Spinal muscular atrophy: clinical classification and disease heterogeneity

The Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP INTEND): test development and reliability

The motor skills of patients with spinal muscular atrophy, type I (SMA-I) are very limited. It is difficult to quantify the motor abilities of these patients and as a result there is currently no validated measure of motor function that can be utilized as an outcome measure in clinical trials of SMA-I. We have developed the Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders ("CHOP INTEND") to evaluate the motor skills of patients with SMA-I. The test was developed following the evaluation of 26 infants with SMA-I mean age 11.5 months (1.4-37.9 months) with the Test of Infant Motor Performance and The Children's Hospital of Philadelphia Test of Strength in SMA, a newly devised motor assessment for SMA. Items for the CHOP INTEND were selected by an expert panel based on item mean and standard deviation, item frequency distribution, and Chronbach's alpha. Intra-rater reliability of the resulting test was established by test-retest of 9 infants with SMA-I over a 2 month period; Intraclass correlation coefficient (ICC) (3,1)=0.96. Interrater reliability was by video analysis of a mixed group of infants with neuromuscular disease by 4 evaluators; ICC (3,4)=0.98 and in a group of 8 typically developing infants by 5 evaluators ICC (3,5)=0.93. The face validity of the CHOP INTEND is supported by the use of an expert panel in item selection; however, further validation is needed. The CHOP INTEND is a reliable measure of motor skills in patients with SMA-I and neuromuscular disorders presenting in infancy.

Differential 3' splice site recognition of SMN1 and SMN2 transcripts by U2AF and U2 snRNP

Spinal Muscular atrophy is a prevalent genetic disease caused by mutation of the SMN1 gene, which encodes the SMN protein involved in assembly of small nuclear ribonucleoprotein (snRNP) complexes. A paralog of the gene, SMN2, cannot provide adequate levels of functional SMN because exon 7 is skipped in a significant fraction of the mature transcripts. A C to T transition located at position 6 of exon 7 is critical for the difference in exon skipping between SMN1 and SMN2. Here we report that this nucleotide difference results in increased ultraviolet light-mediated crosslinking of the splicing factor U2AF(65) with the 3' splice site of SMN1 intron 6 in HeLa nuclear extract. U2 snRNP association, analyzed by native gel electrophoresis, is also more efficient on SMN1 than on SMN2, particularly under conditions of competition, suggesting more effective use of limiting factors. Two trans-acting factors implicated in SMN regulation, SF2/ASF and hnRNP A1, promote and repress, respectively, U2 snRNP recruitment to both RNAs. Interestingly, depending on the transcript and the regulatory factor, the effects on U2 binding not always correlate with changes in U2AF(65) crosslinking. Furthermore, blocking recognition of a Tra2-beta1-dependent splicing enhancer located in exon 7 inhibits U2 snRNP recruitment without affecting U2AF(65) crosslinking. Collectively, the results suggest that both U2AF binding and other steps of U2 snRNP recruitment can be control points in SMN splicing regulation.

The medical and economic roles of pipeline pharmacogenetics: Alzheimer's disease as a model of efficacy and HLA-B(*)5701 as a model of safety

Pharmacogenetics (PGX) is the study of drug response as a function of an individual's DNA. PGX is often viewed as an extension of disease association genetics, and although this information may be related, it is not the study of drug response. Although medicines are used to treat diseases, the value of strategies that identify and incorporate DNA biomarkers associated with clinical efficacy, or DNA biomarkers for untoward clinical responses, can be applied directly to pharmaceutical pipelines. The growth of adverse event PGX studies involving marketed medicines generally uses relatively large numbers of affected patients, but has been productive. However, the two critical strategies for pipeline genetics must make use of fewer patients: (1) the early identification of efficacy signals so that they can be applied early in development for targeted therapies and (2) identification of safety signals that can subsequently be validated prospectively during development using the least number of patients with adverse responses. Assumptions are often made that large numbers of patients are necessary to recognize PGX hypotheses and to validate DNA biomarkers. In some ways, pipeline pharmacogenetics may be viewed as the opposite of current genome-wide scanning designs. The goal is to obtain PGX signals in as few patients as possible, and then validate PGX hypotheses for specificity and sensitivity as development trials go forward--not using hundreds of thousand of markers to detect strong linkage disequilibrium signals in thousands of patients and their controls. Drug development takes 5-7 years for a drug candidate to traverse to registration--and this is similar to the timeframe for validating genetic biomarkers using sequential clinical trials. Two important examples are discussed, the association of APOE genotypes to the demonstration of actionable efficacy signals for the use of rosiglitazone for Alzheimer's disease; and the identification of HLA-B(*)5701 as a highly sensitive and specific predictive marker for abacavir treated patients who will develop hypersensitivity syndrome (HSS). The rosiglitazone study prevented pipeline attrition by changing the interpretation of a critical Phase IIB proof of concept study (2005) from a failed study, to a positive efficacy response in a genetically predictable proportion of patients. Now, three years later, a Phase III program of clinical trials using pharmacogenetic designs is months away from completion (late08). If successfully registered (early09), millions of patients could benefit, and efficacy PGX would have achieved its first prospective block-buster. The use of safety candidate gene association genetics in patients who received abacavir therapy and developed HSS starting in 1998 culminated in a double blind clinical trial that determined sensitivity > 97% and specificity >99% in 2007. Clinical consensus panels rapidly recommended abacavir as the preferred therapy along with HLA-B(*)5701 pre-testing, immediately increasing the market share of abacavir with respect to other reverse transcriptases that are associated with there own adverse events. Targeting of medicines during drug development is now possible, practical, and profitable.

Proximal muscle weakness in a 15-year-old boy

Proximal muscle weakness, often indicative of a myopathy, has a broad differential diagnosis in children. We present a case of an adolescent boy with proximal weakness and a mildly elevated creatine kinase. He was found to have spinal muscular atrophy type III rather than a myopathy.

Spinal muscular atrophy

Spinal muscular atrophy is an autosomal recessive neurodegenerative disease characterised by degeneration of spinal cord motor neurons, atrophy of skeletal muscles, and generalised weakness. It is caused by homozygous disruption of the survival motor neuron 1 (SMN1) gene by deletion, conversion, or mutation. Although no medical treatment is available, investigations have elucidated possible mechanisms underlying the molecular pathogenesis of the disease. Treatment strategies have been developed to use the unique genomic structure of the SMN1 gene region. Several candidate treatment agents have been identified and are in various stages of development. These and other advances in medical technology have changed the standard of care for patients with spinal muscular atrophy. In this Seminar, we provide a comprehensive review that integrates clinical manifestations, molecular pathogenesis, diagnostic strategy, therapeutic development, and evidence from clinical trials.