Unité de Recherches sur les Handicaps Génétiques de l'Enfant, INSERM U-393, et Département de Génétique, Hôpital Necker-Enfants Malades, Paris, France.
A 4-base pair deletion in the neuronal serine protease neurotrypsin gene was associated with autosomal recessive nonsyndromic mental retardation (MR). In situ hybridization experiments on human fetal brains showed that neurotrypsin was highly expressed in brain structures involved in learning and memory. Immuno-electron microscopy on adult human brain sections revealed that neurotrypsin is located in presynaptic nerve endings, particularly over the presynaptic membrane lining the synaptic cleft. These findings suggest that neurotrypsin-mediated proteolysis is required for normal synaptic function and suggest potential insights into the pathophysiological bases of mental retardation.