Refined Characterization of the Expression and Stability of the SMN Gene Products

Jérémie Vitte^*, Coralie Fassier^*, Francesco D. Tiziano, Cécile Dalard^*, Sabrina Soave^*, Natacha Roblot^*, Christine Brahe, Pascale Saugier-Veber, Jean Paul Bonnefont and Judith Melki^*

From the Molecular Neurogenetics Laboratory,^* INSERM U798, Evry and Paris 11 Universities, Evry, France; the Department of Genetics, Rouen University Hospital and INSERM U614, Institute for Biochemical Research, University of Rouen, Rouen, France; Hôpital Necker-Enfants Malades, INSERM, U393, Paris, France; and the Institute of Medical Genetics, Catholic University, Rome, Italy

Spinal muscular atrophy (SMA) is characterized by degenerationof lower motor neurons and caused by mutations of the SMN1 gene.SMN1 is duplicated in a homologous gene called SMN2, which remainspresent in patients. SMN has an essential role in RNA metabolism,but its role in SMA pathogenesis remains unknown. Previous studiessuggested that in neurons the protein lacking the C terminus(SMN⁷), the major product of the SMN2 gene, had a dominant-negativeeffect. We generated antibodies specific to SMN^FL or SMN⁷. Intransfected cells, the stability of the SMN⁷ protein was regulatedin a cell-dependent manner. Importantly, whatever the humantissues examined, SMN⁷ protein was undetectable because of theinstability of the protein, thus excluding a dominant effectof SMN⁷ in SMA. A similar decreased level of SMN^FL was observedin brain and spinal cord samples from human SMA, suggestingthat SMN^FL may have specific targets in motor neurons. Moreover,these data indicate that the vulnerability of motor neuronscannot simply be ascribed to the differential expression ora more dramatic reduction of SMN^FL in spinal cord when comparedwith brain tissue. Improving the stability of SMN⁷ protein mightbe envisaged as a new therapeutic strategy in SMA.