Molecular analysis of survival motor neuron (SMN) and neuronal apoptosis inhibitory protein (NAIP) genes of spinal muscular atrophy patients and their parents

The analysis of the association between the copy numbers of survival motor neuron gene 2 and neuronal apoptosis inhibitory protein genes and the clinical phenotypes in 40 patients with spinal muscular atrophy: Observational study

In this article, the correlation between the copy number of survival motor neuron 2 (SMN2) gene, neuronal apoptosis inhibitory protein (NAIP), and the phenotype of spinal muscular atrophy patients were analyzed.Forty patients with spinal muscular atrophy (SMA) were included in the study at the Department of Medical Genetics of the First People's Hospital and the Department of Neurology of the Second People's Hospital in Yunnan Province from January 2012 to September 2018. Multiplex ligation-dependent probe amplification assay was performed to determine the copy numbers of SMN2 and NAIP genes. Statistical analysis was performed to determine the correlation between copy numbers of the SMN2 and NAIP genes and the clinical phenotypes of SMA.Our results show that among the 40 SMA patients, there were 13 type I cases, 16 type II cases and 11 type III cases. A total of 37 patients possessed a homozygous deletion of SMN1 exons 7 and 8, while the other 3 SMA patients possessed a single copy of SMN1 exon 8. There was no correlation between SMA subtypes and the deletion types of SMN1 exon 7 and 8 (P = .611). The percentage of 2, 3, and 4 copies of SMN2 exon 7 was 25.0%, 62.5%, and 12.5%, respectively. The percentage of 0, 1, and 2 copies of NAIP exon 5 was 10%, 57.5%, and 32.5%, respectively. The distributions of SMN2 and NAIP copy numbers among various SMA types were significantly different (all P < .05). Five combined SMN1-SMN2-NAIP genotypes were detected, of which 0-3-1 genotype had the highest proportion than the others, accounting for 42.5%. The copy number of SMN2 and NAIP gene had synergistic effect on SMA phenotype. The combined SMN1-SMN2-NAIP genotypes with fewer copies were associated with earlier onset age, higher mortality, and smaller average age at death in SMA patients.Therefore, we conclude that the copy number variance of SMN2 and NAIP is correlated with the SMA phenotype. Analysis of the copy number structure of the SMN1-SMN2-NAIP gene is helpful for SMA typing, disease prognosis prediction, and genetic counseling.

Spinal muscular atrophy and the antiapoptotic role of survival of motor neuron (SMN) protein

Spinal muscular atrophy (SMA) is a devastating and often fatal neurodegenerative disease that affects spinal motor neurons and leads to progressive muscle wasting and paralysis. The survival of motor neuron (SMN) gene is mutated or deleted in most forms of SMA, which results in a critical reduction in SMN protein. Motor neurons appear particularly vulnerable to reduced SMN protein levels. Therefore, understanding the functional role of SMN in protecting motor neurons from degeneration is an essential prerequisite for the design of effective therapies for SMA. To this end, there is increasing evidence indicating a key regulatory antiapoptotic role for the SMN protein that is important in motor neuron survival. The aim of this review is to highlight key findings that support an antiapoptotic role for SMN in modulating cell survival and raise possibilities for new therapeutic approaches.

Rapid diagnosis of spinal muscular atrophy using tetra-primer ARMS PCR assay: simultaneous detection of SMN1 and SMN2 deletion

Spinal muscular atrophy (SMA), the leading genetic cause of death in childhood, is an autosomal recessive neuromuscular disorder characterized by progressive muscle weakness, associated with deletions of the survival motor neuron 1 (SMN1) gene. Approximately 94% of SMA patients carry homologous deletions of SMN1 exon(s) 7 (and 8). Because of the high incidence and severity of the disease, precise detection and quantification of SMN1 and SMN2 gene copy numbers is essential for diagnosis and genetic counseling. We have developed a reliable single-tube tetra-primer PCR assay to simultaneously detect both the SMN1 and SMN2 exon 7 deletion using the advantage of C/T difference at nucleotide position of 840 in exon 7. The assay has been optimized and tested in 48 healthy controls, 20 known patients with SMA, 12 carriers (one SMN1 copy), and 8 amniotic fluids suspected of having SMA for whom we had determined the SMN1/SMN2 deletion by an additional PCR-RFLP method. We have observed complete concordance between methods. Our tetra-primer PCR assay is sensitive, low-cost, and easy to use method for simultaneous detection of both SMN1 and SMN2 deletion, which could be used even in "low-tech" laboratories.

Deletion analysis of SMN1 and NAIP genes in Southern Chinese children with spinal muscular atrophy

Spinal muscular atrophy (SMA) is a disorder characterized by degeneration of lower motor neurons and occasionally bulbar motor neurons leading to progressive limb and trunk paralysis as well as muscular atrophy. Three types of SMA are recognized depending on the age of onset, the maximum muscular activity achieved, and survivorship: SMA1, SMA2, and SMA3. The survival of motor neuron (SMN) gene has been identified as an SMA determining gene, whereas the neuronal apoptosis inhibitory protein (NAIP) gene is considered to be a modifying factor of the severity of SMA. The main objective of this study was to analyze the deletion of SMN1 and NAIP genes in southern Chinese children with SMA. Here, polymerase chain reaction (PCR) combined with restriction fragment length polymorphism (RFLP) was performed to detect the deletion of both exon 7 and exon 8 of SMN1 and exon 5 of NAIP in 62 southern Chinese children with strongly suspected clinical symptoms of SMA. All the 32 SMA1 patients and 76% (13/17) of SMA2 patients showed homozygous deletions for exon 7 and exon 8, and all the 13 SMA3 patients showed single deletion of SMN1 exon 7 along with 24% (4/17) of SMA2 patients. Eleven out of 32 (34%) SMA1 patients showed NAIP deletion, and none of SMA2 and SMA3 patients was found to have NAIP deletion. The findings of homozygous deletions of exon 7 and/or exon 8 of SMN1 gene confirmed the diagnosis of SMA, and suggested that the deletion of SMN1 exon 7 is a major cause of SMA in southern Chinese children, and that the NAIP gene may be a modifying factor for disease severity of SMA1. The molecular diagnosis system based on PCR-RFLP analysis can conveniently be applied in the clinical testing, genetic counseling, prenatal diagnosis and preimplantation genetic diagnosis of SMA.

Nonprogressive juvenile-onset spinal muscular atrophy: A clinico-radiological and CAG repeat study of androgen receptor gene

BACKGROUND

Occurrence of nonprogressive juvenile-onset spinal muscular atrophy (SMA) predominantly in males suggests a possibility of X-linked disorder but there is no such report addressing this problem.

AIMS

To evaluate CAG repeat expansion of androgen receptor (AR) gene in patients with nonprogressive juvenile-onset SMA.

SETTING

Tertiary medical teaching institute.

SUBJECTS AND METHODS

Patients fulfilling the diagnostic criteria of nonprogressive juvenile-onset SMA were included. Detailed clinical evaluation and pedigree charting were done in all. Nerve conduction study, electromyography and cervical spinal MRI were carried out. From peripheral venous blood, DNA was separated and AR gene CAG repeat exon polymorphism was assayed using polymerase chain reaction (PCR) in conjugation with genotyping and Gene scan soft ware. Number of CAG repeats was compared with normal controls.

RESULTS

25 patients with nonprogressive juvenile-onset SMA from 24 families were included and their mean age was 22.2 years. Age at the time of disease onset ranged between 15 and 30 years with a mean duration of illness 2.6 years. None of the patients had testicular atrophy or gynecomastia. C7-T1 myotomal wasting and weakness although was unilateral to begin with but became bilateral in 16 and 4 more patients had evidences of subclinical involvement of the other side as revealed by EMG. Spinal MRI revealed cord atrophy at C6-8 vertebral level in 16 patients. CAG repeat study of AR gene was carried out in 16 patients. The number of CAG repeats in patients ranged between 15 and 39 (median 21) which were within the normal range.

CONCLUSION

Abnormal CAG repeat expansion of AR gene is not found in patients with nonprogressive juvenile-onset SMA.

Study of survival of motor neuron (SMN) and neuronal apoptosis inhibitory protein (NAIP) gene deletions in SMA patients

In view of the paucity of deletion studies of survival of motor neuron (SMN) and neuronal apoptosis inhibitor protein (NAIP) genes in Indian SMA patients, this study has been undertaken to determine the status of SMN1, SMN2 and NAIP gene deletions in Indian SMA patients. Clinically and neurophysiologically diagnosed SMA patients were included in the study. A gene deletion study was carried out in 45 proximal SMA patients and 50 controls of the same ethnic group. Both SMN1 and NAIP genes showed homozygous absence in 76% and 31% respectively in proximal SMA patients. It is proposed that the lower deletion frequency of SMN1 gene in Indian patients may be due to mutations present in other genes or population variation, which need further study.

Degradation of survival motor neuron (SMN) protein is mediated via the ubiquitin/proteasome pathway

Homozygous deletion or mutation in the survival motor neuron (SMN)1 gene causes proximal spinal muscular atrophy (SMA), whereas SMN2 acts as a modifying gene that can influence the severity of SMA. It has been suggested that restoration of the SMN protein level in neuronal cells may prevent cell loss and may be helpful for treatment of SMA. Recent studies indicate that the ubiquitin/proteasome pathway is a major system for proteolysis of intracellular proteins. In this study, we investigate whether SMN protein is degraded via the ubiquitin/proteasome pathway. Primary fibroblasts were established from the skin biopsies of SMA patients and the effect of a proteasome inhibitor MG132 and lysosome inhibitor NH(4)Cl on SMN protein level was examined. We found that MG132, but not NH(4)Cl, significantly increased the amount and nuclear accumulation of SMN protein in SMA patient's fibroblasts. Immunoprecipitation/western blot analysis indicated that SMN protein was ubiquitinated in cells. In vitro protein ubiquitination assay also demonstrated that SMN protein could be conjugated with ubiquitin. Taken together, we have provided clear evidences that degradation of SMN protein is mediated via the ubiquitin/proteasome pathway and suggest that proteasome inhibitors may up-regulate SMN protein level and may be useful for the treatment of SMA.

Joint range of motion limitations in children and young adults with spinal muscular atrophy

OBJECTIVES

To elicit descriptive data about limited joint range of motion (ROM) in subjects with type II or III spinal muscular atrophy (SMA) and to examine the relation between the number of motions with limited range and both age and functional ability.

DESIGN

Descriptive cross-sectional study.

SETTING

Neurologic pediatric outpatient clinic at a hospital in Taiwan.

PARTICIPANTS

Twenty-seven subjects with SMA type II (mean age, 9.8+/-6.5y) and 17 with SMA type III (mean age, 12.2+/-8.7y). Intervention Measurement with transparent goniometers of joint ROM bilaterally of the shoulder, elbow, wrist, hip, knee, and ankle.

MAIN OUTCOME MEASURES

The proportion of participants with each ROM limitation compared with all participants with the same SMA type, age distribution of the participants with each ROM limitation, mean range loss of each motion limitation, and the contracture index (risk index of joint contracture).

RESULTS

Eighty-nine percent of the participants with SMA type II experienced knee extension limitation. Approximately 50% of the participants with both types of SMA had ankle dorsiflexion limitation. The motions of knee and hip extension and ankle dorsiflexion also had a relatively high contracture index. The number of motions with limited range positively correlated ( P <.001) with age and upper-extremity functional grade (the higher the functional grade, the poorer the functional ability) for SMA type II.

CONCLUSIONS

We found varying degrees of joint ROM limitation. Certain motions were noted to be high risks for the development of contractures. This risk was higher mostly in younger children.

Molecular analysis of SMN, NAIP and P44 genes of SMA patients and their families

Mutations of the telomeric survival motor neuron gene (SMN1) are related to spinal muscular atrophy (SMA). However, no phenotype-genotype correlation has been observed since the SMN1 gene is lacking in the majority of patients affected with either the severe form (type I) or the milder forms (types II and III). Here, we analyze the SMN, NAIP and P44 genes in 132 Chinese SMA patients and their families. At least three types of normal allele, and four types of mutant allele were found in this study. The combination of one normal allele with one mutant allele resulted in carriers of different types, and the combination of different mutant alleles accounted for the different genotypes among different types of SMA. Deletions of mutant alleles can be further subgrouped into four types, which includes involving SMN1, SMN1 and NAIP(T) (telomeric portion of NAIP gene), SMN1 and NAIP(T) and P44(T) (telomeric portion of P44 gene), and SMN1 and SMN2 (centromeric portion of SMN gene). Some of the severe (type I) SMA cases correlated with the extent of deletions in the SMN, NAIP and P44 genes or the dosage of SMN gene when both SMN1 and SMN2 are deleted. We also found two novel point mutations, an A insertion at codon 8 (AGT-->AAGT) and an A substitution at codon 228 (TTA-->TAA).

Molecular genetic study of a childhood form of spinal muscular atrophy

Molecular genetic studies were performed in 28 cases of childhood-onset spinal muscular atrophy (24 unrelated families). This consisted of type 1 (severe) (n = 5), type 2 (intermediate form) (n = 8), and type 3 (mild) (n = 15). Deletion of exons 7 and 8 of the SMNt gene was found in 100%, 100%, and 93%, respectively, in type 1, 2, and 3 spinal muscular atrophy. Deletion of exons 5 and 6 of the NAIP gene was found in 3 of 5 (60%) of type 1 and none of the type 2 and 3 cases. None of the 32 asymptomatic relatives had homozygous deletions in the SMNt and NAIP genes. Thus, the role of the NAIP gene needs to be specifically defined in spinal muscular atrophy. In a suspected case of spinal muscular atrophy, deletion of the SMNt gene is a useful laboratory marker for confirmation of the diagnosis.

Gene deletion patterns in spinal muscular atrophy patients with different clinical phenotypes

Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterized by degeneration of lower motor neurons. We have assayed deletions in two candidate genes, the survival motor neuron (SMN) and neuronal apoptosis inhibitory protein (NAIP) genes, in 108 samples, of which 46 were from SMA patients, and 62 were from unaffected subjects. The SMA patients included 3 from Bahrain, 9 from South Africa, 2 from India, 5 from Oman, 1 from Saudi Arabia, and 26 from Kuwait. SMN gene exons 7 and 8 were deleted in all type I SMA patients. NAIP gene exons 5 and 6 were deleted in 22 of 23 type I SMA patients. SMN gene exon 7 was deleted in all type II SMA patients while exon 8 was deleted in 19 of 21 type II patients. In 1 type II SMA patient, both centromeric and telomeric copies of SMN exon 8 were deleted. NAIP gene exons 5 and 6 were deleted in only 1 type II SMA patient. In 1 of the 2 type III SMA patients, SMN gene exons 7 and 8 were deleted with no deletion in the NAIP gene, while in the second patient, deletions were detected in both SMN and NAIP genes. None of the 62 unaffected subjects had deletions in either the SMN or NAIP gene. The incidence of biallelic polymorphism in SMN gene exon 7 (BsmAI) was found to be similar (97%) to that (98%) reported in a Spanish population but was significantly different from that reported from Taiwan (0%). The incidence of a second polymorphism in SMN gene exon 8 (presence of the sequence ATGGCCT) was markedly different in our population (97%) and those reported from Spain (50%) and Taiwan (0%).

High-resolution genetic and physical map of the Lgn1 interval in C57BL/6J implicates Naip2 or Naip5 in Legionella pneumophila pathogenesis

Prior genetic and physical mapping has shown that the Naip gene cluster on mouse chromosome 13D1-D3 contains a gene, Lgn1, that is responsible for determining the permissivity of ex vivo macrophages to Legionella pneumophila replication. We have identified differences in the structure of the Naip array among commonly used inbred mouse strains, although these gross structural differences do not correlate with differences in L. pneumophila permissiveness. A physical map of the region employing clones of the C57BL/6J haplotype confirms that there are fewer copies of Naip in this strain than are in the physical map of the 129 haplotype. We have also refined the genetic location of Lgn1, leaving only Naip2 and Naip5 as candidates for Lgn1. Our genetic map suggests the presence of two hotspots of recombination within the Naip array, indicating that the 3' portion of Naip may be involved in the genomic instability at this locus.

Analysis of the mRNA transcripts of the survival motor neuron (SMN) gene in the tissue of an SMA fetus and the peripheral blood mononuclear cells of normals, carriers and SMA patients

Spinal muscular atrophy (SMA) is a disorder characterized by degeneration of the anterior horn cells of the spinal cord. The gene most highly associated with SMA is the survival motor neuron (SMN) gene. In this study, we present an analysis of messenger RNA (mRNA) expression of the SMN gene in peripheral blood mononuclear cells in normal subjects, SMA carriers and patients from 20 SMA families. We found at least 6-8 different transcripts of SMN gene formed by alternative splicing involving exons 3, 5 and 7. We compared transcripts from the different types of SMA and found no definite differences in transcript patterns and amounts. Normal subjects with the telomeric SMN (SMN(T)) gene only had variable splicing resulting in several transcripts, the most dominant being a transcript containing all coding regions. However, SMA patients with the centromeric SMN (SMN(C)) gene only had a higher degree of splice variation and tended to show little or no exon 7. These results demonstrate that SMN(T) and SMN(C) genes participate in alternative splicing phenomena. The different splicing patterns support the view that the SMN(T) gene is responsible for SMA disease. We also analyzed the transcripts from several tissues of an SMA fetus who had a homozygous SMN(T) gene deletion. Different splicing patterns were also found in these tissues, and were similar to the splicing pattern of leukocytes. We compared the major transcripts from exons 4 to 8 of both the SMN(T) and SMN(C) genes and found that the relative proportion varied among normal subjects, SMA carriers and patients. This approach could be used as a novel diagnostic method. We suggest that analyzing the mRNA expression of the SMN gene in peripheral blood mononuclear cells offers an apparently reliable technique for separating SMA patients, carriers, and normal individuals.

A mouse model for spinal muscular atrophy

The survival motor neuron gene is present in humans in a telomeric copy, SMN1, and several centromeric copies, SMN2. Homozygous mutation of SMN1 is associated with proximal spinal muscular atrophy (SMA), a severe motor neuron disease characterized by early childhood onset of progressive muscle weakness. To understand the functional role of SMN1 in SMA, we produced mouse lines deficient for mouse Smn and transgenic mouse lines that expressed human SMN2. Smn-/- mice died during the peri-implantation stage. In contrast, transgenic mice harbouring SMN2 in the Smn-/- background showed pathological changes in the spinal cord and skeletal muscles similar to those of SMA patients. The severity of the pathological changes in these mice correlated with the amount of SMN protein that contained the region encoded by exon 7. Our results demonstrate that SMN2 can partially compensate for lack of SMN1. The variable phenotypes of Smn-/-SMN2 mice reflect those seen in SMA patients, providing a mouse model for this disease.

Progressive spinal muscular atrophies

Spinal muscular atrophy is the most common autosomal-recessive genetic disorder lethal to infants. It was first described in the 1890s. Since then our understanding of the disorder has progressed significantly. Progression of the disease is due to loss of anterior horn cells, thought to be caused by apoptosis. Diagnosis is based on the course of the illness, as well as certain changes seen on nerve and muscle biopsy and electrodiagnostic studies. More recently, our understanding of the genetics of this disorder has provided a noninvasive approach to diagnosis. This method of testing has its downside, but the quest for a more sensitive analysis is still underway. Even though our knowledge of this disease has come a long way since its first recognition, the therapies available to these children are still only supportive. Again, researchers eagerly look for new therapeutic interventions to allow for improved quality of life and an extended life span.

Prenatal prediction of spinal muscular atrophy in Chinese

We used linkage analysis, non-isotope SSCP (single-strand conformation polymorphism) and PCR-RFLP (polymerase chain reaction-restriction fragment length polymorphism) for prenatal diagnosis of spinal muscular atrophy (SMA). A total of 26 cases from 20 SMA families (16, type 1 and 4) were evaluated. 5 out of 26 fetuses were affected and, following genetic counselling, the parents decided to terminate the pregnancies. Aborted fetal tissues were examined and the diagnosis was confirmed in each case. The 21 unaffected cases were either normals (12 cases) or carriers (9 cases). These children have been followed for six months to two and a half years. No false-negative or false-positive results on prenatal testing were found. We conclude that prenatal diagnosis of SMA is reliable and accurate.

Caspase-3-like proteases and 6-hydroxydopamine induced neuronal cell death

Neurotoxicity induced by 6-hydroxydopamine (6-OHDA) is believed to be due, in part, to the production of reactive oxygen species (ROS) and/or an inhibition of mitochondrial function. However, little is known about the ensuing intracellular events which ultimately result in cell death. Here we show that exposure to relatively low concentrations of 6-OHDA induces apoptosis of cerebellar granule neurons (CGN). 6-OHDA-induced apoptosis of CGN is associated with activation of a caspase-3-like protease. Western blots of cytosolic extracts from 6-OHDA-treated CGN reveal a translocation of cytochrome c from mitochondria to the cytosol, which precedes activation of the protease detected by Ac-DEVD-pNA. DNA laddering can be blocked by caspase inhibitors zVAD-FMK and Ac-DEVD-CHO, however cell death can only be attenuated for a short time period in the presence of these inhibitors. Our data suggest that 6-OHDA-induced apoptosis of CGN involves activation of a caspase-3-like protease. In contrast to the neurotoxicity induced by MPP+, however, the peptide inhibitors zVAD-FMK and Ac-DEVD-CHO can only attenuate early neuronal death induced by 6-OHDA. At later time points, neuronal death lacking DNA laddering occurs even in the presence of the peptide inhibitor zVAD-FMK or Ac-DEVD-CHO.