Clinical and molecular cross-sectional study of a cohort of adult type III spinal muscular atrophy patients: clues from a biomarker study

Abstract

Proximal spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by mutations of the SMN1 gene. Based on severity, three forms of SMA are recognized (types I-III). All patients usually have 2-4 copies of a highly homologous gene (SMN2), which produces insufficient levels of functional survival motor neuron (SMN) protein due to the alternative splicing of exon 7. The availability of potential candidates to the treatment of SMA has raised a number of issues, including the availability of biomarkers. This study was aimed at evaluating whether the quantification of SMN2 products in peripheral blood is a suitable biomarker for SMA. Forty-five adult type III patients were evaluated by Manual Muscle Testing, North Star Ambulatory Assessment scale, 6-min walk test, myometry, forced vital capacity, and dual X-ray absorptiometry. Molecular assessments included SMN2 copy number, levels of full-length SMN2 (SMN2-fl) transcripts and those lacking exon 7 and SMN protein. Clinical outcome measures strongly correlated to each other. Lean body mass correlated inversely with years from diagnosis and with several aspects of motor performance. SMN2 copy number and SMN protein levels were not associated with motor performance or transcript levels. SMN2-fl levels correlated with motor performance in ambulant patients. Our results indicate that SMN2-fl levels correlate with motor performance only in patients preserving higher levels of motor function, whereas motor performance was strongly influenced by disease duration and lean body mass. If not taken into account, the confounding effect of disease duration may impair the identification of potential SMA biomarkers.

Solving the puzzle of spinal muscular atrophy: What are the missing pieces?

“…Regarding quantitative approaches, the only available data is that obtained for peripheral blood mononuclear cells (PBMCs); these values probably do not reflect those in diseased target tissues. Additionally, while SMN2 full length transcript levels are lower in patients compared to controls and are grossly related to phenotypic severity, this observation was not recapitulated for SMN protein, which is significantly reduced only in the most severely affected patients [Sumner et al, 2006;Tiziano et al, 2010a;Tiziano et al, 2010b;Piepers et al, 2011;Crawford et al, 2012;Tiziano et al, 2013]. This being said, studies in SMA mice suggest that as little as 70% of control SMN protein might be sufficient to revert disease pathology if introduced during the pre-symptomatic period [Foust et al, 2010].…”

Assays for the Identification and Prioritization of Drug Candidates for Spinal Muscular Atrophy

“…The different technical approaches utilized are analyzed in detail throughout this section. [121][122][123][124][125][126][127] A greater number of studies belong to the second category, as the majority of therapeutic approaches have aimed to increase SMN protein levels by increasing the level of SMN transcripts. An exception to this transcript ''rule'' is the modification of SMN protein stability, 117 where a change in SMN transcript levels is not expected.…”

“…Second, baseline SMN transcript and protein from patient PBMC samples have not always shown high correlation. 122,125 Despite this, the hope is that drugs designed for SMN2 modulation will induce an increase in transcript expression with resultant SMN protein increases, prerequisite for therapeutic effectiveness in SMA. Third, SMN protein and transcript analysis is not indicated for the evaluation of therapeutic compounds that are not intended to modify SMN levels, such as neuroprotecting compounds.…”

“…109,122,125,133 SMN is expressed ubiquitously, but its levels vary greatly across tissues, with the lowest reported levels being in the muscle, nerve, and spinal cord and with the skin and blood at up to 50-fold higher levels. 154 The broad range of SMN protein levels by tissue is further complicated by data on differences in cellular subpopulations such as motor neurons, which have been reported to be inherently deficient in the full-length protein due to inefficient splicing of SMN exon 7.…”

The influence of storage parameters on measurement of survival motor neuron (SMN) protein levels: Implications for pre-clinical studies and clinical trials for spinal muscular atrophy

“…Indeed, several of these approaches are entering clinical trials in patients. In the absence of validated, robust biomarkers [21][22][23][24][25] SMN levels are often used as a major read-out to evaluate therapeutic efficacy. However, proteins can be unstable when outside their native environment.…”