La secuenciación masiva (NGS) como método diagnóstico en la enfermedad de Stargardt / Next generation sequencing in the diagnosis of Stargardt's disease

Introducción: La enfermedad de Stargardt es la maculopatía más frecuente en la edad infantil y adulta. Presenta un origen genético por afectación principalmente del gen ABCA4 con herencia autosómica recesiva. Se trata de un gen con características especiales por su gran tamaño y comportamiento, mostrando una elevada tasa de mutaciones. La aparición, desarrollo y accesibilidad económica de las técnicas de secuenciación masiva permiten realizar el diagnóstico genético de la enfermedad de Stargardt. Pacientes y métodos: Se presentan 2 casos clínicos diagnosticados genéticamente de enfermedad de Stargardt mediante la realización de un panel de secuenciación masiva de 298 genes. Resultados: Los pacientes presentaban un fenotipo de maculopatía de ojo de buey con ausencia de flecks y las siguientes mutaciones: c.G5882A:p.Gly1961Glu y c.C3056T:p.T1019M para el caso 1; c.G5882A:p.Gly1961Glu y c.287del:p.Asn96Thrfs·19 para el caso 2. Ambos pacientes comparten la mutación c.G588A:2p.Gly1961Glu que explica su fenotipo similar característico. Conclusiones: La secuenciación masiva es especialmente útil en la enfermedad de Stargardt, pues el gen ABCA4 presenta un gran tamaño y elevada heterogeneidad polimórfica, que se traduce en una amplia variabilidad clínica (AU)

Introduction: Stargardt's disease is the most frequent form of inherited macular dystrophy in children and adults. It is a genetic eye disorder caused by mutations in ABCA4 gene with an autosomal recessive inheritance. ABCA4 is a very polymorphic and large gene containing 50 exons. The development of next generation sequencing (NGS) can be used for the genetic diagnosis of this disease. Patients and methods: A report is presented on two patients with a clinical diagnosis of Stargardt's disease whose genetic confirmation was performed by a NGS panel of 298 genes. Results: Clinically, the patients showed bull's eye maculopathy and absence of flecks, and genetically they shared the Gly1961Glu mutation that could explain their common phenotype, together with c.C3056T:p.T1019M for case 1, and c.287del:p.Asn96Thrfs·19 for case 2. Conclusions: NGS is particularly useful in the diagnosis of Stargardt's disease as ABCA4 is a large gene with a high allelic heterogeneity that causes a wide range of clinical manifestations (AU)

Next generation sequencing in the diagnosis of Stargardt's disease. / La secuenciación masiva (NGS) como método diagnóstico en la enfermedad de Stargardt.

INTRODUCTION: Stargardt's disease is the most frequent form of inherited macular dystrophy in children and adults. It is a genetic eye disorder caused by mutations in ABCA4 gene with an autosomal recessive inheritance. ABCA4 is a very polymorphic and large gene containing 50 exons. The development of next generation sequencing (NGS) can be used for the genetic diagnosis of this disease. PATIENTS AND METHODS: A report is presented on two patients with a clinical diagnosis of Stargardt's disease whose genetic confirmation was performed by a NGS panel of 298 genes. RESULTS: Clinically, the patients showed bull's eye maculopathy and absence of flecks, and genetically they shared the Gly1961Glu mutation that could explain their common phenotype, together with c.C3056T:p.T1019M for case 1, and c.287del:p.Asn96Thrfs*19 for case 2. CONCLUSIONS: NGS is particularly useful in the diagnosis of Stargardt's disease as ABCA4 is a large gene with a high allelic heterogeneity that causes a wide range of clinical manifestations.

mTOR mutations in Smith-Kingsmore syndrome: Four additional patients and a review.

Smith-Kingsmore syndrome (SKS) OMIM #616638, also known as MINDS syndrome (ORPHA 457485), is a rare autosomal dominant disorder reported so far in 23 patients. SKS is characterized by intellectual disability, macrocephaly/hemi/megalencephaly, and seizures. It is also associated with a pattern of facial dysmorphology and other non-neurological features. Germline or mosaic mutations of the mTOR gene have been detected in all patients. The mTOR gene is a key regulator of cell growth, cell proliferation, protein synthesis and synaptic plasticity, and the mTOR pathway (PI3K-AKT-mTOR) is highly regulated and critical for cell survival and apoptosis. Mutations in different genes in this pathway result in known rare diseases implicated in hemi/megalencephaly with epilepsy, as the tuberous sclerosis complex caused by mutations in TSC1 and TSC2, or the PIK3CA-related overgrowth spectrum (PROS). We here present 4 new cases of SKS, review all clinical and molecular aspects of this disorder, as well as some characteristics of the patients with only brain mTOR somatic mutations.

PLP1 gene analysis in 88 patients with leukodystrophy.

Pelizaeus-Merzbacher disease (PMD) is caused in most cases by either duplications or point mutations in the PLP1 gene. This disease, a dysmyelinating disorder affecting mainly the central nervous system, has a wide clinical spectrum and its causing mutations act through different molecular mechanisms. Eighty-eight male patients with leukodystrophy were studied. PLP1 gene analysis was performed by the Multiplex Ligation-dependent Probe Amplification technique and DNA sequencing, and, in duplicated cases of PLP1, gene dosage was completed by using array-CGH. We have identified 21 patients with mutations in the PLP1 gene, including duplications, short and large deletions and several point mutations in our cohort. A customized array-CGH at the Xq22.2 area identified several complex rearrangements within the PLP1 gene region. Mutations found in the PLP1 gene are the cause of PMD in around 20% of the patients in this series.

Frequency of ABCA4 mutations in 278 Spanish controls: an insight into the prevalence of autosomal recessive Stargardt disease.

AIM: To determine the carrier frequency of ABCA4 mutations in order to achieve an insight into the prevalence of autosomal recessive Stargardt disease (arSTGD) in the Spanish population. METHODS: arSTGD patients (n = 133) were analysed using ABCR400 microarray and sequencing. Control subjects were analysed by two different strategies: 200 individuals were screened for the p.Arg1129Leu mutation by denaturing-HPLC and sequencing; 78 individuals were tested for variants with the microarray and sequencing. RESULTS: For the first strategy in control subjects, the p.Arg1129Leu variant was found in two heterozygous individuals, which would mean a carrier frequency for any variant of approximately 6.0% and a calculated arSTGD prevalence of 1:1000. For the second strategy, carrier frequency was 6.4% and therefore an estimated prevalence of the disease of 1:870. CONCLUSION: Calculated prevalence of arSTGD based on the ABCA4 carrier frequency could be considerably higher than previous estimation. This discrepancy between observed (genotypic) and estimated (phenotypic) prevalence could be due to the existence of non-pathological or low penetrance alleles, which may result in late-onset arSTGD or may be implicated in age-related macular degeneration. This situation should be regarded with special care when genetic counselling is given and further follow-up of these patients should be recommended.

Molecular analysis of the ABCA4 gene for reliable detection of allelic variations in Spanish patients: identification of 21 novel variants.

BACKGROUND/AIMS: Mutations in ABCA4 have been associated with autosomal recessive Stargardt disease (STGD), a few cases with autosomal recessive cone-rod dystrophy (arCRD) and autosomal recessive retinitis pigmentosa (arRP). The purpose of the study was threefold: to molecularly characterise families with no mutations or partially characterised families; to determine the specificity and sensitivity of the genotyping microarray; and to evaluate the efficiency of different methodologies. METHODS: 23 STGD, five arCRD and three arRP Spanish patients who were previously analysed with the ABCR400 microarray were re-evaluated. Results were confirmed by direct sequencing. In patients with either none or only one mutant allele, ABCA4 was further analysed by denaturing high-performance liquid chromatography (dHPLC) and multiplex ligation-dependent probe amplification (MLPA). Haplotype analysis was also performed. RESULTS: In the first analysis performed with the microarray, 27 ABCA4 variants (27/62; 43.5%) were found. By dHPLC scanning, 12 novel mutations were additionally identified. In addition, two previously described mutations, one false negative (1/62; 1.6%) and one false positive (1.6%), were detected. MLPA analysis did not reveal additional substitutions. The new strategy yielded an increment of 21% compared with the approach used in the first round. CONCLUSION: ABCA4 should be analysed by optimal combination of high-throughput screening techniques such as microarray, dHPLC and direct sequencing. To the best of our knowledge, this strategy yielded significant mutational spectrum identification in Spanish patients with ABCA4-associated phenotypes. Follow-up of patients, presenting an early onset of the disease and severe mutations, seems essential to perform accurate genotype-phenotype correlations and further characterisation of pathological ABCA4 alleles.

Foetal sex determination in maternal blood from the seventh week of gestation and its role in diagnosing haemophilia in the foetuses of female carriers.

The existence of foetal DNA in maternal blood, discovered in 1997, opened new possibilities for noninvasive prenatal diagnosis. This includes foetal sex assessment by the detection of specific Y chromosome sequences in maternal blood, particularly important when a foetus may be affected by an X-linked disorder such as haemophilia. This study aims to validate this sex assessment method and to test its clinical utility in the diagnosis of 15 potentially affected pregnancies in female carriers of haemophilia. In the validation study, 316 maternal blood samples from 196 pregnant women at gestations ranging from 5 weeks to 12 weeks were analysed. In the clinical study, 15 pregnancies at risk of having a haemophilic foetus were tested. All pregnancies in the validation study were correctly diagnosed. The accuracy and specificity of the methodology from the seventh week of gestation was 100%. The sex of all 15 pregnancies identified as being at risk of bearing a haemophilic foetus was correctly diagnosed. Foetal sex assessment by detecting specific Y chromosome sequences in maternal blood is now routinely used in our hospital because of its high accuracy from the seventh week of gestation. Reliable foetal gender determination from maternal blood of pregnant women carriers of haemophilia in the first trimester of gestation can avoid more conventional, invasive methods of prenatal diagnosis.