“Holt-Oram Syndrome”, Victor McKusick, Mendelian Inheritance in Man, 1966. (TBX5)

Holt-Oram syndrome is an autosomal dominant disorder characterized by abnormalities of the upper limbs and shoulder girdle, associated with a congenital heart lesion. The typical combination is considered to be a triphalangeal thumb with a secundum atrial septal defect (ASD), but there is a great range in the severity of both the heart and skeletal lesions.

The TBX5 gene encodes the T-box transcription factor-5  protein. This protein serves as a vital genetic master switch during early embryonic growth. 

Heart Development: It directly controls the transcription networks that form the heart chambers and the electrical system coordinating heartbeats. 

Limb Formation: It acts as the key initiator for the structural positioning and physical outgrowth of the forelimbs (arms and hands). 

Clinical Relevance: Genetic mutations or deletions starting at this locus disrupt these pathways, causing Holt-Oram Syndrome. This condition is medically characterized by skeletal malformations in the upper limbs paired with congenital heart septal defects.