Elena Maestrini

A Quantitative-Trait Locus on Chromosome 6p Influences Different Aspects of Developmental Dyslexia

Recent application of nonparametric-linkage analysis to reading disability has implicated a putative quantitative-trait locus (QTL) on the short arm of chromosome 6. In the present study, we use QTL methods to evaluate linkage to the 6p25-21.3 region in a sample of 181 sib pairs from 82 nuclear families that were selected on the basis of a dyslexic proband. We have assessed linkage directly for several quantitative measures that should correlate with different components of the phenotype, rather than using a single composite measure or employing categorical definitions of subtypes. Our measures include the traditional IQ/reading discrepancy score, as well as tests of word recognition, irregular-word reading, and nonword reading. Pointwise analysis by means of sib-pair trait differences suggests the presence, in 6p21.3, of a QTL influencing multiple components of dyslexia, in particular the reading of irregular words (P=.0016) and nonwords (P=.0024). A complementary statistical approach involving estimation of variance components supports these findings (irregular words, P=.007; nonwords, P=.0004). Multipoint analyses place the QTL within the D6S422-D6S291 interval, with a peak around markers D6S276 and D6S105 consistently identified by approaches based on trait differences (irregular words, P=.00035; nonwords, P=.0035) and variance components (irregular words, P=.007; nonwords, P=.0038). Our findings indicate that the QTL affects both phonological and orthographic skills and is not specific to phoneme awareness, as has been previously suggested. Further studies will be necessary to obtain a more precise localization of this QTL, which may lead to the isolation of one of the genes involved in developmental dyslexia.

A molecular defect in loricrin, the major component of the cornified cell envelope, underlies Vohwinkel's syndrome

Terminal keratinocyte differentiation involves coordinated expression of several functionally interdependent genes, many of which have been mapped to the epidermal differentiation complex (EDC) on chromosome 1q21. We have identified linkage of Vohwinkels syndrome in an extended pedigree to markers flanking the EDC region with a maximum multipoint lod score of 14.3. Sequencing of the loricrin gene revealed an insertion that shifts the translation frame of the C-terminal Gly– and Gln/Lys–rich domains, and is likely to impair cornification. Our findings provide the first evidence for a defect in an EDC gene in human disease, and disclose novel insights into perturbations of cornified cell envelope formation.

Serotonin transporter (5‐HTT) and γ‐aminobutyric acid receptor subunit β3 (GABRB3) gene polymorphisms are not associated with autism in the IMGSA families

Previous studies have suggested that the serotonin transporter (5-HTT) gene and the gamma-aminobutyric acid receptor subunit beta3 (GABRB3) gene, or other genes in the 15q11-q13 region, are possibly involved in susceptibility to autism. To test this hypothesis we performed an association study on the collection of families from the International Molecular Genetic Study of Autism (IMGSA) Consortium, using the transmission disequilibrium test. Two polymorphisms in the 5-HTT gene (a functional insertion-deletion polymorphism in the promoter and a variable number tandem repeat in the second intron) were examined in 90 families comprising 174 affected individuals. Furthermore, seven microsatellite markers spanning the 15q11-q13 region were studied in 94 families with 182 affected individuals. No significant evidence of association or linkage was found at any of the markers tested, indicating that the 5-HTT and the GABRB3 genes are unlikely to play a major role in the aetiology of autism in our family data set.

Mutation analysis of the coding sequence of the MECP2 gene in infantile autism

Abstract. Mutations in the coding region of the methyl-CpG-binding proteinxa02 (MECP2) gene cause Rett syndrome and have also been reported in a number of X-linked mental retardation syndromes. Furthermore, such mutations have recently been described in a few autistic patients. In this study, a large sample of individuals with autism was screened in order to elucidate systematically whether specific mutations in MECP2 play a role in autism. The mutation analysis of the coding sequence of the gene was performed by denaturing high-pressure liquid chromatography and direct sequencing. Taken together, 14 sequence variants were identified in 152 autistic patients from 134 German families and 50 unrelated patients from the International Molecular Genetic Study of Autism Consortium affected relative-pair sample. Eleven of these variants were excluded for having an aetiological role as they were either silent mutations, did not cosegregate with autism in the pedigrees of the patients or represented known polymorphisms. The relevance of the three remaining mutations towards the aetiology of autism could not be ruled out, although they were not localised within functional domains of MeCP2 and may be rare polymorphisms. Taking into account the large size of our sample, we conclude that mutations in the coding region of MECP2 do not play a major role in autism susceptibility. Therefore, infantile autism and Rett syndrome probably represent two distinct entities at the molecular genetic level.

Identifying Autism Susceptibility Genes

Recent molecular genetic studies indicate that the goal of identifying autism susceptibility genes may soon be attainable. Taken together, the linkage findings suggest that there is a high probability that a region on chromosome 7q contains a susceptibility locus for autism. Nevertheless, the isolation of a putative gene in this region still represents a considerable challenge. The reported linkage peaks cover a very large genomic region (45 cM), and, although more than one locus might be implicated, the variation in location estimates could have arisen by chance or be due to other factors that may confound genetic mapping, such as an increased recombination rate. Clearly a metaanalysis offers one approach to extracting the maximum information from the different studies. At present, the 15q region shows weaker evidence for linkage, but the relatively high incidence of chromosomal abnormalities supports a role for this region in autism etiology, although the effect of this putative locus may be restricted to a smaller subgroup of autism cases. Interestingly, both the 15q and 7q regions contain imprinted genes suggesting that abnormal imprinting may be implicated in the pathogenesis of autism.Over the next few years, we can expect new genome screens and the enlargement of existing family collections. Larger sample sizes will help all groups to increasingly distinguish between true and spurious linkages and further overlapping areas of linkage are likely to emerge. We can also anticipate the use of more sophisticated analytical approaches to extract the full genetic information from family data by accurately measuring milder autism phenotypes and dimensionalizing the different phenotypic components for use in Quantitative Trait Loci analysis. The identification of the first autism susceptibility gene is likely to accelerate the discovery of other genes and will hopefully provide pointers to the mechanisms underlying abnormal brain development and function. That leap in our knowledge will be crucial for developing new preventative and treatment strategies.§To whom correspondence should be addressed (e-mail: maestrin@alma.unibo.it).

Mutation screening and imprinting analysis of four candidate genes for autism in the 7q32 region

Genetic studies indicate that chromosome 7q is likely to contain an autism susceptibility locus (AUTS1). We have followed a positional candidate gene approach to identify the relevant gene and report the analysis of four adjacent genes localised to a 800 kb region in 7q32 that contains an imprinted domain: PEG1/MEST, COPG2, CPA1 and CPA5—a previously uncharacterised member of the carboxypeptidase gene family. Screening these genes for DNA changes and association analysis using intragenic single nucleotide polymorphisms (SNPs) provided no evidence for an etiological role in IMGSAC families. We also searched for imprinting mutations potentially implicated in autism: analysis of both DNA methylation and replication timing indicated a normal imprinting regulation of the PEG1/COPG2 domain in blood lymphocytes of all patients tested. The analysis of these four genes strongly suggests that they do not play a major role in autism aetiology, and delineates our strategy to screen additional candidate genes in the AUTS1 locus.

Molecular Genetic Investigations of Autism

Genetic factors are likely to play a major role in the etiology of autism. The genetics of the disorder is however complex, probably involving the action of several genes. In an attempt to identify autism susceptibility loci we are currently undertaking a systematic screening of the whole human genome using multiplex families. We describe the resources and the methods needed to achieve such a task, including extensive collection of family data, semiautomated genotyping technology, and specialized statistical approaches for linkage analysis of complex traits.

Probes for CpG islands on the distal long arm of the human X chromosome are clustered in Xq24 and Xq28

We have isolated and characterized 55 EagI-containing genomic DNA clones from the distal long arm of the human X chromosome. The presence of additional sites for rare-cutter restriction enzymes and the demethylation of the corresponding genomic DNA demonstrate that at least 30 clones correspond to CpG islands of the Xq24-Xqter region. All clones were regionally mapped with a hybrid panel. The majority are in Xq28 and Xq24 (18 and 14 clones, respectively), 15 are in the Xq26-Xq27 interval, and none is in Xq25. This analysis demonstrates a nonuniform distribution of CpG islands that may reflect the distribution of coding regions in this part of the genome.