located on chromosome 18q, with an autosomal dominant inheritance pattern and 50% transmission risk to offspring 1, 2, 3
SMAD4 encodes a protein in the TGF-β signaling pathway, and mutations cause both phenotypes to manifest simultaneously, with up to 76% of SMAD4 mutation carriers developing features of HHT 2, 4

Patients with SMAD4 mutations develop few to hundreds of juvenile polyps throughout the gastrointestinal tract, particularly in the colon, stomach, and small intestine, with a cumulative lifetime colorectal cancer risk of 39-68% and gastric cancer risk of 21% in those with gastric polyps 1, 3, 4, 6
The syndrome is characterized by spontaneous and recurrent epistaxis, multiple mucocutaneous telangiectasias, and arteriovenous malformations in brain, lungs, gastrointestinal tract, liver, and retina, creating risk of stroke, cerebral abscess, or hemorrhage 1, 2, 3, 4

Diagnosis requires meeting criteria for BOTH conditions: JPS criteria and HHT Curaçao criteria, with SMAD4 genetic testing critical for identifying the overlap syndrome 2, 6
Patients may initially present with only one phenotype, making genetic testing essential for identifying the overlap syndrome 2

All SMAD4 mutation carriers must undergo screening for pulmonary AVMs using contrast echocardiography or chest CT, as well as MRI of brain to detect cerebral vascular malformations, and Doppler ultrasonography for hepatic involvement 2, 7
Liver biopsy must be strictly avoided due to high hemorrhage risk, and thoracoabdominal contrast CT may be used to identify additional arteriovenous malformations 2, 7
SMAD4 mutation carriers require upper GI tract surveillance every 1-3 years starting at age 18 years, and colonoscopy with polypectomy at 2-year intervals for colorectal surveillance 2, 6, 8

Primary treatment consists of repeated endoscopic polypectomy at 2-year intervals for both colorectal and upper GI polyps, with colectomy with ileorectal anastomosis appropriate for patients with high polyp burden 6, 8
Percutaneous transcatheter embolization is recommended for pulmonary AVMs, and a stepwise approach for bleeding includes nasal moisturization, oral tranexamic acid, local ablative therapies, and systemic bevacizumab for refractory cases 2
Iron replacement therapy is essential for anemia from chronic bleeding, and all patients with SMAD4 mutations should be managed in conjunction with a specialist HHT center 2

Patients with SMAD4 mutations may lack overt clinical symptoms of HHT but remain at risk of asymptomatic AVMs that can present suddenly with catastrophic complications, and failure to screen for juvenile polyposis in SMAD4-positive HHT patients leads to missed gastrointestinal cancers 2
Pregnancy poses particular risk as hormonal and hemodynamic changes cause rapid PAVM growth with higher complication risk 7