Evidence of a segregation ratio distortion of SMN1 alleles in spinal muscular atrophy.

Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterised by degeneration and loss of the motor neurons of the anterior horn of the spinal cord. The absence of SMN1 is determinant to have SMA and parents of SMA patients are regarded as carriers of the disease. We compared the segregation ratio of the mutated allele and the wild-type allele of all the confirmed carrier parents assuming Mendelian proportions. Results of transmissions in 235 prenatal tests and in 128 unaffected siblings showed a statistically significant deviation in favour of the wild-type SMN1 allele. The number of affected foetuses and carriers were lower than that expected. No significant differences in the sex ratio or in the progenitor origin of the transmitted allele to the carriers were found. One hypothesis that has been advanced to account for the distortion observed in affected foetuses is the negative postzygote selection due to early miscarriage. However, given that the number of carriers in our series was lower than expected, prezygote events such as meiotic drive, survival of gametes or preferential fertilisation should also be considered.

Investigation of the role of SMN1 and SMN2 haploinsufficiency as a risk factor for Hirayama's disease: clinical, neurophysiological and genetic characteristics in a Spanish series of 13 patients.

OBJECTIVE: The effect of the number of copies in the SMN1 and SMN2 genes - the most extensively studied susceptibility and modifying genetic factors in adult onset motor neuron diseases - as a genetic risk factor for Hirayama's disease (HirD) has never been studied. The purpose of this study was to investigate the influence of the number of copies of the SMN1/SMN2 genes on the resulting phenotype in 13 HirD Spanish patients. PATIENTS AND METHODS: We performed a qualitative and quantitative SMN1/SMN2 gene analysis in 13 unrelated HirD patients. The phenotype-genotype correlation was investigated, paying particular attention to the effect of the SMN1/SMN2 copy number on the disease's phenotype. RESULTS: No patient had a homozygous deletion of the SMN1 or SMN2. No differences were found when comparing the SMN1 and SMN2 copy number distributions of the healthy population and HirD patients, and they do not therefore appear to be a susceptibility factor. There was also no correlation found between the number of copies of the SMN1 and SMN2 and the severity of the resulting phenotype. CONCLUSION: Our results suggest that SMN1 and SMN2 are not predisposing factors for HirD and therefore support a lack of association between these genes and the resulting phenotype.

Rapid identification of female haemophilia A carriers with deletions in the factor VIII gene by quantitative real-time PCR analysis.

Large deletions of the factorVIII gene account for approximately 5% of severe haemophilia A patients. Although deletions are readily detectable in males, the identification of heterozygosity in possible carriers of these families still constitutes a challenge. In order to identify a deleted allele over the background of the normal allele in these carriers, we developed a rapid real-time quantitative PCR approach by means of LightCycler technology and SYBR green I for monitoring product formation. The method was applied to families with independent deletions (one in exon 14 and the other in exons 23-24) of the Factor VIII gene, thereby allowing a reliable determination of carrier or non-carrier status. The method is extremely versatile and can be adapted to other deletions within the factorVIII gene as well as to other diseases whose molecular pathology consists of deletions or duplications.

Implementation of SMA carrier testing in genetic laboratories: comparison of two methods for quantifying the SMN1 gene.

The degeneration and loss of motor neurons of the anterior horn characterize children affected with spinal muscular atrophy (SMA). Mutations in the survival motor neuron gene (SMN1) are determinant for the development of the disease whereas the number of copies of SMN2, the highly homologous copy of SMN1, plays a role as a phenotypic modifier factor. The detection of SMN1 homozygous deletions is the typical test for SMA diagnosis. Owing to the limitation of this test for carrier and heterozygous deletion analysis, the demand of SMN1 quantitative tests is permanently growing. The high incidence of SMA, the notable carrier frequency, the severity of the disease, and the lack of effective treatment may justify the implementation of such an analysis in DNA diagnostic labs. The advantages and disadvantages of two reliable quantitative methods were evaluated. One of these is a competitive PCR protocol using internal standards and a genomic sequence as a reference. The other method is a real-time PCR employing an external standard as a reference. Both methods present sufficient advantages for incorporation into molecular genetic diagnostic labs. The possibility of studying samples from different labs, the versatility and reproducibility of the analysis, and cost-benefit calculations must be considered in the final choice.

Analysis of the C9orf72 gene in spinal muscular atrophy patients.

Spinal muscular atrophy and amyotrophic lateral sclerosis are both motor neuron disorders. Several studies have tried to establish a link between the two diseases but the subject is still under debate. In amyotrophic lateral sclerosis, large expansions of the hexanucleotide GGGGCC in intron 1 of the C9orf72 gene are responsible for a variable percentage of familial and sporadic cases. We investigated whether the number of the hexanucleotide repeat in C9orf72 was associated with the phenotype and the number of SMN2 copies in a group of 162 SMA patients. Conventional PCR, repeat primed-PCR and Southern blot were used to determine repeat number and characterize large expansions. Results showed that no pathological (> 30 repeats) or premutated alleles (20-30 repeats) were found. The allelic distribution of the C9orf72 gene in spinal muscular atrophy patients overlapped with the data obtained in our control population, discarding putative repeats that may be associated with the disease. No association was observed with either the SMA phenotype or the number of SMN2 copies. In conclusion, the involvement of C9orf72 as a genetic modifier in spinal muscular atrophy is unlikely. Current investigation of modifier genes in SMA and of the link between ALS and SMA should consider other possible candidates.

Treatment of spinal muscular atrophy cells with drugs that upregulate SMN expression reveals inter- and intra-patient variability.

Spinal muscular atrophy (SMA) is a genetic neuromuscular disorder caused by mutations in the SMN1 gene. The homologous copy (SMN2) is always present in SMA patients. SMN1 gene transcripts are usually full-length (FL), but exon 7 is spliced out in a high proportion of SMN2 transcripts (delta7) (Δ7). Advances in drug therapy for SMA have shown that an increase in SMN mRNA and protein levels can be achieved in vitro. We performed a systematic analysis of SMN expression in primary fibroblasts and EBV-transformed lymphoblasts from seven SMA patients with varying clinical severity and different SMN1 genotypes to determine expression differences in two accessible tissues (skin and blood). The basal expression of SMN mRNA FL and Δ7 in fibroblasts and lymphoblasts was analyzed by quantitative real-time PCR. The FL-SMN and FL/Δ7 SMN ratios were higher in control cells than in patients. Furthermore, we investigated the response of these cell lines to hydroxyurea, valproate and phenylbutyrate, drugs previously reported to upregulate SMN2. The response to treatments with these compounds was heterogeneous. We found both intra-patient and inter-patient variability even within haploidentical siblings, suggesting that tissue and individual factors may affect the response to these compounds. To optimize the stratification of patients in clinical trials, in vitro studies should be performed before enrolment so as to define each patient as a responder or non-responder to the compound under investigation.

Accuracy of marker analysis, quantitative real-time polymerase chain reaction, and multiple ligation-dependent probe amplification to determine SMN2 copy number in patients with spinal muscular atrophy.

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by absence of or mutations in the survival motor neuron1 gene (SMN1). All SMA patients have a highly homologous copy of SMN1, the SMN2 gene. Severe (type I) SMA patients present one or two SMN2 copies, whereas milder chronic forms (type II-III) usually have three or four SMN2 copies. SMN2 dosage is important to stratify patients for motor function tests and clinical trials. Our aim was to compare three methods, marker analysis, real-time quantitative polymerase chain reaction using the LightCycler instrument, and multiple ligation-dependent probe amplification (MLPA), to characterize their accuracy in quantifying SMN2 genes. We studied a group of 62 genetically confirmed SMA patients, 54 with homozygous absence of exons 7 and 8 of SMN1 and 8 with SMN2-SMN1 hybrid genes. A complete correlation using the three methods was observed in 32 patients (51.6%). In the remaining 30 patients, discordances between the three methods were found, including under or overestimation of SMN2 copies by marker analysis with respect to the quantitative methods (LightCycler and MLPA) because of lack of informativeness of markers, 3' deletions of SMN genes, and breakpoints in SMN2-SMN1 hybrid genes. The technical limitations and advantages and disadvantages of these methods are discussed. We conclude that the three methods complement each other in estimating the SMN2 copy number in most cases. However, MLPA offers additional information to characterize SMA cases with particular rearrangements such as partial deletions and hybrid genes.