Germline mosaicism in Rett syndrome identified by prenatal diagnosis

Rett syndrome is an X-linked neurodevelopmental dominant disorder that affects almost exclusively girls. The vast majority of cases are sporadic and are caused by de novo mutations in the MECP2 gene, located in Xq28. Only few familial cases have been reported: in four cases, the mother was an asymptomatic carrier and in other four cases, the germline mosaicism in the mother was postulated. Owing to the above reported cases of germline mosaicism, we decided to offer prenatal diagnosis to all expectant mothers with a Rett daughter despite the absence of the causative mutation in parents' blood. We describe here the outcome of the first nine cases of prenatal diagnosis followed by our center. In eight cases, the fetus did not carry the mutation. In one case, the female fetus did carry the same mutation of the affected sister. The couple decided to interrupt the pregnancy and to devolve fetal tissues for research purposes. Our results indicate that prenatal diagnosis should be proposed to all couples with a Rett daughter, even when the mutation is apparently de novo. Moreover, one positive prenatal test among the first nine cases indicates that germline mosaicism may be seriously considered for the assessment of recurrence risk during genetic counseling

Common Variants in the COL4A4 Gene Confer Susceptibility to Lattice Degeneration of the Retina

Lattice degeneration of the retina is a vitreoretinal disorder characterized by a visible fundus lesion predisposing the patient to retinal tears and detachment. The etiology of this degeneration is still uncertain, but it is likely that both genetic and environmental factors play important roles in its development. To identify genetic susceptibility regions for lattice degeneration of the retina, we performed a genome-wide association study (GWAS) using a dense panel of 23,465 microsatellite markers covering the entire human genome. This GWAS in a Japanese cohort (294 patients with lattice degeneration and 294 controls) led to the identification of one microsatellite locus, D2S0276i, in the collagen type IV alpha 4 (COL4A4) gene on chromosome 2q36.3. To validate the significance of this observation, we evaluated the D2S0276i region in the GWAS cohort and in an independent Japanese cohort (280 patients and 314 controls) using D2S0276i and 47 single nucleotide polymorphisms covering the region. The strong associations were observed in D2S0276i and rs7558081 in the COL4A4 gene (Pc = 5.8×10−6, OR = 0.63 and Pc = 1.0×10−5, OR = 0.69 in a total of 574 patients and 608 controls, respectively). Our findings suggest that variants in the COL4A4 gene may contribute to the development of lattice degeneration of the retina

Expression profiling of clonal lymphocyte cell cultures from Rett syndrome patients

BACKGROUND: More than 85% of Rett syndrome (RTT) patients have heterozygous mutations in the X-linked MECP2 gene which encodes methyl-CpG-binding protein 2, a transcriptional repressor that binds methylated CpG sites. Because MECP2 is subject to X chromosome inactivation (XCI), girls with RTT express either the wild type or mutant MECP2 in each of their cells. To test the hypothesis that MECP2 mutations result in genome-wide transcriptional deregulation and identify its target genes in a system that circumvents the functional mosaicism resulting from XCI, we performed gene expression profiling of pure populations of untransformed T-lymphocytes that express either a mutant or a wild-type allele. METHODS: Single T lymphocytes from a patient with a c.473C>T (p.T158M) mutation and one with a c.1308-1309delTC mutation were subcloned and subjected to short term culture. Gene expression profiles of wild-type and mutant clones were compared by oligonucleotide expression microarray analysis. RESULTS: Expression profiling yielded 44 upregulated genes and 77 downregulated genes. We compared this gene list with expression profiles of independent microarray experiments in cells and tissues of RTT patients and mouse models with Mecp2 mutations. These comparisons identified a candidate MeCP2 target gene, SPOCK1, downregulated in two independent microarray experiments, but its expression was not altered by quantitative RT-PCR analysis on brain tissues from a RTT mouse model. CONCLUSION: Initial expression profiling from T-cell clones of RTT patients identified a list of potential MeCP2 target genes. Further detailed analysis and comparison to independent microarray experiments did not confirm significantly altered expression of most candidate genes. These results are consistent with other reported data

Type-IV collagen related diseases.

Alport syndrome (ATS) is a progressive inherited glomerulonephritis accounting for 1-2% of all patients who start renal replacement therapy, with an estimated gene frequency of approximately 1 in 5000. ATS is a genetically heterogeneous disease, commonly inherited as an X-linked semi-dominant trait, caused by mutations in COL4A5, on the X-chromosome, and only rarely (less than 10% of cases) caused by the COL4A3 or the COL4A4 gene on chromosome 2q. In the X-linked form females are generally less affected than males, microhematuria being the only sign present throughout life, although approximately 30% can progress to end-stage renal disease. It became evident in recent years that mutations in the COL4A3 or the COL4A4 gene can give rise not only to autosomal recessive ATS syndrome, in which males and females are severely affected, but also to an autosomal dominant form, where the clinical progression towards impaired renal function can be very slow and also to benign familial hematuria (BFH) in which renal function is preserved

Clinical and molecular characterization of a patient with a 2q31.2-32.3 deletion identified by array-CGH

We report on a patient with a de novo interstitial deletion of the long arm of chromosome 2 involving bands 2q31.2-2q32.3. The patient shows severe mental retardation, absence of speech, sleep disturbances, behavioral problems, and some dysmorphic features. In particular, he presents with macrocephaly, high forehead, thick and coarse hair, thick eyebrows, synophrys, increased inner and outer canthal distance, bifid nasal tip, high palate, micrognathia, dysmorphic right ear, and long and tapering fingers. Array-CGH analysis allowed us to identify and characterize a 2q interstitial deletion of about 13 Mb, involving the segment between cytogenetic bands 2q31.2 and 2q32.3. The deletion was confirmed by quantitative PCR. We compare the phenotype of our patient with those already reported in literature. In particular, we discuss the similarities shared with two recently reported patients, studied by array-CGH, who show an overlapping deletion. The common clinical features are: long face, high forehead, abnormal teeth and ears, midface hypoplasia, high palate, micrognathia, transparent and thin skin, high frequency of inguinal hernia, severe development impairment, and behavioral problems. Some genes located in the deleted region may be good candidates for the neurological phenotype such as ZNF533 and MYO1B, which are both involved in neuronal function. Furthermore, the GLS gene could be a good candidate in generating the behavioral phenotype in the patient. In fact, it encodes for the major enzyme yielding glutamate from glutamine and it can be implicated in behavioral disturbances in which glutamate acts as a neurotransmitter

Le 'sindromi' di Alport: dalla clinica alla genetica [Clinical and genetic features of the Alport 'syndromes']

Alport syndrome (ATS) is a clinically and genetically heterogeneous progressive nephropathy often associated with deafness and/or ocular lesions. The histological aspect is characterized by thinning, thickening and splitting of the glomerular basement membrane (GBM). Alport syndrome is caused by mutations in COL4A3 gene (type IV collagen, alfa-3 chain), or COL4A4 gene (type IV collagen, alfa-4 chain) or COL4A5 gene (type IV collagen, alfa-5 chain) genes. Alport syndrome accounts for 1-2% of renal failure cases in Europe, and for 2-3% of transplanted patients in United States. This review focuses on the three types of Alport syndrome which differ in the clinical progression and in the mode of inheritance. The common X-linked form is caused by mutations in the COL4A5 gene and it accounts for 85% of cases. The autosomal dominant and the autosomal recessive forms are caused by mutations in either COL4A3 or COL4A4 genes. The autosomal recessive form which is responsible for the 10-15% of Alport cases, has been known since several years. On the contrary, the autosomal dominant form has only recently been identified in some families. Furthermore, this review will focus on the difficulties encountered during the genetic counselling related to the differential diagnosis between Alport syndrome and Thin Basement Membrane Disease (TBMD). We will report direct experiences of our group showing the difficulties to give an exact prognosis and a correct recurrence risk to the family

Rett syndrome: the complex nature of a monogenic disease.

Rett syndrome (RTT) is a severe neurodevelopmental disorder affecting almost exclusively girls. It is currently considered a monogenic X-linked dominant disorder due to mutations in MECP2 gene, encoding the methyl-CpG binding protein 2. A few RTT male cases, resulting from mosaicism for MECP2 mutations, have been reported. Male germline MECP2 mutations cause either severe encephalopathy with death at birth (usually in brothers of classical RTT females) or X-linked recessive mental retardation (XLMR). To date the wide phenotypic heterogeneity associated with MECP2 mutations in females (from classical RTT to healthy carriers) has been explained by differences in X chromosome inactivation. However, conflicting results have been obtained in different studies, with both random and highly skewed X-inactivation reported in healthy carrier females. Consequently it is possible that mechanisms other than X-inactivation play a role in the expressivity of MECP2 mutations. To explain the phenotypic heterogeneity associated with MECP2 mutations we propose a digenic model in which the presence of a "mutated" allele in a second gene, leading to a less functional protein, determines the clinical severity of the MECP2 mutation. The model is supported by the identification of the same mutation in XLMR and RTT cases. The carrier mothers of XLMR families are clinically asymptomatic and present balanced X chromosome inactivation. Therefore the same mutation arising in different genetic backgrounds can cause XLMR in males, remain silent in the carrier females and cause classic RTT in females. MECP2 mutations account for approximately 70-80% of classic RTT cases. MECP2 negative cases might result from mutations in noncoding regions of MECP2 gene. Alternatively, these cases might be due to mutations in other genes (locus heterogeneity). This hypothesis is supported by the identification of several chromosomal rearrangements in MECP2 negative patients with RTT and RTT-like phenotypes. MeCP2 is considered a general transcriptional repressor. However, conditional mouse mutants with selective loss of Mecp2 in the brain develop clinical manifestations similar to RTT, indicating that MECP2 is exclusively required for central nervous system function. The involvement of MeCP2 in methylation-specific transcriptional repression suggests that MECP2 related disorders result from dysregulated gene expression. Studies on gene expression have been performed in mouse and human brains. A relatively small number of gene expression changes were identified. It is possible that MeCP2 causes dysregulation of a very small subset of genes that are not detected with this method of analysis, or that very subtle changes in many genes cause the neuronal phenotype