Novel Identical Likely Pathogenic ACTB Variant in Congenital Smooth Muscle Hamartoma: A Report of Two Pediatric Cases

Congenital smooth muscle hamartomas (CSMH) are benign hamartomatous proliferations (of the smooth muscles of the arrector pili muscles). They are most often present at birth and associated with many clinical and pathological signs common to Becker's nevus (BN). Pathogenic mosaic variants in the beta actin gene (ACTB) have been described in both entities. We present two children with CSMH in whom an identical and previously unreported likely pathogenic variant of ACTB was identified. This supports the hypothesis that Becker's nevus and CSMH are part of the same nosological spectrum; therefore, the term CSMH is more appropriate.