Tuberous sclerosis complex: A Drosophila connection

Recent findings based on experiments with Drosophila melanogaster significantly advance our understanding of a human disease known as tuberous sclerosis complex (TSC). The present note begins with background information and goes on to explain what these findings are

Primary microcephaly: microcephalin and ASPM determine the size of the human brain

In microcephaly (small head), the size of the head as measured by the occipito-frontal circumference of an affected individual is greater than three standard deviations below the population age-related mean. The cranial vault in a microcephaly patient is smaller than normal relative to the facial skeleton and the rest of the body. The small cranial capacity results from underlying hypoplasia of the cerebral cortex rather than abnormal development of the overlying skull and there is no major abnormality in cortical architecture (Jackson et al 1998; Mochida and Walsh 2001). Microcephaly is known to have a heterogeneous etiology with environmental and genetic causes. Among the environmental causes are intrauterine infections, drugs (alcohol) taken during pregnancy, prenatal radiation exposure, maternal phenylketonuria and birth asphyxia. All of these except birth asphyxia are known to be rare causes of microcephaly. The majority of microcephalic cases are caused by a variety of genetic mechanisms including cytogenetic abnormalities and single-gene disorders (Jackson et al 1998)

Promoter characterization of the TSC1 gene

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder with loci on chromosome 9q34 (TSC1) and chromosome 16p13 (TSC2). Genes for both loci have been isolated and characterized. The promoters of both genes have not been characterized so far and little is known about the regulation of these genes. This study reports the characterization of the human TSC1 promoter region. We have used PCR methodology to isolate approximately 1.6 kb genomic DNA 5’ to the TSC1 cDNA. This sequence has directed a high level of expression of luciferase activity in both HeLa and HepG2 cells. Successive 5’ and 3’ deletion analysis has suggested that a ~587 bp region, from position +77 to -510 from the TSS (transcription start site), contains the promoter activity. Interestingly, this region contains no consensus TATA box or CAAT box. However, a 521 bp fragment surrounding the TSS exhibits the characteristics of a CpG island which overlaps with the promoter region. Electrophoretic mobility shift assay indicated interaction of the predicted transcription factors binding elements like GC-box, E-box and E2F binding element with the transcription factors, within the core promoter. The identification of the TSC1 promoter region will help in designing a suitable strategy to identify mutations in this region in patients who do not show any mutations in the coding regions. It will also help to study the regulation of the TSC1 gene and its role in tumorigenesis

Genetic analysis of primary microcephaly in Indian families: novel ASPM mutations

Patients with primary microcephaly, an autosomal recessive trait, have mild to severe mental retardation without any other neurological deficits. It is a genetically heterogeneous disorder with six known loci: MCPH1 to MCPH6. Only the genes for MCPH1 and MCPH5 have been identified so far.We have ascertained nine consanguineous families with primary microcephaly from India. To establish linkage of these nine families to known MCPH loci,microsatellite markers were selected from the candidate regions of each of the six known MCPH loci and used to genotype the families. The results were suggestive of linkage of three families to the MCPH5 locus and one family to the MCPH2 locus. The remaining five families were not linked to any of the known loci. DNA-sequence analysis identified one known (Arg117X) and two novel (Trp1326X and Gln3060X) mutations in the three MCPH5-linked families in a homozygous state. Three novel normal population variants (i.e., c.7605G>A, c.4449G>A, and c.5961A>G) were also detected in the ASPM gene

Genetic variation in the TSC1 and TSC2 genes in 24 TSC families from India

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder with loci on chromosome 9q34.3 (TSC1) and chromosome 16p13.3 (TSC2). Genes for both loci have been isolated and characterized. Clinical symptoms of TSC include cortical tubers, subependymal nodules, mental retardation, seizures, autism, shagreen patches and angiofibromas on the skin, cardiac rhabdomyomas, retinal hamartomas, and angiomyolipomas in the kidneys. Several mutations have been reported in both TSC genes in patients mainly from the western and Japanese populations. However, there is no report on the mutation analysis of TSC genes in patients from the Indian population. We report here the mutational analysis of the TSC1 and TSC2 genes in 24 TSC families from India. Using PCR-SSCP and DNA sequence analyses, we have screened all 21 coding exons of the TSC1 gene and all 41 coding exons of the TSC2 gene in seven familial and 17 sporadic TSC cases. We have also sequenced promoters of both the TSC genes in 24 probands. We have identified a total of 12 mutations. Of these, seven mutations are novel. We have identified a single previously known deletion in the TSC1 gene. Of 11 mutations identified in the TSC2 gene, 3 are deletions, 2 are insertions, 3 are missense, 2 are splice site and 1 is a nonsense mutation. In addition, we have also detected three and eight variants/polymorphisms in the TSC1 and TSC2 genes respectively. Of these, three are novel SNPs. There was no correlation between the types of mutations (missense, nonsense, etc.) and the severity of the disease. As observed in the western and Japanese populations, the mutations were scattered across the TSC2 gene. DNA sequence analysis of promoter regions of both TSC genes in 24 families did not show any variation. (This work was financially supported by a grant from DBT, New Delhi to AK and SCG and a CSIR JRF to MA)

Mutational analysis of the TSC2 gene in 23 TSC families from India

Tuberous sclerosis complex (TSC) is an autosomal dominant neuro-cutaneous disorder with loci on chromosome 9q34.3 (TSC1) and chromosome 16p13.3 (TSC2). Genes for both loci have been isolated and characterized. Pathologically, tuberous sclerosis complex is a disorder of cell proliferation, differentiation and migration. Clinical symptoms of TSC include cortical tubers, subependymal nodules, mental retardation, seizures, autism, shagreen patches and angiofibromas on the skin, cardiac rhabdomyomas, retinal hamartomas, ungual and peringual fibromas, and cysts and angiomyolipomas in the kidneys. More than 350 mutations have been reported in both TSC genes in patients mainly from the western and Japanese populations. However, there is no report on the mutation analysis of TSC genes in patients from the Indian population. We report here the mutational analysis of the TSC2 gene in 23 TSC families from India. Using PCR-SSCP and DNA sequence analyses, we have screened all 41 exons and promoter region of the TSC2 gene in seven familial and 16 sporadic TSC cases. We have identified a total of 12 mutations representing 25% and 75% of familial and sporadic cases respectively. Of these, eight mutations are novel. Of 12 mutations, 3 are deletions, 2 are insertions, 4 are missense, 2 are splice site and 1 is a nonsense mutation. In addition, we have also detected nine single-nucleotide polymorphisms (SNPs) in the TSC2 gene. Of these three are novel SNPs. There was no correlation between the types of mutations (missense, nonsense, etc.) and the severity of the disease. The mutations were found to be distributed across the gene without any clustering, suggesting that mutation analysis requires scanning of the complete coding sequence of the TSC2 gene (This work was financially supported by a grant from DBT, New Delhi to AK and SCG and a CSIR JRF to MA

Mutation and polymorphism analysis of TSC1 and TSC2 genes in Indian patients with tuberous sclerosis complex

Objective - To find the mutation and polymorphism spectrum of TSC1 and TSC2 genes in patients affected with tuberous sclerosis complex from the Indian population. Material and methods - All coding exons and promoter regions of both TSC genes were screened for mutations and polymorphisms in 24 TSC families using polymerase chain reaction-single strand conformation polymorphism and DNA sequencing techniques. Results - A single previously known mutation, c.2111_2112delAT was identified in the TSC1 gene. A total of 11 mutations were identified in the TSC2 gene. Of these, seven mutations, c.137_138delGA, c.2070delC, c.2087_2088insAA, c.3080T>C (p.L1027P), c.648+1G>A, c.3131+1G>A and c.5034C>G were novel. The remaining four mutations, c.4544_4547delACAA, c.1941_1942insT, c.1831C>T (p.R611W) and c.1832G>A (p.R611Q) had been reported previously in other populations. The novel mutation, c.137_138delGA was predicted to result in the production of a very small tuberin protein of 64 amino acids lacking all seven functional domains. In addition, we also detected three and 10 polymorphisms in the TSC1 and TSC2 genes respectively. DNA sequence analysis of promoter regions of both TSC genes in 24 families did not show any variation. Conclusions - This is the first molecular genetic study of TSC in an Indian population. A total of 12 mutations were detected in 24 Indian TSC families in TSC genes. All except one mutation were detected in the TSC2 gene. No variation was found in the promoter regions of either gene. As observed in the western and Japanese populations, the mutations were scattered across the TSC2 gene

Mutations in WDR62, encoding a centrosomal and nuclear protein, in Indian primary microcephaly families with cortical malformations

Primary microcephaly is an autosomal recessive disorder characterized by smaller than normal brain size and mental retardation. It is genetically heterogeneous with seven loci: MCPH1-MCPH7. We have previously reported genetic analysis of 35 families, including the identification of the MCPH7 gene STIL. Of the 35 families, three families showed linkage to the MCPH2 locus. Recent whole-exome sequencing studies have shown that the WDR62 gene, located in the MCPH2 candidate region, is mutated in patients with severe brain malformations. We therefore sequenced the WDR62 gene in our MCPH2 families and identified two novel homozygous protein truncating mutations in two families. Affected individuals in the two families had pachygyria, microlissencephaly, band heterotopias, gyral thickening, and dysplastic cortex. Using immunofluorescence study, we showed that, as with other MCPH proteins, WDR62 localizes to centrosomes in A549, HepG2, and HaCaT cells. In addition, WDR62 was also localized to nucleoli. Bioinformatics analysis predicted two overlapping nuclear localization signals and multiple WD-40 repeats in WDR62. Two other groups have also recently identified WDR62 mutations in MCPH2 families. Our results therefore add further evidence that WDR62 is the MCPH2 gene. The present findings will be helpful in genetic diagnosis of patients linked to the MCPH2 locus