Familial pneumothorax in twins with Tatton-Brown-Rahman DNMT3A overgrowth syndrome.

Spontaneous pneumothorax is a common respiratory presentation that may signal underlying genetic disease. Familial pneumothorax occurs in ~10% of primary cases, yet 75% remain genetically unclassified. We report identical twin brothers presenting with spontaneous pneumothoraces in adulthood, leading to a diagnosis of Tatton-Brown-Rahman syndrome (TBRS), a DNMT3A-related overgrowth disorder not previously associated with pneumothorax. Both individuals exhibited tall stature, mild intellectual disability, hypermobility, and cardiac abnormalities. Whole genome sequencing identified a rare de novo DNMT3A missense variant (c.1585 G > A, p.D529N) absent from population databases and predicted to be damaging. Methylation profiling confirmed genome-wide hypomethylation consistent with impaired DNMT3A function, supporting pathogenicity. No variants were found in known familial pneumothorax genes. Apical blebs observed at surgery and connective tissue features suggest a mechanistic link between TBRS and pneumothorax, analogous to other monogenic connective tissue disorders. This case expands the phenotypic spectrum of TBRS and highlights the importance of genetic evaluation in familial pneumothorax. Diagnosis enables personalised care, including surveillance for extrapulmonary complications such as aortic root dilatation and haematological malignancy. Our findings suggest that TBRS should be considered in patients presenting with pneumothorax, tall stature, and neurodevelopmental features. Further cases are needed to confirm this association and refine clinical management strategies.