Drosophila melanogaster homolog of Down syndrome critical region 1 is critical for mitochondrial function

Mitochondrial dysfunction has emerged as a common theme that underlies numerous neurological disorders, including Down syndrome. Down syndrome cultures and tissues show mitochondrial damage such as impaired mitochondrial enzyme activities, defective mitochondrial DNA repairs and accumulation of toxic free radicals, but the cause of mitochondrial dysfunction remains elusive. Here we demonstrate that the Drosophila melanogaster homolog of human Down syndrome critical region gene 1 (DSCR1), nebula (also known as sarah, sra), has a crucial role in the maintenance of mitochondrial function and integrity. We report that nebula protein is located in the mitochondria. An alteration in the abundance of nebula affects mitochondrial enzyme activities, mitochondrial DNA content, and the number and size of mitochondria. Furthermore, nebula interacts with the ADP/ATP translocator and influences its activity. These results identify nebula/DSCR1 as a regulator of mitochondrial function and integrity and further suggest that an increased level of DSCR1 may contribute to the mitochondrial dysfunction seen in Down syndrome.     

The human Achaete-Scute homologue 2 (ASCL2,HASH2) maps to chromosome 11p15.5, close to IGF2 and is expressed in extravillus trophoblasts

Here we describe the cloning of the human Achaete Scute Homologue 2 (HASH2) gene, officially designated ASCL2 (Achaete Scute complex like 2), a homologue of the Drosophila Achaete and Scute genes. In mouse, this gene is imprinted and maps to chromosome 7. We mapped the human homologue close to IGF2 and H19 at 11p15.5, the human region syntenic with mouse chromosome 7, indicating that this imprinted region is highly conserved in mouse and man. HASH2 is expressed in the extravillus trophoblasts of the developing placenta only. The lack of HASH2 expression in non-malignant hydatidiform (androgenetic) moles indicates that HASH2 is also imprinted in man.      

Bone density phenotypes in mice aneuploid for the Down syndrome critical region

Down syndrome (trisomy 21) is associated with reduced bone density in humans, but it is unclear whether this is due to specific effects of chromosome 21 genes or lifestyle factors. Mouse models with aneuploidy of segments of mouse chromosome 16 that are homologous to human chromosome 21 can be used to elucidate the mechanism by which Down syndrome phenotypes arise. Ts1Rhr and Ms1Rhr mice are trisomic and monosomic, respectively, for the hypothesized "Down syndrome critical region" containing approximately 33 genes. We assessed the skeletons of these mice from 3 to 16 weeks of age using dual X-ray absorptiometry. Ts1Rhr mice were unexpectedly similar to normal controls, showing that a larger region of trisomy is necessary to recapitulate the Down syndrome phenotype. Ms1Rhr mice, in contrast, showed decreases in weight, bone mineral content, bone mineral density, and bone area from weaning to adulthood. Regional bone density was also decreased in the femur, tibia, and lower lumbar spine. The microarchitecture of 3 week old Ms1Rhr femurs was then analyzed using μCT. Volumetric density, total tissue volume, bone volume, and bone fraction were all reduced in both cortical and trabecular bone. Ms1Rhr trabeculae were thinner and had decreased connectivity. A 31.5% reduction in the level of insulin-like growth factor I in the serum was found, and we hypothesize that this is responsible for the bone density phenotype. We discuss bone-related genes in the region and propose that humans with distal chromosome 21 deletions may exhibit reduced bone density.     

Brain-specific angiogenesis inhibitor 1 (BAI1) is expressed in human cerebral neuronal cells

Brain-specific angiogenesis inhibitor 1 (BAI1) is a p53-target gene specifically expressed in the brain. We examined the distribution of the endogenous BAI1 protein in normal human brain tissue using a polyclonal antibody against the extracellular region of BAI1. Immunohistochemical study demonstrated that BAI1 was expressed in neuronal cells of the cerebral cortex but not in astrocytes. BAI1 protein was localized in the cellular cytoplasm and membrane. It was predominantly localized in the cellular membrane when expressed in cultured cells by means of gene transfection. BAI1 protein may play an important role in neuronal functions such as synapse formation and signal transduction.     

Clinical manifestations of the deletion of Down syndrome critical region including DYRK1A and KCNJ6

A relatively small region of human chromosome 21 (Hsa21) is considered to play a major role in Down syndrome (DS) phenotypes, and the concept of a Down syndrome critical region (DSCR) has been proposed. The goal of the phenotype–genotype correlation study is to discover which genes are responsible for each DS phenotype. Loss of the genomic copy numbers of Hsa21 can give us important suggestion to understand the functions of the involved genes. Genomic copy number aberrations were analyzed by micro‐array‐based comparative genomic hybridization (aCGH) in 300 patients with developmental delay. Partial deletions of Hsa21 were identified in three patients with developmental delay, epilepsy, microcephaly, and distinctive manifestations. Two of the patients had mosaic deletions of 21q22‐qter including a part of DSCR; one of whom whose mosaic ratio was higher than the other showed more severe brain morphogenic abnormality with colpocephaly, which was similar to the previously reported patients having pure deletions of 21q22‐qter, indicating the critical region for cortical dysplasia at this region. The remaining patient had the smallest microdeletion with 480 kb in DSCR including DYRK1A and KCNJ6. Although we could not identify any nucleotide alteration in DYRK1A and KCNJ6 in our cohort study for 150 patients with mental retardation with/without epilepsy, this study underscores the clinical importance of DSCR not only for DS but also for developmental disorders. © 2010 Wiley‐Liss, Inc.     

Isolation of a novel gene from the DiGeorge syndrome critical region with homology to Drosophila gdl and to human LAMC1 genes

DiGeorge syndrome, and more widely the CATCH 22 syndrome, are associated with microdeletions in chromosomal region 22q11.2. A critical region of 500 kb has been delimited within which maps the breakpoint of a balanced translocation associated with mild CATCH 22 phenotypes. We report the isolation from this critical region of a novel gene, DGCR6, which maps 115 kb centromeric to the balanced translocation breakpoint. The DGCR6 gene product shares homology with the Drosophila melanogaster gonadal protein, which participates in gonadal and germ-line cells development, and with the human laminin. gamma-1 chain, which upon polymerization with alpha- and beta- chains forms the laminin molecule. Laminin binds to cells through interaction with a receptor and has functions in cell attachment, migration and tissue organization during development. DGCR6 could be a candidate for involvement in the DiGeorge syndrome pathology by playing a role in neural crest cell migration into the third and fourth pharyngeal pouches, the structures from which derive the organs affected in DiGeorge syndrome.      

Keipert syndrome (Nasodigitoacoustic syndrome) is X-linked and maps to Xq22.2-Xq28

Keipert syndrome is a rare condition comprising sensorineural deafness associated with facial and digital abnormalities. To date, Keipert syndrome has been reported in six male patients including two sib pairs; however the genetic basis of Keipert syndrome is yet to be elucidated. We report on the diagnosis of Keipert syndrome in the nephew of the brothers in the first report of Keipert syndrome, with a pedigree consistent with X-linked recessive inheritance. Linkage analysis using microsatellite markers along the X-chromosome suggests that the gene for Keipert syndrome is located in the region Xq22.2-Xq28. We postulate the Keipert syndrome is caused by a novel gene at Xq22.2-Xq28.     

A novel human homologue of the Drosophila frizzled wnt receptor gene binds wingless protein and is in the Williams syndrome deletion at 7q11.23

Williams syndrome (WS) is a developmental disorder with a characteristic personality and cognitive profile that is associated, in most cases, with a 2 Mb deletion of part of chromosome band 7q11.23. By applying CpG island cloning methods to cosmids from the deletion region, we have identified a new gene, called FZD3. Dosage blotting of DNA from 11 WS probands confirmed that it is located within the commonly deleted region. Sequence comparisons revealed that FZD3, encoding a 591 amino acid protein, is a novel member of a seven transmembrane domain receptor family that are mammalian homologs of the Drosophila tissue polarity gene frizzled. FZD3 is expressed predominantly in brain, testis, eye, skeletal muscle and kidney. Recently, frizzled has been identified as the receptor for the wingless (wg) protein in Drosophila. We show that Drosophila as well as human cells, when transfected with FZD3 expression constructs, bind Wg protein. In mouse, the wg homologous Wnt1 gene is involved in early development of a large domain of the central nervous system encompassing much of the midbrain and rostral metencephalon. The potential function of FZD3 in transmitting a Wnt protein signal in the human brain and other tissues suggests that heterozygous deletion of the FZD3 gene could contribute to the WS phenotype.      

Molecular and cytogenetic characterization of a recurrent unbalanced translocation (4;21) (p16.3;q22.1): Relevance to the Wolf-Hirschhorn and Down syndrome critical regions

We report on an aneuploidy syndrome due to the unbalanced segregation of a familial translocation (4;21)(p16.3;q22.1) causing a partial 4p monosomy and a partial 21q trisomy. The three affected children presented with severe failure to thrive, short stature, microcephaly, profound hypotonia, and mental retardation. The face, very similar in the three children, is characterized by frontal bossing, upslanting of the palpebral fissures, short nose, and deep set ears, giving the overall appearance of the Down syndrome. The molecular study has defined the aneuploid segment on both 4p and 21q. Most of the Down syndrome critical region was found to be trisomic, while only part of the candidate Wolf-Hirschhorn syndrome critical region was deleted, suggesting that this region is not critical for the major malformations characteristic for WHS. © 1996 Wiley-Liss, Inc.     

Mutations in the human homologue of Drosophila patched in Japanese nevoid basal cell carcinoma syndrome patients

Mutations in the human homologue of Drosophila patched (PTCH) have been identified in patients with nevoid basal cell carcinoma syndrome (NBCCS; also called Gorlin syndrome) as well as sporadic basal cell carcinomas and medulloblastomas. However, using PCR-SSCP analysis, mutations in PTCH have been found in only a fraction (about one third to a half) of NBCCS patients. In this study, we determined the whole genomic organizations of the PTCHgene and developed a new set of more accurate primers for the analysis of mutations in PTCH. Using these primers, we examined 8 Japanese NBCCS patients for mutations in all PTCH exons by direct sequencing of the PCR products. As a result, we identified 5 novel PTCH mutations in 6 out of 8 patients including 2 sisters as well as 5 polymorphisms, two of them, 1704G>C and 2928G>C were novel. Four of these mutations, 900delC, 1247insT, 1999delC and 933+5G>T, cause protein truncation due to the insertion or deletion of a single nucleotide or aberrant splicing. The remaining mutation, 1514G>A was a missense alteration (G509D). Interestingly, the amino acid substitution, G509V, has been reported previously in an NBCCS patient, suggesting an important role of this amino acid residue in the function of PTCH protein. The difference in the detection rate of PTCH mutations among NBCCS between previous reports and ours is due to the difference either in ethnicity or in the detection methods.     

A microdeletion proximal of the critical deletion region is associated with mild Wolf-Hirschhorn syndrome

It is generally accepted that the facial phenotype of Wolf-Hirschhorn syndrome is caused by deletions of either Wolf-Hirschhorn critical regions 1 or 2 (WHSCR 1-2). Here, we identify a 432?kb deletion located 600?kb proximal to both WHSCR1-2 in a patient with a WHS facial phenotype. Seven genes are underlying this deletion region including FAM193a, ADD1, NOP14, GRK4, MFSD10, SH3BP2, TNIP2. The clinical diagnosis of WHS facial phenotype was confirmed by 3D facial analysis using dense surface modeling. Our results suggest that the WHSCR1-2 flanking sequence contributes directly or indirectly to the severity of WHS. Sequencing the Wolf-Hirschhorn syndrome candidate 1 and 2 genes did not reveal any mutations. Long range position effects of the deletion that could influence gene expression within the WHSCR were excluded in EBV cell lines derived from patient lymphoblasts. We hypothesize that either (1) this locus harbors regulatory sequences which affect gene expression in the WHSCR1-2 in a defined temporal and spatial developmental window or (2) that this locus is additive to deletions of WHSCR1-2 increasing the phenotypic expression.