The clinical landscape for SMA in a new therapeutic era.

Despite significant advances in basic research, the treatment of degenerative diseases of the nervous system remains one of the greatest challenges for translational medicine. The childhood onset motor neuron disorder spinal muscular atrophy (SMA) has been viewed as one of the more tractable targets for molecular therapy due to a detailed understanding of the molecular genetic basis of the disease. In SMA, inactivating mutations in the SMN1 gene can be partially compensated for by limited expression of SMN protein from a variable number of copies of the SMN2 gene, which provides both a molecular explanation for phenotypic severity and a target for therapy. The advent of the first tailored molecular therapy for SMA, based on modulating the splicing behaviour of the SMN2 gene provides, for the first time, a treatment which alters the natural history of motor neuron degeneration. Here we consider how this will change the landscape for diagnosis, clinical management and future therapeutic trials in SMA, as well as the implications for the molecular therapy of other neurological diseases.

Improving detection and genetic counseling in carriers of spinal muscular atrophy with two copies of the SMN1 gene.

Spinal muscular atrophy (SMA) is an autosomal recessive disease caused by mutations in the survival motor neuron1 gene (SMN1). Global carrier frequency is around 1 in 50 and carrier detection is crucial to define couples at risk to have SMA offspring. Most SMA carriers have one SMN1 copy and are currently detected using quantitative methods. A few, however, have two SMN1 genes in cis (2/0 carriers), complicating carrier diagnosis in SMA. We analyzed our experience in detecting 2/0 carriers from a cohort of 1562 individuals, including SMA parents, SMA relatives, and unrelated individuals of the general population. Interestingly, in three couples who had an SMA child, both the parents had two SMN1 copies. Families of this type have not been previously reported. Our results emphasize the importance of performing a detailed carrier study in SMA parents with two SMN1 copies. Expanding the analysis to other key family members might confirm potential 2/0 carriers. Finally, when a partner of a known carrier presents two SMN1 copies, the study of both parents will provide a more accurate diagnosis, thus optimizing genetic counseling.

F8 gene dosage defects in atypical patients with severe haemophilia A.

We performed molecular analysis of the factor 8 gene (F8) in 272 unrelated Spanish patients with haemophilia A (HA) and detected a mutation by routine analysis in 267 of them (98.1%). No mutation was detected in the remaining five patients despite clinical and laboratory confirmation of HA. The aim is to describe the molecular alterations in F8 discovered by gene dosage methodologies in three of these patients. For methodology, F8 sequencing, intragenic marker analysis, multiplex ligation-dependent probe amplification and quantitative real time-PCR were followed. One patient had Klinefelter syndrome (47,XXY) and a large deletion spanning exons 1-12 masked by the other F8 allele; the second patient showed a large duplication spanning exons 2-10 and the third patient revealed a non-contiguous double duplication of exons 14 and 23-25. The remaining two patients had mild HA and dosage results were normal. The application of gene dosage methods is useful to define haemophilic patients in whom mutations are not detected using other routine methods. Nevertheless, in a small percentage of patients (<1%), no molecular pathology can be identified after testing several genetic methodologies.

The c.859G>C variant in the SMN2 gene is associated with types II and III SMA and originates from a common ancestor.

Homozygous mutations of the telomeric SMN1 gene lead to degeneration of motor neurons causing spinal muscular atrophy (SMA). A highly similar centromeric gene (SMN2) can only partially compensate for SMN1 deficiency. The c.859G>C variant in SMN2 has been recently reported as a positive disease modifier. We identified the variant in 10 unrelated chronic SMA patients with a wide spectrum of phenotypes ranging from type II patients who can only sit to adult walkers. Haplotype analysis strongly suggests that the variant originated from a common ancestor. Our results confirm that the c.859G>C variant is a milder SMN2 allele and predict a direct correlation between SMN activity and phenotypic severity.

Carrier frequency of SMA by quantitative analysis of the SMN1 deletion in the Iranian population.

BACKGROUND AND PURPOSE: Spinal muscular atrophy (SMA) is a common autosomal recessive neuromuscular disorder. Carrier frequency studies of SMA have been reported for various populations. Although no large-scale population-based studies of SMA have been performed in Iran, previous estimates have indicated that the incidence of autosomal recessive disorder partly because of the high prevalence of consanguineous marriage is much higher in the Iranian population than in other populations. METHODS: In this study, we used a reliable and highly sensitive quantitative real-time PCR assay with SYBR green I dye to detect the copy number of the SMN1 gene to determine the carrier frequency of SMA in 200 healthy unrelated, non-consanguineous couples from different part of Iran. RESULTS: To validate the method in our samples, we determined the relative quantification (RQ) of patients with homozygous deletion (0.00) and hemyzygous carriers (0.29-0.55). The RQ in 10 of 200 normal individuals were within the carrier range of 0.31-0.57, estimating a carrier frequency of 5% in the Iranian population. CONCLUSIONS: Our data show that the SMA carrier frequency in Iran is higher than in the European population and that further programs of population carrier detection and prenatal testing should be implemented.

Severe haemophilia A in a female resulting from an inherited gross deletion and a de novo codon deletion in the F8 gene.

Haemophillia A (HA) is an X-linked bleeding disorder caused by mutations in the F8 gene. While the disease affects 1 in 5000 males, phenotypic expression of haemophilia A is rare in females, similar to other X-linked recessive disorders. We describe a 5-year-old female with severe haemophilia A. We determined the underlying molecular defect in the F8 genes of the proband and her closest family members by direct DNA sequencing, marker analysis and quantitative real-time polymerase chain reaction. The patient showed two different mutations in the F8 gene: the paternal copy of the F8 gene had a de novo p.Phe652/653 deletion in exon 13 while the maternally inherited gene showed a large deletion encompassing exons 1 to 22. The structural analysis of residues Phe652/Phe653 based on a three-dimensional model of activated factor VIII provides evidence of the impact of the mutant factor VIII protein in the clinical manifestations of the patient. This unusual finding highlights the need to perform a thorough molecular analysis including sequencing, marker and quantitative analyses to identify compound heterozygous females with HA.

Application of intron 9 and intron 25 dinucleotide repeats of the factor VIII gene for carrier diagnosis in haemophilia A.

We describe the usefulness of two dinucleotide repeats located in intron 9 and in intron 25 of the factor VIII gene for carrier diagnosis of haemophilia A. We analyzed 100 unrelated Spanish women and 34 women from haemophilia A (HA) families in whom known intragenic markers were unhelpful in determining their carrier status. The heterozygosity rate of intron 9 and intron 25 markers in the 100 control women was lower (0.28 and 0.38, respectively) than the values obtained with common markers routinely used in our laboratory. However, the application of intron 9 and intron 25 markers was effective in identifying the at-risk X chromosome in 11 of 34 (32%) of the uninformative women from HA families. The combined use of these repeats with current markers may facilitate the identification of the X chromosome in HA families for application in carrier, prenatal and pre-implantation diagnoses.

Analysis of the involvement of the NR2E3 gene in autosomal recessive retinal dystrophies.

The nuclear receptor protein NR2E3 is postulated to play an important role in rod and cone photoreceptor development. NR2E3 gene mutational analyses were carried out in 103 unrelated subjects with different retinal diseases. A total of 14 different sequence variants were identified, including 3 mutations, 6 rare sequence variants and five polymorphisms. One of three mutations is novel (a frameshift mutation: c.1034_1038del5bp). Five of the six rare sequence variants and one of the polymorphisms identified are novel. Splice prediction programs and functional splicing assays were performed to study three of these variants. The c.119-2 A>C mutant allele construction produces, in addition to the normal one, an abnormal transcript of 180 bp resulting from an aberrant splicing with skipping of exon 2 and the generation of a premature stop codon in exon 3. These experimental data confirm the splice predictions made by the computer programs. The obtained results reinforce the idea that NR2E3 gene is involved in several retinal diseases without a clear genotype-phenotype correlation.

First report of a patient with a mixoploidy 47,XXX/94,XXXXXX.

We present a 16 years old female with a chromosomal mixoploidy and multiple phenotypic anomalies. Peripheral blood G-band karyotype was 47,XXX and her skin fibroblast karyotype revealed a mosaic with a 47,XXX cell line in 88% of metaphases and a 94,XXXXXX cell line in 12% of metaphases, consistent with a hypertetraploidy. The most prominent clinical signs were: short stature, left upper limb asymmetry, senile-like appearance, generalized hypertrichosis, and small hands and feet. Radiological examination showed bone dysplasia. The result of molecular studies demonstrated that the patient inherited the two X chromosomes from the mother and one from the father, indicating that her 47,XXX trisomy resulted from an oogenesis error in the first meiotic division. The 94,XXXXXX cell line was likely the result of a cytokinesis error. To our knowledge, this is the first documented patient with a trisomy and a hypertetraploidy.

SMN2 copy number predicts acute or chronic spinal muscular atrophy but does not account for intrafamilial variability in siblings.

Spinal muscular atrophy (SMA) is an autosomal recessive disorder that affects motor neurons. It is caused by mutations in the survival motor neuron gene 1 (SMN1). The SMN2 gene, which is the highly homologous SMN1 copy that is present in all the patients, is unable to prevent the disease. An SMN2 dosage method was applied to 45 patients with the three SMA types (I-III) and to four pairs of siblings with chronic SMA (II-III) and different phenotypes. Our results confirm that the SMN2 copy number plays a key role in predicting acute or chronic SMA. However, siblings with different SMA phenotypes show an identical SMN2 copy number and identical markers, indicating that the genetic background around the SMA locus is insufficient to account for the intrafamilial variability. In our results, age of onset appears to be the most important predictor of disease severity in affected members of the same family. Given that SMN2 is regarded as a target for potential pharmacological therapies in SMA, the identification of genetic factors other than the SMN genes is necessary to better understand the pathogenesis of the disease in order to implement additional therapeutic approaches.

Utility of a (GT) dinucleotide repeat in intron 1 of the factor 8 gene for haemophilia A carrier diagnosis.

We report the usefulness of a dinucleotide GT repeat in intron 1 of the factor 8 gene in carrier diagnosis of haemophilia A (HA). We analyzed 47 women from HA families in which the mutation was not identified in the index case and in which the common intragenic polymorphic markers were uninformative. The intron 1 GT repeat was useful to identify the X chromosome with the mutation in 19 of them. The analysis of this easily detectable marker in conjunction with other current markers may facilitate X chromosome identification in a large proportion of HA families.

Detection of novel mutations in the SMN Tudor domain in type I SMA patients.

The authors present a complete SMN gene analysis in four type I unrelated spinal muscular atrophy patients who retained one copy of the SMN1 gene. Two intragenic point mutations were identified in exon 3 (I116F, Q136E), affecting a very conserved region with the Tudor domain of SMN1. The remaining two patients showed no alterations in the SMN1 coding sequences although a transcription defect was detected in one of them, corroborating the existence of non-functional SMN1 genes.

Exclusion of mosaicism in Spanish haemophilia A families with inversion of intron 22.

Inversion of intron 22, the most frequent mutation event in haemophilia A (HA), was tested in our HA families to diagnose the females at risk of being carriers, to trace the origin of the mutation and to investigate the presence of germinal or somatic mosaicism. A total of 166 females belonging to 54 families with inversion, were analysed. All but one of the mothers tested were carriers and the inversion originated almost exclusively in male germ cells. Somatic or germline mosaicisms were excluded in 53 of these women and in 20 grandfathers, suggesting that such mosaicisms may be a rare event in families with inversion of intron 22.

Prenatal diagnosis for risk of spinal muscular atrophy.

OBJECTIVES: Prenatal diagnosis of spinal muscular atrophy is usually performed in high risk couples by detection of a homozygous deletion in the survival motor neurone gene (SMN1). However, other relatives at risk of being carriers very often request genetic counselling and the possibility of prenatal diagnosis. The aim of this study was to validate a SMN1 gene quantitative test to help the couples formed by one spinal muscular atrophy carrier and a partner of the general population (1/200 potential risk) to achieve a less ambiguous risk result for the pregnancy. DESIGN: Spinal muscular atrophy carrier studies in at-risk individuals. SETTING: Department of Genetics and Gynaecology and Obstetrics in a large university hospital. POPULATION: Seventy-nine obligate carriers (more than one affected child with deletion in the offspring) and 58 non-carriers (relatives of spinal muscular atrophy families defined by marker studies) were tested to set up a quantitative analysis. The method was applied in different situations in 126 members from 34 families with spinal muscular atrophy patients. METHODS: DNA studies of the SMNI gene by marker analysis and quantitative assay. MAIN OUTCOME MEASURES: To determine double (non-carrier) or single dose (carrier) of exon 7 of the SMN1 gene in relatives of spinal muscular atrophy patients. Bayesian calculation of risk. RESULTS: The sensitivity and specificity of the method were 96% and 100%, respectively. Studies on different couples with an a priori risk of 1/200 allowed us to reduce the final risk to 1/5000 or to increase it to 1/4. CONCLUSIONS: The quantitative method can be used to achieve a less ambiguous risk in pregnancies with a 1/200 risk and in families where no sample is available to study the index case. Screening of gamete donors when the recipient is a known carrier should also be considered.

Survival and respiratory decline are not related to homozygous SMN2 deletions in ALS patients.

The presence of the SMN2 deletion in 124 patients with ALS was investigated. Eleven patients had the homozygous deletion of SMN2 (8.8%) in comparison with 20 of 200 (10%) of the healthy control population. No significant differences in sex, age at onset, initial symptoms, form of inheritance, decline in ventilatory function, or survival time were found between patients with and without the deletion. The hypothesis that SMN2 is a prognostic factor in sporadic or familial ALS was not confirmed in this study.

Modifier genes in haemophilia: their expansion in the human genome.

Mutations in factor VIII and IX genes have a determinant effect on the severity of haemophilia. Modulation of clinical manifestations depends on other genetic factors, including modifier genes. In the context of haemophilia, such genes could be the ones involved in thrombophilia. Factor V Leiden and prothrombin 20210A were studied as possible phenotypic modifiers. Inhibitor development after therapeutic factor replacement depends on the type of mutation and on the genetic factors related to the immune response of each patient. The study of all these variants in haemophiliacs constitutes an important step in prevention, prognosis and therapeutic alternatives of the disease.

Characterisation of SMN hybrid genes in Spanish SMA patients: de novo, homozygous and compound heterozygous cases.

Autosomal recessive spinal muscular atrophy (SMA) is classified, by age of onset and maximal motor milestones achieved, into type I (severe form), type II (intermediate form) and type III (mild/moderate form). SMA is caused by mutations in the survival motor neuron telomeric gene (SMN1) and a centromeric functional copy of this gene (SMN2) exists, both genes being located at 5q13. Homozygous deletion of exons 7 and 8 of SMN1 has been detected in approx 85% of Spanish SMA patients regardless of their phenotype. Nineteen cases with the sole deletion of exon 7 but not exon 8 (2 cases of type I, 13 cases of type II, four cases of type III) were further analysed for the presence of SMN2-SMN1 hybrid genes. We detected four different hybrid structures. Most of the patients were carriers of a hybrid structure: centromeric intron 6- centromeric exon 7- telomeric exon 8 (CCT), with or without neuronal apoptosis-inhibitor protein (NAIP). In two patients, a different hybrid structure, viz. telomeric intron 6- centromeric exon 7- telomeric exon 8 (TCT), was detected with or without NAIP. A phenotype-genotype correlation comparing the different structures of the hybrid alleles was delineated. Type I cases in our series are attributable to intrachromosomal deletion with a smaller number of SMN2 copies. Most cases with hybrid genes are type II occurring by a combination of a classical deletion in one chromosome and a hybrid gene in the other. Type III cases are closely associated with homozygozity or compound heterozygozity for hybrid genes resulting from two conversion events and have more copies of hybrid genes and SMN2 than type I or II cases.

Bases moleculares de la atrofia muscular espinal: el gen SMN / Molecular bases of spinal muscular atrophy: the SMN gen

La atrofia muscular espinal (AME) es una enfermedad neuromuscular que se transmite con un patrón de herencia autosómico recesivo y se caracteriza por la degeneración y pérdida de las motoneuronas del asta anterior de la médula espinal. Se manifiesta con debilidad proximal y simétrica y con atrofia progresiva de los grupos musculares. La AME se clasifica en tres grupos de acuerdo con la gravedad de los síntomas, la edad de aparición de los mismos y su evolución: una forma grave tipo I o enfermedad de Werdnig-Hoffmann, una forma intermedia tipo II y una forma crónica tipo III o enfermedad de Kugelberg-Welander. La identificación del gen SMN1(survival motor neuron) como determinante de la AME ha ofrecido nuevas posibilidades para el estudio de la enfermedad. La patología molecular del gen SMN1 observada e los afectados incluye mayoritariamente deleciones y conversiones congénitas, habiéndose descrito también algunas mutaciones puntuales. No existe una correlación con el tipo de mutación y el fenotipo, puesto que se han descrito pacientes con grave afectación congénita hasta individuos asintomáticos con la misma deleción. Estas observaciones han llevado a postular la existencia de factores modificadores del fenotipo, algunos comprobados, como el número de copias del ggen SMN2, y otros posibles, aún en estudio. El gen SMN se expresa en varias poblaciones neuronales del sistema nervioso; sin embargo, solo las motoneuronas son responsables de las manifestaciones clínicas de la enfermedad. La proteína Smn forma un complejo nuclear en el que existen varias proteínas que intervendrían en distintas fases de la reacción del splicing (reacción que produce la unión de los exones con pérdida de los intrones de un gen). Esta función, aparentemente esencial en todas las células del organismo, debe ser crítica en las motoneuronas que son, probablemente, más sensibles a la ausencia o disfunción de SMN. Esto podría ser debido a que las motoneuronas poseen un nivel de transcripción mayor que otros tipos celulares o porque en otras células o tejidos existirían mecanismos compensatorios, genéticos o celulares, todavía desconocidos (AU)