Ataxia-telangiectasia: linkage analysis in highly inbred Arab and Druze families and differentiation from an ataxia-microcephaly-cataract syndrome

SIL1 mutations and clinical spectrum in patients with Marinesco-Sjogren syndrome

Marinesco-Sjögren syndrome is a rare autosomal recessive multisystem disorder featuring cerebellar ataxia, early-onset cataracts, chronic myopathy, variable intellectual disability and delayed motor development. More recently, mutations in the SIL1 gene, which encodes an endoplasmic reticulum resident co-chaperone, were identified as the main cause of Marinesco-Sjögren syndrome. Here we describe the results of SIL1 mutation analysis in 62 patients presenting with early-onset ataxia, cataracts and myopathy or combinations of at least two of these. We obtained a mutation detection rate of 60% (15/25) among patients with the characteristic Marinesco-Sjögren syndrome triad (ataxia, cataracts, myopathy) whereas the detection rate in the group of patients with more variable phenotypic presentation was below 3% (1/37). We report 16 unrelated families with a total of 19 different SIL1 mutations. Among these mutations are 15 previously unreported changes, including single- and multi-exon deletions. Based on data from our screening cohort and data compiled from the literature we found that SIL1 mutations are invariably associated with the combination of a cerebellar syndrome and chronic myopathy. Cataracts were observed in all patients beyond the age of 7 years, but might be missing in infants. Six patients with SIL1 mutations had no intellectual disability, extending the known wide range of cognitive capabilities in Marinesco-Sjögren syndrome to include normal intelligence. Modestly constant features were somatic growth retardation, skeletal abnormalities and pyramidal tract signs. Examination of mutant SIL1 expression in cultured patient lymphoblasts suggested that SIL1 mutations result in severely reduced SIL1 protein levels irrespective of the type and position of mutations. Our data broaden the SIL1 mutation spectrum and confirm that SIL1 is the major Marinesco-Sjögren syndrome gene. SIL1 patients usually present with the characteristic triad but cataracts might be missing in young children. As cognitive impairment is not obligatory, patients without intellectual disability but a Marinesco-Sjögren syndrome-compatible phenotype should receive SIL1 mutation analysis. Despite allelic heterogeneity and many families with private mutations, the phenotype related to SIL1 mutations is relatively homogenous. Based on SIL1 expression studies we speculate that this may arise from a uniform effect of different mutations on protein expression.

Phenotype-genotype correlations in patients with Marinesco-Sjögren syndrome

Marinesco-Sjögren syndrome (MSS; MIM 248800) is an autosomal recessive disorder characterized by congenital cerebellar ataxia, early cataracts, developmental delay, myopathy and short stature. Alterations in the gene SIL1 cause MSS in some patients with typical findings. In this study, molecular investigations including sequencing of the SIL1 gene, western blotting and microscopic investigations in fibroblast cultures were carried out in a cohort of 15 patients from 14 unrelated families, including the large, inbred family reported by Superneau et al., having the clinical features of MSS to provide insights into the pathophysiology of the disorder. A total of seven different mutations were found in eight of the patients from seven families. The mutations caused loss of the BIP-associated protein (BAP) protein in four patients by western blot. Novel clinical features such as dental abnormalities, iris coloboma, eczema and hormonal abnormalities were noticed in some patients, but there was no clear way to distinguish those with and without SIL1 mutations. Cultured fibroblasts contained numerous cytoplasmic inclusion bodies, similar to those identified in the brain of the whoozy mouse in five unrelated patients, three with and two without SIL1 mutations, suggesting some SIL1 negative patients share a common cellular pathogenesis with those who are SIL1 positive.

The molecular basis of autosomal recessive diseases among the Arabs and Druze in Israel

The Israeli population mainly includes Jews, Muslim and Christian Arabs, and Druze In the last decade, data on genetic diseases present in the population have been systematically collected and are available online in the Israeli national genetic database ( http://www.goldenhelix.org/server/israeli ). In the non-Jewish population, up to 1 July 2010, the database included molecular data on six diseases relatively frequent in the whole population: thalassemia, familial Mediterranean fever (FMF), cystic fibrosis, deafness, phenylketonuria and congenital adrenal hyperplasia, as well as data on 195 autosomal recessive diseases among Muslim Israeli Arabs, 11 among the Christian Arabs and 31 among Druze. A single mutation was characterized in 149 out of the 238 rare disorders for which the molecular basis was known. In many diseases, mutation had never been observed in any other population and was present in one family only suggesting that it occurred as a de novo event. In other diseases, the mutation was present in more than one community or even in other populations such as Bedouins from the Arab peninsula or Christians from Lebanon. In the 89 other disorders, more than one mutation was characterized either in the same gene or in more than one gene. While it is probable that most of these cases represent random events in some cases such as Bardet Biedl among the Bedouins, the reason may be a selective advantage to the heterozygotes.

A novel function of DNA repair molecule Nbs1 in terminal differentiation of the lens fibre cells and cataractogenesis

The Nbs1 protein, hypomorphic mutant in Nijmegen breakage syndrome (NBS), is a component of the Mre11/Rad50/Nbs1 (M/R/N) complex that acts as a DNA double-strand break sensor and functions in cell cycle checkpoint in response to DNA damage and DNA repair. Here we report that targeted disruption of murine NBS1 gene (Nbn) in the lens alters the M/R/N complex nuclear localization and results in microphthalmia in mice due to reduced proliferation of the lens epithelial cells. Unexpectedly, all Nbn-deficient lenses develop cataracts at an early age due to altered lens fibre cell differentiation, including disruption of normal lens epithelial and fibre cell architecture and incomplete denucleation of fibre cells, and these changes are independent of the p53 pathway. In addition, Nbn-deficient lenses show dysregulated transcription of various crystallins. Thus, this study implicates a novel function of Nbs1 in terminal differentiation of the lens fibre cells and in cataractogenesis.

Postnatal growth failure, microcephaly, mental retardation, cataracts, large joint contractures, osteoporosis, cortical dysplasia, and cerebellar atrophy

We describe two sibs with postnatal-onset growth deficiency, microcephaly, cataract, prominent supraorbital ridge, large joint contractures, severe osteoporosis, cortical dysplasia, cerebellar atrophy, and mental retardation. The combination appears to constitute a previously undescribed syndrome inherited in an autosomal recessive pattern.

Mice lacking the cell adhesion molecule Thy-1 fail to use socially transmitted cues to direct their choice of food

BACKGROUND

Thy-1 is a major cell-surface glycoprotein of mature neurons and certain other cells, including those of the lymphoreticular system. Despite being the simplest member of the immunoglobulin superfamily, the biological role of Thy-1 has proved elusive. Analysis of Thy-1 null mice has shown the presence of excessive GABAergic inhibition of neurotransmission in the dentate gyrus of the hippocampal formation selectively, without any neurological or behavioural effects being apparent.

RESULTS

We show here that Thy-1 null mice are unable to make the appropriate dietary choice in the test for social transmission of food preference, despite showing a normal level of social interaction with the demonstrator mouse, normal neophobia, and normal learning in a T-maze using scented food as cues. The mice also performed normally in tests of anxiety, locomotor activity, exploration of a novel environment, habituation to novelty and spatial learning. This phenotype is maintained on two different strain backgrounds, is rescued by transgenic expression of Thy-1 and by administration of the GABA(A) receptor antagonist pentylenetetrazole.

CONCLUSIONS

The test for social transmission of food preference is based on the normal ability of mice in a colony to learn from each other which foods are safe to eat. The lack of this key survival behaviour in Thy-1 null mice could act as an evolutionary pressure point to conserve expression of Thy-1. Furthermore, the specific cognitive defect caused by inactivation of the Thy-1 gene suggests that it would be worthwhile to determine the role of Thy-1 in certain human familial forms of mental retardation that map to chromosome 11q22-23 in the region of the Thy-1 locus rather than the nearby ataxia telangiectasia locus.

Congenital cataract and multisystem disorders

A knowledge of those syndromes associated with congenital cataract is essential for the paediatric ophthalmologist, as congenital cataracts are manifest in a large number of syndromes. It is important to have the correct diagnosis in such cases, not only for genetic and prognostic information, but also in order to help the parents to understand their child's condition. This paper describes the more common syndromes seen in association with congenital cataract, and emphasises the importance of looking at the whole child and family. We aim to provide a practical clinical guide to the diagnosis of hereditary and non-hereditary syndromes associated with congenital cataract.

Consanguineous marriage and its clinical consequences in migrants to Australia

Marriage between close biological relatives is strongly favoured in many countries in Asia and Africa. Although substantial numbers of migrants from these regions are now living in Australia, little information is available either on the prevalence of consanguineous unions among migrants or on their clinical outcome. Data are presented on a range of generally rare autosomal recessive genetic disorders diagnosed in the children of parents of Eastern Mediterranean origin who attended Westmead Hospital, Sydney for genetic counselling during the period 1990 to 1994. The effects of parental consanguinity are assessed both in terms of the specific recessive disorders detected, and the perceived role of consanguineous marriage in the communities investigated.

A single ataxia telangiectasia gene with a product similar to PI-3 kinase

A gene, ATM, that is mutated in the autosomal recessive disorder ataxia telangiectasia (AT) was identified by positional cloning on chromosome 11q22-23. AT is characterized by cerebellar degeneration, immunodeficiency, chromosomal instability, cancer predisposition, radiation sensitivity, and cell cycle abnormalities. The disease is genetically heterogeneous, with four complementation groups that have been suspected to represent different genes. ATM, which has a transcript of 12 kilobases, was found to be mutated in AT patients from all complementation groups, indicating that it is probably the sole gene responsible for this disorder. A partial ATM complementary DNA clone of 5.9 kilobases encoded a putative protein that is similar to several yeast and mammalian phosphatidylinositol-3' kinases that are involved in mitogenic signal transduction, meiotic recombination, and cell cycle control. The discovery of ATM should enhance understanding of AT and related syndromes and may allow the identification of AT heterozygotes, who are at increased risk of cancer.

Human cDNA clones that modify radiomimetic sensitivity of ataxia-telangiectasia (group A) cells

Genes responsible for genetic diseases with increased sensitivity to DNA-damaging agents can be identified using complementation cloning. This strategy is based on in vitro complementation of the cellular sensitivity by gene transfer. Ataxia-telangiectasia (A-T) is a multisystem autosomal recessive disorder involving cellular sensitivity to ionizing radiation and radiomimetic drugs. A-T is genetically heterogeneous, with four complementation groups. We attempted to identify cDNA clones that modify the radiomimetic sensitivity of A-T cells assigned to complementation group [A-T(A)]. The cells were transfected with human cDNA libraries cloned in episomal vectors, and various protocols of radiomimetic selection were applied. Thirteen cDNAs rescued from survivor cells were found to confer various degrees of radiomimetic resistance to A-T(A) cells upon repeated introduction, and one of them also partially influenced another feature of the A-T phenotype, radioresistant DNA synthesis. None of the clones mapped to the A-T locus on chromosome 11q22-23. Nine of the clones were derived from known genes, some of which are involved in cellular stress responses. We concluded that a number of different genes, not necessarily associated with A-T, can influence the response of A-T cells to radiomimetic drugs, and hence the complementation cloning approach may be less applicable to A-T than to other diseases involving abnormal processing of DNA damage.

Non-progressive familial congenital cerebellar hypoplasia

A syndrome is reported of congenital non-progressive, gradually slightly improving, ataxia in 3 out of 5 male sibs, issues of a first-order consanguineous mating. Additional characteristic features included: moderate microcephaly, generalised muscle weakness and hypotonia, nystagmus, and moderate mental retardation. A pyramidal syndrome of hyperreflexia and Babinski signs, without any spasticity, became manifest in the 2nd or 3rd year of life. In all three, the caudal part of the vermis was absent, the enlarged IVth ventricle opening up via Magendie's foramen into the cisterna magna. The middle and rostral vermian parts as well as the sagittal paravermian parts of the cerebellar hemispheres were hypoplastic. The differential diagnosis of this syndrome is analysed and the developmental pathogenetic mechanisms likely to produce the typifying distribution of aplasia are indicated.

A new human brain cDNA molecule: assignment to chromosome 11q21-q23.1 and description of two polymorphisms studied by the polymerase chain reaction

A new human brain cDNA molecule was mapped by in situ hybridization to the 11q21-q23.1 region of the human genome, probably to the 11q22 band. An EcoRI restriction site and a (GT)n repeat element within the gene were shown to be polymorphic. Both polymorphisms were readily studied by the polymerase chain reaction. A two-allele polymorphism was described for the EcoRI restriction site, whereas four different alleles were detected for the second genetic marker. The observed heterozygosities were 37% and 42% for the former and the latter polymorphism, respectively. The combined heterozygosity index was estimated to be 0.56. These new genetic markers will be useful for linkage analysis of neurogenetic diseases that have been mapped to this chromosomal region.