Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by homozygous mutations of the SMN1 gene. Three forms of SMA are recognized (type I-III) on the basis of clinical severity. SMN1 and a nearly identical copy, SMN2, are located in a duplicated and inverted region at 5q13. Both genes encode the SMN protein but, because of alternative splicing, the majority of SMN2 transcripts lack exon 7, and are unable to produce a sufficient amount of protein to prevent the onset of the disease. Recently, evidence has been provided that SMN2 expression can be enhanced by pharmacological treatment. However, no reliable biomarkers are available to test the molecular efficacy of the treatments. At present, the only potential molecular biomarker is the dosage of SMN gene products in peripheral blood leukocytes (both protein and transcript levels).

In the study of F.D. Tiziano et al. (European Journal of Human Genetics, 2010) it has been shown that SMN2-fl (full-length) transcripts are reduced in leukocytes of SMA patients compared to controls and it has been provided the rationale for SMN transcripts dosage in clinical trials. In this paper the Authors have demonstrated that SMN2-fl transcripts levels are related to clinical severity, as in type III patients SMN2-fl levels are significantly higher compared with type II, directly correlate with functional ability in a small group of type II patients, and with age of onset in type III patients.

Moreover, in haploidentical SMA siblings with discordant phenotype, the less severely affected individuals showed significantly higher SMN2-fl transcript levels. The SMN1/SMN2 mRNA Quantum Kit is based on the use of absolute standard curves (constructed by means of serial dilutions of plasmid DNA used as external standards), which allows the quantification of full length SMNl transcripts (SMN1 and SMN2) as number of mRNA molecules per nanogram of total RNA, independently from the use of endogenous controls, thus avoiding possible biases due to variations in endogenous control transcript levels. In this assay, GAPDH transcript quantification assay has been included as positive control, to rule out that possible differences between patients and controls could be related to RT or real time PCR efficiency or RNA quality. RT - PCR reagents are sufficient for over 35 reactions. Each reaction mix for real-time PCR is sufficient for about 150 amplifications. External standards are sufficient for over 45 amplifications each. We strongly suggest to amplify each sample in triplicate.

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