RESUMO
The spinal muscular atrophy (SMA) region on chromosome 5q13 contains an inverted duplication of about 500 kb, and deleterious mutations in the survival motor neuron 1 (SMN1) gene cause SMA, a common lethal childhood neuropathy. We have used a number of approaches to probe the evolutionary history of these genes and show that SMN gene duplication and the appearance of SMN2 occurred at very distinct evolutionary times. Molecular fossil and molecular clock data suggest that this duplication may have occurred as recently as 3 million years ago in that the position and identity repetitive elements are identical for both human SMN genes and overall sequence divergence ranged from 0.15% to 0.34%. However, these approaches ignore the possibility of sequence homogenization by means of gene conversion. Consequently, we have used quantitative polymerase chain rection and analysis of allelic variants to provide physical evidence for or against SMN gene duplication in the chimpanzee, mankind's closest relative. These studies have revealed that chimpanzees have 2-7 copies of the SMN gene per diploid genome; however, the two nucleotides diagnostic for exons 7-8 and the SMNdelta7 mRNA product of the SMN2 gene are absent in non-human primates. In contrast, the SMN2 gene has been detected in all extant human populations studied to date, including representatives from Europe, the Central African Republic, and the Congo. These data provide conclusive evidence that SMN gene duplication occurred more than 5 million years ago, before the separation of human and chimpanzee lineages, but that SMN2 appears for the first time in Homo sapiens.
Assuntos
Duplicação Gênica , Proteínas do Tecido Nervoso/genética , Alelos , Animais , Sequência de Bases , Linhagem Celular Transformada , Proteína de Ligação ao Elemento de Resposta ao AMP Cíclico , DNA/química , DNA/genética , DNA Complementar/química , DNA Complementar/genética , Feminino , Variação Genética , Humanos , Dados de Sequência Molecular , Pan troglodytes/genética , Polimorfismo Genético , Proteínas de Ligação a RNA , Proteínas do Complexo SMN , Alinhamento de Sequência , Análise de Sequência de DNA , Homologia de Sequência do Ácido Nucleico , Proteína 1 de Sobrevivência do Neurônio Motor , Proteína 2 de Sobrevivência do Neurônio MotorRESUMO
The telomeric survival motor neuron (SMN(T)) gene is a valuable molecular diagnostic tool for childhood-onset spinal muscular atrophy (SMA) as homozygous deletions of SMN(T) exon 7 (delta7SMN(T)) are present in approximately 94% of patients. In this report, we provide the first comprehensive study of 32 unrelated non-deletion SMA patients. Quantitative polymerase chain reaction (PCR) studies established that 90% had two intact copies of SMN(T) exon 7 suggesting that these patients do not have 5q SMA. Once 5q SMA is confirmed, the SMN(T) gene can be screened for subtle mutations. Using single strand conformation analysis, we identified two missense mutations (P245L and Y272C) in exon 6 of the SMN(T) gene of two SMA patients shown to have a single copy of SMN(T) exon 7. Y272 is most likely critical for SMN(T) function as it is a target for recurring mutations and is associated with type I SMA. These results emphasize the need for dosage analysis in the differential diagnosis of 5q SMA in nondeletion patients, consistent with extensive clinical heterogeneity and some genetic heterogeneity in this disease. Homozygosity or heterozygosity for a delta7SMN(T) allele confirms the diagnosis of 5q SMA with greater precision than clinical examination alone.
