PubMed Health⌕ Search

Biomedical subjects

D H Ledbetter

Publications and source records attributed to D H Ledbetter.

At least 55 records · Page 3Linked to original sources
PubMed1998

Maternal meiosis I non-disjunction of chromosome 15: dependence of the maternal age effect on level of recombination.

Non-disjoined chromosomes 15 from 115 cases of uniparental disomy (ascertained through Prader-Willi syndrome) and 13 cases of trisomy of maternal origin were densely typed for microsatellite loci spanning chromosome 15q. Of these 128 cases a total of 97 meiosis I (MI) errors, 19 meiosis II (MII) errors and 12 mitotic errors were identified. The genetic length of a map created from the MI errors was 101 cM, as compared with a maternal length of 137 cM based on CEPH controls. No significant differences were detected in the distribution of recombination events along the chromosome arm and a reduction was seen for most of the chromosome 15 intervals examined. It was estimated that 21% of tetrads leading to MI non-disjunction were achiasmate, which may account for most or all of the reduction in recombination noted. The mean age of mothers of cases involving MI errors which showed no transitions from heterodisomy to isodisomy was significantly lower (32.7) than cases showing one or more observable transitions (36.3) (P < 0.003, t -test). However, even among chiasmate pairs the highest mean maternal age was seen for multiple exchange tetrads. Chromosome-specific differences in maternal age effects may be related to the normal distribution of exchanges (and their individual susceptibilities) for each chromosome. However, they may also reflect the presence of multiple factors which act to ensure normal segregation, each affected by maternal age in a different way and varying in importance for each chromosome.

Chromosomes, Human, Pair 15↗
PubMed1998

Integrated YAC contig map of the Prader-Willi/Angelman region on chromosome 15q11-q13 with average STS spacing of 35 kb.

Prader-Willi syndrome and Angelman syndrome are associated with parent-of-origin-specific abnormalities of chromosome 15q11-q13, most frequently a deletion of an approximately 4-Mb region. Because of genomic imprinting, paternal deficiency of this region leads to PWS and maternal deficiency to AS. Additionally, this region is frequently involved in other chromosomal rearrangements including duplications, triplications, or supernumerary marker formation. A detailed physical map of this region is important for elucidating the genes and mechanisms involved in genomic imprinting, as well as for understanding the mechanism of recurrent chromosomal rearrangments. An initial YAC contig extended from D15S18 to D15S12 and was comprised of 23 YACs and 21 STSs providing an average resolution of about one STS per 200 kb. To close two gaps in this contig, YAC screening was performed using two STSs that flank the gap between D15S18 and 254B5R and three STSs located distal to the GABRA5-149A9L gap. Additionally, we developed 11 new STSs, including seven polymorphic markers. Although several groups have developed whole-genome genetic and radiation hybrid maps, the depth of coverage for 15q11-q13 has been somewhat limited and discrepancies in marker order exist between the maps. To resolve the inconsistencies and to provide a more detailed map order of STSs in this region, we have constructed an integrated YAC STS-based physical map of chromosome 15q11-q13 containing 118 YACs and 118 STSs, including 38 STRs and 49 genes/ESTs. Using an estimate of 4 Mb for the size of this region, the map provides an average STS spacing of 35 kb. This map provides a valuable resource for identification of disease genes localized to this region as well as a framework for complete DNA sequencing.

Angelman Syndrome↗
PubMed1998

Isolation of BAC clones spanning the Xq22.3 translocation breakpoint in a lissencephaly patient with a de novo X;2 translocation.

X linked lissencephaly and subcortical band heterotopia (XLIS/SBH) is a disorder of cortical development, which causes classical lissencephaly with severe mental retardation and epilepsy in hemizygous males and SBH associated with milder mental retardation and epilepsy in heterozygous females. Here we report the fine mapping of a breakpoint involved in a de novo X;autosomal balanced translocation (46,XX,t(X;2) (q22.3;p25.1)) previously described in a female with classical lissencephaly. We constructed a complete 490 kb BAC contig around the Xq22.3 breakpoint with 11 novel STSs and isolated three BAC clones spanning the breakpoint. This mapping information and BAC contig will be useful in the detailed characterisation of the XLIS gene and other contiguous genes which may also be involved in brain development or function.

Brain↗
PubMed1998

Classical lissencephaly syndromes: does the face reflect the brain?

Both Miller-Dieker syndrome and isolated lissencephaly sequence are associated with classical lissencephaly. Both have been shown to be associated with deletions and mutations in LIS1 on 17p. Traditionally, the two disorders have been distinguished by the presence of a characteristic facial appearance in Miller-Dieker syndrome. The forehead is tall and prominent and may have vertical furrowing. There is narrowing at the temples. Eyes are widely spaced with upward slanting fissures. The nose is very short with anteverted nares. The upper lip is long, wide, and thick. The ears may have minor flattening of the helices. By contrast, these features are not seen in isolated lissencephaly sequence. We have measured five children with Miller-Dieker syndrome (MDS) and 25 children and adolescents with isolated lissencephaly sequence (ILS). Z score (standard deviation score) pattern profiles have been formulated and compared. Patients with ILS at all ages show reduced head circumference, a round head, and a wide and flat face with a broad nose and widely spaced eyes. The most unexpected finding is the similarity of pattern profiles of ILS and MDS in the age group 6 months to 4 years. Correlation coefficient is 0.812 (p<0.001). In MDS there are a few distinguishing features, including brachycephaly, a slightly wider face, and a considerably shorter nose. Given the striking similarity of these objective pattern profiles, it seems likely that the principal diagnostic discriminators are qualitative features, specifically the tall, furrowed forehead and the long, broad, thickened upper lip, which is so thick that the vermilion border of the upper lip is inverted and angled down.

Adolescent↗
PubMed1998

Two 22q telomere deletions serendipitously detected by FISH.

Cryptic telomere deletions have been proposed to be a significant cause of idiopathic mental retardation. We present two unrelated subjects, with normal G banding analysis, in whom 22q telomere deletions were serendipitously detected at two different institutions using fluorescence in situ hybridisation (FISH). Both probands presented with several of the previously described features associated with 22q deletions, including hypotonia, developmental delay, and absence of speech. Our two cases increase the total number of reported 22q telomere deletions to 19, the majority of which were identified by cytogenetic banding analysis. With the limited sensitivity of routine cytogenetic studies (approximately 2-5 Mb), these two new cases suggest that the actual prevalence of 22q telomere deletions may be higher than currently documented. Of additional interest is the phenotypic overlap with Angelman syndrome (AS) as it raises the possibility of a 22q deletion in patients in whom AS has been ruled out. The use of telomeric probes as diagnostic reagents would be useful in determining an accurate prevalence of chromosome 22q deletions and could result in a significantly higher detection rate of subtelomeric rearrangements.

Child, Preschool↗
PubMed1998

Molecular screening for proximal 15q abnormalities in a mentally retarded population.

Paternal or maternal deletions in the 15q11.2-q13 region are known to result in Prader-Willi syndrome (PWS) or Angelman syndrome (AS), respectively. Maternal duplications in 15q11.2-q13 have been found in patients with autism. A population of adults with moderate to profound mental retardation was studied to examine the usefulness of PCR based molecular methods in screening for proximal chromosome 15 abnormalities. Two hundred and eighty-five subjects were initially screened at five microsatellite markers with average heterozygosity values of 0.74 (range 0.54-0.82). Of these subjects, four had a single allele at all five loci, suggestive of a deletion or uniparental isodisomy. The four samples were further screened with additional markers located within 15q11.2-q13 as well as markers telomeric to this region. One subject had uniparental disomy (UPD) and three subjects had a deletion. To determine the parental origin of the 15q11-q13 region containing the single haplotype, samples were analysed with a newly developed methylation specific PCR technique at the SNRPN locus. Each of the four subjects showed presence of the paternal allele and absence of the maternal allele. All cases had a phenotype consistent with Angelman syndrome as expected for the level of mental retardation, but the subject with UPD was distinct from the other subjects with an absence of a history of seizures and presence of bilateral undescended testes and Parkinsonism. Although Angelman syndrome has an estimated population prevalence of 0.008%, at least 1.4% of the moderately to profoundly mentally retarded subjects screened were found to have Angelman syndrome.

Adult↗
PubMed1997

Rox, a novel bHLHZip protein expressed in quiescent cells that heterodimerizes with Max, binds a non-canonical E box and acts as a transcriptional repressor.

Proteins of the Myc and Mad family are involved in transcriptional regulation and mediate cell differentiation and proliferation. These molecules share a basic-helix-loop-helix leucine zipper domain (bHLHZip) and bind DNA at the E box (CANNTG) consensus by forming heterodimers with Max. We report the isolation, characterization and mapping of a human gene and its mouse homolog encoding a new member of this family of proteins, named Rox. Through interaction mating and immunoprecipitation techniques, we demonstrate that Rox heterodimerizes with Max and weakly homodimerizes. Interestingly, bandshift assays demonstrate that the Rox-Max heterodimer shows a novel DNA binding specificity, having a higher affinity for the CACGCG site compared with the canonical E box CACGTG site. Transcriptional studies indicate that Rox represses transcription in both human HEK293 cells and yeast. We demonstrate that repression in yeast is through interaction between the N-terminus of the protein and the Sin3 co-repressor, as previously shown for the other Mad family members. ROX is highly expressed in quiescent fibroblasts and expression markedly decreases when cells enter the cell cycle. Moreover, ROX expression appears to be induced in U937 myeloid leukemia cells stimulated to differentiate with 12-O-tetradecanoylphorbol-13-acetate. The identification of a novel Max-interacting protein adds an important piece to the puzzle of Myc/Max/Mad coordinated action and function in normal and pathological situations. Furthermore, mapping of the human gene to chromosome 17p13.3 in a region that frequently undergoes loss of heterozygosity in a number of malignancies, together with the biochemical and expression features, suggest involvement of ROX in human neoplasia.

Amino Acid Sequence↗
PubMed1997

Deletion within the D17S34 locus in a primitive neuroectodermal tumor.

Loss of heterozygosity on chromosome 17p13.3 is frequently observed in solid tumors, and the presence of a tumor suppressor gene has been predicted in this region of chromosome 17. We have analyzed a primitive neuroectodermal tumor sample exhibiting loss of heterozygosity at the D17S34 locus, a commonly used telomeric marker on the short arm of chromosome 17. The remaining allele showed a rearrangement. Cosmids spanning the D17S34 locus and probes from that region were used to demonstrate a 9-kb deletion within the D17S34 locus and were found to contain evolutionary, conserved sequences. Genetic alterations in this region may also affect expression of immediately adjacent genes, such as ABR, and could be a common mechanism in the causation of primitive neuroectodermal tumors.

Chromosomes, Human, Pair 17↗
PubMed1997

Refined molecular characterization of the breakpoints in small inv dup(15) chromosomes.

Inv dup(15) is the most common supernumerary marker chromosome in humans. To investigate the mechanism responsible for this frequent chromosome rearrangement, we characterized the breakpoints in 18 individuals with small inv dup(15) chromosomes [i.e., negative for the Prader-Willi (PWS)/Angelman syndrome (AS) critical region]. Since two proximal breakpoint regions ("hotspots") for PWS/AS deletions have been previously identified with the most proximal 15q markers D15S541/S542 and S543, we hypothesized that formation of the small inv dup(15) chromosomes may involve one or both of these breakpoint hotspots. By analysis with S542, both breakpoint regions were found to be involved in approximately equal frequencies. In ten cases, the inv dup(15) was negative for S542 (Class I), indicating the breakpoint is between the centromere and the most proximal marker on chromosome 15. For the other eight cases, S542 was positive by fluorescence in situ hybridization (5/5) and/or microsatellite analysis (7/7), but S543 was negative (Class II). These two breakpoint regions appear to be the same as the two proximal breakpoints reported in the common PWS/AS deletions. To initiate cloning and sequencing of the Class II breakpoint, the gap in the yeast artificial chromosome (YAC) contig between S541/S542 and S543 was filled by screening the CEPH YAC and mega-YAC libraries. YACs 705C2 and 368H3 were found to bridge this gap, and therefore contain the more distal breakpoint region. The finding of consistent breakpoints in small inv dup(15), like that found in PWS/AS deletions, provides strong evidence for hotspots for chromosome breakage in this region. In addition, our results show that two extra copies (tetrasomy) of the region from 15cen to the euchromatic region containing S542 are present in individuals with Class II breakpoints. Since most individuals carrying a small inv dup(15) are phenotypically normal, the euchromatin region included in the small inv dup(15) chromosomes does not appear to contain genes with clinically significant dosage effects.

Adolescent↗
PubMed1997

A revision of the lissencephaly and Miller-Dieker syndrome critical regions in chromosome 17p13.3.

Miller-Dieker syndrome (MDS) is a multiple malformation syndrome characterized by classical lissencephaly and a characteristic facies. It is associated with visible or submicroscopic deletions within chromosome band 17p13.3. Lissencephaly without facial dysmorphism has also been observed and is referred to as isolated lissencephaly sequence (ILS). Apparently partial and non-overlapping deletions of the 5' or 3' end of a candidate gene LIS1 in one ILS and one MDS patient had suggested that MDS was a single gene disorder, and that LIS1 spans in excess of 400 kb. However, the originally presumed 5' end of LIS1 was found to belong to the 14-33 epsilon gene residing more distally on 17p13.3. We have now isolated the correct 5' end of LIS1, constructed a approximately 500 kb genomic contig encompassing LIS1, and estimated its gene to be approximately 80 kg. Fluorescence in situ hybridization analysis of an ILS patient with a de novo balanced translocation, as well as analysis of several other key MDS and ILS deletion patients, localizes the lissencephaly critical region within the LIS1 gene. Therefore, LIS1 remains the strongest candidate gene for the lissencephaly phenotype in ILS and MDS. Our analyses also suggest that additional genes distal to LIS1 may be responsible for the facial dysmorphology and other abnormalities seen in MDS but not in ILS patients, supporting our original concept MDS as a contiguous gene deletion syndrome.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗
PubMed1997

Point mutations and an intragenic deletion in LIS1, the lissencephaly causative gene in isolated lissencephaly sequence and Miller-Dieker syndrome.

Classical lissencephaly (smooth brain) or generalized agyria-pachygyria is a severe brain malformation which results from an arrest of neuronal migration at 9-13 weeks gestation. It has been observed in several malformation syndromes including Miller-Dieker syndrome (MDS) and isolated lissencephaly sequence (ILS). A gene containing beta-transducin like repeats, now known as LIS1, was previously mapped to the ILS/MDS chromosome region on 17p13.3. We recently localized the classical lissencephaly critical region to the LIS1 gene locus by molecular analysis of key ILS and MDS patients. We have now characterized the structure of LIS1, which consists of 11 exons, and have searched for the presence of subtle mutations in 19 ILS patients who showed no gross rearrangements of LIS1. Single strand conformational polymorphism (SSCP) analysis revealed band-shifts for three patients, each involving a different coding exon, which were not observed in their respective parental DNAs. Sequence analysis identified these de novo mutations as dA --> dG transition in exon VI at nucleotide 446, a dC --> dT transition in exon VIII at nucleotide 817, and a 22 bp deletion at the exon IX-intron 9 junction from nucleotide 988 to 1,002+7, which causes skipping of exon IX in the mature LIS1 transcript. These changes are predicted to result in an H149R amino acid substitution, an R273X premature translation termination, and abolition of amino acids 301-334, in the respective LIS1 proteins. These data thus confirm LIS1 as the gene responsible for classical lissencephaly in ILS and MDS.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗
PubMed1997

Linkage and physical mapping of X-linked lissencephaly/SBH (XLIS): a gene causing neuronal migration defects in human brain.

While disorders of neuronal migration are associated with as much as 25% of recurrent childhood seizures, few of the genes required to establish neuronal position in cerebral cortex are known. Subcortical band heterotopia (SBH) and lissencephaly (LIS), two distinct neuronal migration disorders producing epilepsy and variable cognitive impairment, can be inherited alone or together in a single pedigree. Here we report a new genetic locus, XLIS, mapped by linkage analysis of five families and physical mapping of a balanced X;2 translocation in a girl with LIS. Linkage places the critical region in Xq21-q24, containing the breakpoint that maps to Xq22.3-q23 by high-resolution chromosome analysis. Markers used for somatic cell hybrid and fluorescence in situ hybridization analyses place the XLIS region within a 1 cM interval. These data suggest that SBH and X-linked lissencephaly are caused by mutation of a single gene, XLIS, that the milder SBH phenotype in females results from random X-inactivation (Lyonization), and that cloning of genes from the breakpoint region on X will yield XLIS.

Cerebral Cortex↗
PubMed1997

The E6-Ap ubiquitin-protein ligase (UBE3A) gene is localized within a narrowed Angelman syndrome critical region.

Angelman syndrome (AS) and Prader-Willi syndrome (PWS) are distinct clinical phenotypes resulting from maternal and paternal deficiencies, respectively, in human chromosome 15qll-q13. Although several imprinted, paternally expressed transcripts have been identified within the PWS candidate region, no maternally expressed gene has yet been identified within the AS candidate region. We have developed an integrated physical map spanning the PWS and AS candidate regions and localized two breakpoints, including a cryptic t(14;15) translocation associated with AS and a non-AS 15q deletion, which substantially narrow the AS candidate region to approximately 250 kb. Mapping data indicate that the entire transcriptional unit of the E6-AP ubiquitin-protein ligase (UBE3A) gene lies within the AS region. The UBE3A locus expresses a transcript of approximately 5 kb at low to moderate levels in all tissues tested. The mouse homolog of UBE3A was cloned and sequenced revealing a high degree of conservation at nucleotide and protein levels. Northern and RT-PCR analysis of Ube3a expression in mouse tissues from animals with segmental, paternal uniparental disomy failed to detect substantially reduced or absent expression compared to control animals, failing to provide any evidence for maternal-specific expression from this locus. Recent identification of de novo truncating mutations in UBE3A taken with these observations indicates that mutations in UBE3A can lead to AS and suggests that this locus may encode both imprinted and biallelically expressed products.

Amino Acid Sequence↗
PubMed1997

Genomic organization of the murine Miller-Dieker/lissencephaly region: conservation of linkage with the human region.

Several human syndromes are associated with haploinsufficiency of chromosomal regions secondary to microdeletions. Isolated lissencephaly sequence (ILS), a human developmental disease characterized by a smooth cerebral surface (classical lissencephaly) and microscopic evidence of incomplete neuronal migration, is often associated with small deletions or translocations at chromosome 17p13.3. Miller-Dieker syndrome (MDS) is associated with larger deletions of 17p13.3 and consists of classical lissencephaly with additional phenotypes including facial abnormalities. We have isolated the murine homologs of three genes located inside and outside the MDS region: Lis1, Mnt/Rox, and 14-3-3 epsilon. These genes are all located on mouse chromosome 11B2, as determined by metaphase FISH, and the relative order and approximate gene distance was determined by interphase FISH analysis. The transcriptional orientation and intergenic distance of Lis1 and Mnt/Rox were ascertained by fragmentation analysis of a mouse yeast artificial chromosome containing both genes. To determine the distance and orientation of 14-3-3 epsilon with respect to Lis1 and Mnt/Rox, we introduced a super-rare cutter site (VDE) that is unique in the mouse genome into 14-3-3 epsilon by gene targeting. Using the introduced VDE site, the orientation of this gene was determined by pulsed field gel electrophoresis and Southern blot analysis. Our results demonstrate that the MDS region is conserved between human and mouse. This conservation of linkage suggests that the mouse can be used to model microdeletions that occur in ILS and MDS.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗