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B Horsthemke

Publications and source records attributed to B Horsthemke.

At least 55 records · Page 3Linked to original sources
PubMed1999

The human gene for the poly(A)-specific ribonuclease (PARN) maps to 16p13 and has a truncated copy in the Prader-Willi/Angelman syndrome region on 15q11-->q13.

The deadenylation nuclease or poly(A)-specific ribonuclease (PARN) is a 3' exonuclease, which degrades the poly(A)-tail of eukaryotic mRNA molecules. By DNA sequence analysis of cDNA and genomic clones, fluorescence in situ hybridization, and reverse transcriptase-PCR, we have determined that the active human PARN gene is located in 16p13 and that a truncated copy lacking the 5' end is located in 15q11. The truncated gene maps close to a copy of the D15F37 gene family at the proximal Prader-Willi/Angelman (PWS/AS) deletion breakpoint region. Other copies of the F37 gene family are located at the distal PWS/AS deletion breakpoint region and on 16p11.2. Although PARN and F37 gene sequences are present on 15q and 16p, our data suggest that the synteny of these loci is the result of independent genetic events.

3' Untranslated Regions↗
PubMed1999

Worldwide ethnic distribution of the G protein beta3 subunit 825T allele and its association with obesity in Caucasian, Chinese, and Black African individuals.

Recently, it was demonstrated that one allele (825T) of the gene encoding the G protein beta3 subunit (GNB3) is associated with hypertension in Germans. This study investigates a possible association with obesity in young male Germans, Chinese, and black South Africans with low, intermediate, and high 825T allele frequencies, respectively. In each of these three distinct cohorts, the 825T allele frequency was increased significantly in overweight (body mass index [BMI] > or =25 kg/m2) and obese individuals (BMI >27 kg/m2) compared to those with normal weight. The 825T allele frequencies in these three BMI groups were, respectively, 29.5, 39.3, and 47.7% in Germans, 46.8, 53.9, and 58.6% in Chinese, and 83.1, 87.7, and 90.9% in South Africans. In each of these three distinct groups, the 825T allele was significantly associated with obesity with odds ratios between 2 and 3. More urban than rural black Africans were overweight despite similar 825T allele frequencies in both populations, which underscores the role of both genetic and environmental factors. BP values in young male whites increased significantly with increasing BMI values but were independent of the C825T polymorphism, suggesting that hypertension associated with the 825T allele could be a consequence of obesity. Genotyping of 5254 individuals from 55 native population samples from Africa, the Americas, Europe, Asia, Australia, and New Guinea demonstrated highest 825T allele frequencies in black Africans (82%) and intermediate values in east Asians (47%). It is anticipated that high frequencies of the 825T allele in Africans and Asians may contribute to an obesity and hypertension epidemic if Westernization of lifestyles continues.

Adolescent↗
PubMed1998

Identification of a silencing element in the human 15q11-q13 imprinting center by using transgenic Drosophila.

Prader-Willi syndrome (PWS) and Angelman syndrome are neurogenetic disorders caused by the lack of a paternal or a maternal contribution from human chromosome 15q11-q13, respectively. Deletions in the transcription unit of the imprinted SNRPN gene have been found in patients who have PWS or Angelman syndrome because of a parental imprint switch failure in this chromosomal domain. It has been suggested that the SNRPN exon 1 region, which is deleted in the PWS patients, contains an imprint switch element from which the maternal and paternal epigenotypes of the 15q11-q13 domain originate. Using the model organism Drosophila, we show here that a fragment from this region can function as a silencer in transgenic flies. Repression was detected specifically from this element and could not be observed with control human sequences. Additional experiments allowed the delineation of the silencer to a fragment of 215 bp containing the SNRPN promoter region. These results provide an additional link between genomic imprinting and an evolutionary conserved silencing mechanism. We suggest that the identified element participates in the long range regulation of the imprinted 15q11-q13 domain or locally represses SNRPN expression from the maternal allele.

Alleles↗
PubMed1998

Mutations of the human P gene associated with Type II oculocutaneous albinism (OCA2). Mutations in brief no. 205. Online.

Mutations in the human P gene lead to oculocutaneous albinism type 2 (OCA2, MIM #203200), the most common type of albinism in humans. The P gene encodes a 110 kDa protein that is associated with melonosomal membranes and contains 12 potential membrane spanning domains. The specific function of the P protein is currently unknown. We report 7 new mutations in the P gene associated with OCA2. This includes 6 missense mutations (S86R, C112F, A368V, T592I, A724P and A787V) and one frameshift mutation (1047del7). We also report 8 polymorphisms including one amino acid substitution, D/A257. We and others have found many polymorphisms of the P gene in the coding region, several of which result in amino acid substitutions, making molecular diagnosis problematic. In contrast to this is the tyrosinase gene associated with OCA1, with a limited number of polymorphic variations in the coding region. There is also no apparent clustering of P gene missense mutations in contrast to the clustering observed by the tyrosinase gene missense mutations that define functional domains of the protein. Further mutational analysis is needed to help define the critical functional domains of the P protein and to allow a definitive diagnosis of OCA2.

Albinism, Oculocutaneous↗
PubMed1998

Imprinting in Prader-Willi and Angelman syndromes.

Imprinted genes are marked in the germline and retain molecular memory of their parental origin, resulting in allelic expression differences during development. Abnormalities in imprinted inheritance occur in several genetic diseases and cancer, and are exemplified by the diverse genetic defects involving chromosome 15q11-q13 in Prader-Willi (PWS) and Angelman (AS) syndromes. PWS involves loss of function of multiple paternally expressed genes, while mutations in a single gene, UBE3A, which is subject to spatially restricted imprinting, occur in some AS patients. Identification of mutations in the imprinting process in PWS and AS has led to a definition of an imprinting center (IC), involving the promoter (in PWS) or an alternative transcript of the SNRPN gene (in AS). The IC regulates initiation of imprint switching for all genes in a 2 Mb imprinted domain during gametogenesis. Imprinting mutations define a novel mechanism of genetic disease because they have no direct effect in the affected patient but, rather, it is the parental germline effect of an IC mutation that leads to disease in the offspring.

Angelman Syndrome↗
PubMed1998

Association of a human G-protein beta3 subunit variant with hypertension.

Hypertension is a common disorder of multifactorial origin that constitutes a major risk factor for cardiovascular events such as stroke and myocardial infarction. Previous studies demonstrated an enhanced signal transduction via pertussis toxin-sensitive G proteins in lymphoblasts and fibroblasts from selected patients with essential hypertension. We have detected a novel polymorphism (C825T) in exon 10 of the gene encoding the beta3 subunit of heterotrimeric G proteins (GNB3). The T allele is associated with the occurrence of a splice variant, GNB3-s (encoding G beta3-s), in which the nucleotides 498-620 of exon 9 are deleted. This in-frame deletion causes the loss of 41 amino acids and one WD repeat domain of the G beta subunit. By western-blot analysis, G beta3-s appears to be predominantly expressed in cells from individuals carrying the T allele. Significant enhancement of stimulated GTPgammaS binding to Sf9 insect cells expressing G beta3-s together with G alpha(i)2 and G gamma5 indicates that this splice variant is biologically active. Genotype analysis of 427 normotensive and 426 hypertensive subjects suggests a significant association of the T allele with essential hypertension.

Alleles↗
PubMed1998

[Genetic and molecular diagnostics in retinoblastoma].

Retinoblastoma is a childhood malignancy of the eye. Almost all patients with familial or bilateral disease suffer from the hereditary form of the disease that is caused by germline mutations in one allele of the RB1 gene. Tumor development is initiated by the loss of the second RB allele in a retinal progenitor cell. Most patients with isolated unilateral disease have nonhereditary retinoblastoma and thus do not carry a mutant allele in their germline. In no patient, the presence of a germline mutation can be excluded clinically. Consequently, relatives are at an increased risk for retinoblastoma. Molecular testing, however, enables accurate risk prediction provided that samples are available. In some relatives an increased risk can be excluded by segregation analysis. Most often, however, identification of the disease causing mutation is necessary for accurate risk prediction. Mutation analysis, which is impeded by the size and complexity of the RB gene, is facilitated by use of efficient screening methods. Using these methods, the oncogenic mutation can be identified in most patients. Therefore, predictive testing has become an integral part of contemporary management of retinoblastoma.

Female↗
PubMed1998

Sporadic imprinting defects in Prader-Willi syndrome and Angelman syndrome: implications for imprint-switch models, genetic counseling, and prenatal diagnosis.

The Prader-Willi syndrome (PWS) and the Angelman syndrome (AS) are caused by the loss of function of imprinted genes in proximal 15q. In approximately 2%-4% of patients, this loss of function is due to an imprinting defect. In some cases, the imprinting defect is the result of a parental imprint-switch failure caused by a microdeletion of the imprinting center (IC). Here we describe the molecular analysis of 13 PWS patients and 17 AS patients who have an imprinting defect but no IC deletion. Heteroduplex and partial sequence analysis did not reveal any point mutations of the known IC elements, either. Interestingly, all of these patients represent sporadic cases, and some share the paternal (PWS) or the maternal (AS) 15q11-q13 haplotype with an unaffected sib. In each of five PWS patients informative for the grandparental origin of the incorrectly imprinted chromosome region and four cases described elsewhere, the maternally imprinted paternal chromosome region was inherited from the paternal grandmother. This suggests that the grandmaternal imprint was not erased in the father's germ line. In seven informative AS patients reported here and in three previously reported patients, the paternally imprinted maternal chromosome region was inherited from either the maternal grandfather or the maternal grandmother. The latter finding is not compatible with an imprint-switch failure, but it suggests that a paternal imprint developed either in the maternal germ line or postzygotically. We conclude (1) that the incorrect imprint in non-IC-deletion cases is the result of a spontaneous prezygotic or postzygotic error, (2) that these cases have a low recurrence risk, and (3) that the paternal imprint may be the default imprint.

Angelman Syndrome↗
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

Expressed copies of the MN7 (D15F37) gene family map close to the common deletion breakpoints in the Prader-Willi/Angelman syndromes.

Approximately 70% of patients with Prader-Willi syndrome or Angelman syndrome have a similar sized de novo deletion of 3-4 Mb in the proximal region of 15q. The distal breakpoints appear to cluster between the P gene (OCA2) and D15S24, whereas two deletion breakpoint clusters have been identified on the proximal side (one centromeric to D15S541 and one between D15S541 and D15S9). Based on the identification of a gene family in 15q11-->q13 (MN7, D15F37), we have previously proposed that the presence of multiple copies of this sequence may be related to the instability of this region. Using fluorescence in situ hybridization and YAC mapping, we have found that at least one D15F37 locus is centromeric to D15S9 and at least two are between OCA2 and D15S24. As determined by cDNA cloning and sequence analysis, each of the individual loci is expressed. The close proximity of the D15F37 loci and the deletion breakpoints suggests that the common deletions arise by unequal crossover events at or near these loci.

Adult↗
PubMed1997

Hypermethylation in the retinoblastoma gene is associated with unilateral, sporadic retinoblastoma.

We previously reported 9 unilateral, sporadic retinoblastomas with hypermethylation in the 5' region of the RB gene, and we found that CpG methylation in the RB promoter inhibits the binding of the retinoblastoma binding factor 1 (RBF-1) and the activating transcription factor (ATF)-like factors, thereby resulting in a considerable reduction in RB promoter activity. In this study, we screened for hypermethylation in 121 additional cases of retinoblastoma, and found 5 tumors with hypermethylation, including 4 unilateral, sporadic tumors, and one hereditary tumor. The hereditary tumor had a germline deletion of one allele, and the hypermethylation was an acquired, epigenetic change in the other allele. Another tumor had hypermethylation restricted to approximately 800 base pairs in the RB promoter region including the essential RBF-1 and ATF sites. The frequency of hypermethylation in unilateral, sporadic tumors was 9.3% combining our previous and present examinations (13 among 140), whereas the frequency was 1.0% in bilateral hereditary tumors (one among 101). The statistical analyses using the chi-square test indicated significant correlation between hypermethylation and unilateral, sporadic tumors (p < 0.05). These results suggest that hypermethylation in the RB gene is always an acquired, epigenetic change and causes about 9% of unilateral, sporadic tumors.

Blotting, Southern↗
PubMed1997

Genomic organization and promoter structure of the human EXT1 gene.

Hereditary predisposition to multiple exostoses is a genetically heterogeneous disease. Recently, we have reported the identification of the EXT1 gene on human chromosome 8. We have now isolated a cDNA clone from a human adult lung cDNA library and have determined the genomic organization and promoter structure of the EXT1 gene. The gene is composed of 11 exons, ranging from 90 to 1735 bp, and spans approximately 350 kb of genomic DNA. Sequence analysis of the promoter region revealed the presence of a CpG island containing GC and CAAT boxes, but no TATA box. Such a promoter is characteristic for housekeeping genes. This finding is in good agreement with the ubiquitous expression of the EXT1 gene.

Adult↗
PubMed1997

Identification of novel exons 3' to the human SNRPN gene.

The gene for the small nuclear ribonucleoprotein N (SNRPN) maps to human chromosome 15 and has 10 exons. Using cDNA cloning, direct cDNA selection, and exon-connection reverse transcriptase (RT)-PCR, we have identified three novel 3' exons of SNRPN, which have no protein coding potential. Like the other SNRPN exons, the novel exons are expressed from the paternal allele only. In contrast to several cDNA clones and RT-PCR products, however, the 3.4-kb transcript detected by Northern blot hybridization with a probe for the novel exons does not contain SNRPN. It is possible that the steady-state RNA observed in fetal tissues and in adult testis, ovary, brain, and muscle is initiated at an as yet unidentified transcription start site downstream of SNRPN or is generated by endonucleolytic cleavage of a precursor transcript, as has been shown for another imprinted gene, the insulin-like growth factor II gene.

Adult↗
PubMed1997

Clinical spectrum and molecular diagnosis of Angelman and Prader-Willi syndrome patients with an imprinting mutation.

Recent studies have identified a new class of Prader-Willi syndrome (PWS) and Angelman syndrome (AS) patients who have biparental inheritance, but neither the typical deletion nor uniparental disomy (UPD) or translocation. However, these patients have uniparental DNA methylation throughout 15q11-q13, and thus appear to have a mutation in the imprinting process for this region. Here we describe detailed clinical findings of five AS imprinting mutation patients (three families) and two PWS imprinting mutation patients (one new family). All these patients have essentially the classical clinical phenotype for the respective syndrome, except that the incidence of microcephaly is lower in imprinting mutation AS patients than in deletion AS patients. Furthermore, imprinting mutation AS and PWS patients do not typically have hypopigmentation, which is commonly found in patients with the usual large deletion. Molecular diagnosis of these cases is initially achieved by DNA methylation analyses of the DN34/ZNF127, PW71 (D15S63), and SNRPN loci. The latter two probes have clear advantages in the simple molecular diagnostic analysis of PWS and AS patients with an imprinting mutation, as has been found for typical deletion or UPD PWS and AS cases. With the recent finding of inherited microdeletions in PWS and AS imprinting mutation families, our studies define a new class of these two syndromes. The clinical and molecular identification of these PWS and AS patients has important genetic counseling consequences.

Adult↗
PubMed1997

Structure and function of the human chromosome 15 imprinting center.

The Prader-Willi syndrome (PWS) and the Angelman syndrome (AS) are distinct neurogenetic disorders that are caused by a deficiency of paternal (PWS) or maternal (AS) contributions to chromosome 15. The affected genes are located in an imprinted chromosomal domain of 2 Mb, which is controlled by an imprinting center (IC). The IC has been mapped to a 100-kb region including the SNRPN gene and appears to have a bipartite structure. Mutations of the proximal part of the IC block the paternal-->maternal imprint switch during female gametogenesis, whereas mutations of the distal part of the IC block the maternal-->paternal imprint switch during, male gametogenesis. Imprinting involves differential DNA methylation, which appears to be instrumental in the regulation of gene activity and can be used for diagnostic purposes.

Angelman Syndrome↗
PubMed1997

Imprinting mutations on human chromosome 15.

Genomic imprinting is an epigenetic process by which the male and the female germline of viviparous taxa confer a sex-specific mark (imprint) on certain chromosomal regions. The imprint is reset in the germline of each generation, inherited through somatic cell divisions during postzygotic development and used to regulate parent-of-origin specific expression of susceptible genes. Aberrant imprinting leading to aberrant gene expression patterns represents a novel class of mutations and was first identified in patients with Angelman syndrome and Prader-Willi syndrome. The finding of inherited cis-acting mutations in some of these cases has led to the identification of an imprinting center, which is involved in resetting of the imprint during gametogenesis. Other mutations may interfere with the somatic inheritance of the imprint during postzygotic development.

Chromosomes, Human, Pair 15↗
PubMed1997

[Molecular genetics and diagnosis of retinoblastoma. Significance for ophthalmologic practice].

Retinoblastoma (RB) is initiated by loss of function of both copies of the retinoblastoma susceptibility gene (RB 1). Hereditary predisposition to RB is caused by germline mutations in the RB 1 gene. Tumor formation is initiated by the somatic loss of the second allele. Most patients with hereditary RB develop multiple tumors that usually affect both eyes. In nonhereditary disease, however, both RB 1 mutations are somatic events that cause the formation of a single tumor focus. Knowledge of the germline mutation is often essential for accurate risk prediction. Applying strategies for efficient mutation detection, germline mutations can be identified in most individuals with hereditary RB. The vast majority of mutant alleles cause premature termination of translation owing to frameshift or nonsense mutations. In patients carrying these mutant alleles, penetrance is almost complete (> 95%) and numerous tumor foci are observed. However, some 5% of the mutations result in comparatively mild alterations at the protein level. Patients with mutations of this kind often show a lower mean number of tumor foci (reduced expressivity) or no tumor at all (incomplete penetrance). Reduced expressivity and incomplete penetrance are also observed in patients with large cytogenetic deletions. By mutation analysis in DNA from fresh frozen tumor samples and peripheral blood, we have detected RB 1 germline mutations in some 20% of patients with unilateral RB. These results emphasize the importance of molecular analysis in patients with isolated unilateral RB.

Chromosomes, Human, Pair 13↗