A human model for multigenic inheritance: Phenotypic expression in Hirschsprung disease requires both the RET gene and a new 9q31 locus

Reduced penetrance in genetic disorders may be either dependent or independent of the genetic background of gene carriers. Hirschsprung disease (HSCR) demonstrates a complex pattern of inheritance with ≈50% of familial cases being heterozygous for mutations in the receptor tyrosine kinase RET. Even when identified, the penetrance of RET mutations is only 50–70%, gender-dependent, and varies with the extent of aganglionosis. We searched for additional susceptibility genes which, in conjunction with RET, lead to phenotypic expression by studying 12 multiplex HSCR families. Haplotype analysis and extensive mutation screening demonstrated three types of families: six families harboring severe RET mutations (group I); and the six remaining families, five of which are RET-linked families with no sequence alterations and one RET-unlinked family (group II). Although the presence of RET mutations in group I families is sufficient to explain HSCR inheritance, a genome scan reveals a new susceptibility locus on 9q31 exclusively in group II families. As such, the gene at 9q31 is a modifier of HSCR penetrance. These observations imply that identification of new susceptibility factors in a complex disease may depend on classification of families by mutational type at known susceptibility genes.

Thyroid hormone receptor-associated proteins and general positive cofactors mediate thyroid hormone receptor function in the absence of the TATA box-binding protein-associated factors of TFIID

Coactivators previously implicated in ligand-dependent activation functions by thyroid hormone receptor (TR) include p300 and CREB-binding protein (CBP), the steroid receptor coactivator-1 (SRC-1)-related family of proteins, and the multicomponent TR-associated protein (TRAP) complex. Here we show that two positive cofactors (PC2 and PC4) derived from the upstream stimulatory activity (USA) cofactor fraction act synergistically to mediate thyroid hormone (T3)-dependent activation either by TR or by a TR-TRAP complex in an in vitro system reconstituted with purified factors and DNA templates. Significantly, the TRAP-mediated enhancement of activation by TR does not require the TATA box-binding protein-associated factors of TFIID. Furthermore, neither the pleiotropic coactivators CBP and p300 nor members of the SRC-1 family were detected in either the TR-TRAP complex or the other components of the in vitro assay system. These results show that activation by TR at the level of naked DNA templates is enhanced by cooperative functions of the TRAP coactivators and the general coactivators PC2 and PC4, and they further indicate a potential functional redundancy between TRAPs and TATA box-binding protein-associated factors in TFIID. In conjunction with earlier studies on other nuclear receptor-interacting cofactors, the present study also suggests a multistep pathway, involving distinct sets of cofactors, for activation of hormone responsive genes.

The conserved SOCS box motif in suppressors of cytokine signaling binds to elongins B and C and may couple bound proteins to proteasomal degradation

The suppressors of cytokine signaling (SOCS) family of proteins act as intracellular inhibitors of several cytokine signal transduction pathways. Their expression is induced by cytokine activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway and they act as a negative feedback loop by subsequently inhibiting the JAK/STAT pathway either by direct interaction with activated JAKs or with the receptors. These interactions are mediated at least in part by the SH2 domain of SOCS proteins but these proteins also contain a highly conserved C-terminal homology domain termed the SOCS box. Here we show that the SOCS box mediates interactions with elongins B and C, which in turn may couple SOCS proteins and their substrates to the proteasomal protein degradation pathway. Analogous to the family of F-box-containing proteins, it appears that the SOCS proteins may act as adaptor molecules that target activated cell signaling proteins to the protein degradation pathway.

The natural killer gene complex genetic locus Chok encodes Ly-49D, a target recognition receptor that activates natural killing

Previously, we established that natural killer (NK) cells from C57BL/6 (B6), but not BALB/c, mice lysed Chinese hamster ovary (CHO) cells, and we mapped the locus that determines this differential CHO-killing capacity to the NK gene complex on chromosome 6. The localization of Chok in the NK gene complex suggested that it may encode either an activating or an inhibitory receptor. Here, results from a lectin-facilitated lysis assay predicted that Chok is an activating B6 NK receptor. Therefore, we immunized BALB/c mice with NK cells from BALB.B6–Cmv1r congenic mice and generated a mAb, designated 4E4, that blocked B6-mediated CHO lysis. mAb 4E4 also redirected lysis of Daudi targets, indicating its reactivity with an activating NK cell receptor. Furthermore, only the 4E4+ B6 NK cell subset mediated CHO killing, and this lysis was abrogated by preincubation with mAb 4E4. Flow cytometric analysis indicated that mAb 4E4 specifically reacts with Ly-49D but not Ly-49A, B, C, E, G, H, or I transfectants. Finally, gene transfer of Ly-49DB6 into BALB/c NK cells conferred cytotoxic capacity against CHO cells, thus establishing that the Ly-49D receptor is sufficient to activate NK cells to lyse this target. Hence, Ly-49D is the Chok gene product and is a mouse NK cell receptor capable of directly triggering natural killing.

The Arabidopsis thaliana HY1 locus, required for phytochrome-chromophore biosynthesis, encodes a protein related to heme oxygenases

The hy1 mutants of Arabidopsis thaliana fail to make the phytochrome-chromophore phytochromobilin and therefore are deficient in a wide range of phytochrome-mediated responses. Because this defect can be rescued by feeding seedlings biliverdin IXα, it is likely that the mutations affect an enzyme that converts heme to this phytochromobilin intermediate. By a combination of positional cloning and candidate-gene isolation, we have identified the HY1 gene and found it to be related to cyanobacterial, algal, and animal heme oxygenases. Three independent alleles of hy1 contain DNA lesions within the HY1 coding region, and a genomic sequence spanning the HY1 locus complements the hy1–1 mutation. HY1 is a member of a gene family and is expressed in a variety of A. thaliana tissues. Based on its homology, we propose that HY1 encodes a higher-plant heme oxygenase, designated AtHO1, responsible for catalyzing the reaction that opens the tetrapyrrole ring of heme to generate biliverdin IXα.

A male germ cell tumor-susceptibility-determining locus, pgct1, identified on murine chromosome 13

Inbred 129 strain mice are predisposed to developing male germ cell tumors (GCTs) of the testes. The inherent genetic defects that underlie male GCT susceptibility in the 129 mouse strain are unknown. GCT incidence is increased in 129 strain males that lack functional p53 protein, and we have used this finding to facilitate the generation of panels of GCT-bearing intercross and backcross mice for genetic mapping analysis. A 129 strain locus, designated pgct1, that segregates with the male GCT phenotype has been identified on chromosome 13 near D13Mit188. This region of murine chromosome 13 may be syntenic to a portion of human chromosome 5q that is implicated in male GCT susceptibility in humans.

V(D)J recombination is not activated by demethylation of the kappa locus

V(D)J recombination is thought to be regulated by changes in the accessibility of target sites, such as modulation of methylation. To test whether demethylation of the kappa locus can activate recombination, we generated two recombinationally active B cell lines in which the gene for maintenance of genomic DNA methylation, Dnmt1, was flanked with loxP sites. Transduction with a retrovirus expressing both the cre recombinase and green fluorescent protein allowed us to purify recombinationally active cells devoid of methylation. Loss of methylation of the kappa locus was not sufficient to activate recombination, although transcription was activated in one line. It appears that demethylation of the kappa locus is not the rate-limiting step for altering accessibility and thus regulated demethylation does not generate specificity of recombination.

An initiation element in the yeast CUP1 promoter is recognized by RNA polymerase II in the absence of TATA box-binding protein if the DNA is negatively supercoiled

Purified RNA polymerase II initiated transcription from the yeast CUP1 promoter fused to a C-less cassette if the DNA was negatively supercoiled. Relaxed plasmid was not transcribed. Transcription did not require addition of any other transcription factors. TATA box-binding protein (TBP) was not detectable in the polymerase preparation and the TATA box was not required. Deletion analysis of the CUP1 promoter revealed that a 25-bp element containing the initiation region was sufficient for recognition by polymerase. Two transcription start sites were mapped, one of which is identical to one of the two major start sites observed in vivo. Our observations can be accounted for by using a theoretical analysis of the probability of DNA melting within the plasmid as a function of superhelix density: the CUP1 initiation element is intrinsically unstable to superhelical stress, permitting entry of the polymerase, which then scans the DNA to locate the start site. In support of this analysis, the CUP1 promoter was sensitive to mung bean nuclease. These observations and a previous theoretical analysis of yeast genes support the idea that promoters are stress points within the DNA superhelix. The role of transcription factors might be to mark the promoter and to regulate specific melting of promoter DNA.

Crystal structure of yeast initiation factor 4A, a DEAD-box RNA helicase

The eukaryotic translation initiation factor 4A (eIF4A) is a member of the DEA(D/H)-box RNA helicase family, a diverse group of proteins that couples an ATPase activity to RNA binding and unwinding. Previous work has provided the structure of the amino-terminal, ATP-binding domain of eIF4A. Extending those results, we have solved the structure of the carboxyl-terminal domain of eIF4A with data to 1.75 Å resolution; it has a parallel α-β topology that superimposes, with minor variations, on the structures and conserved motifs of the equivalent domain in other, distantly related helicases. Using data to 2.8 Å resolution and molecular replacement with the refined model of the carboxyl-terminal domain, we have completed the structure of full-length eIF4A; it is a “dumbbell” structure consisting of two compact domains connected by an extended linker. By using the structures of other helicases as a template, compact structures can be modeled for eIF4A that suggest (i) helicase motif IV binds RNA; (ii) Arg-298, which is conserved in the DEA(D/H)-box RNA helicase family but is absent from many other helicases, also binds RNA; and (iii) motifs V and VI “link” the carboxyl-terminal domain to the amino-terminal domain through interactions with ATP and the DEA(D/H) motif, providing a mechanism for coupling ATP binding and hydrolysis with conformational changes that modulate RNA binding.

Subgenome chromosome walking in wheat: A 450-kb physical contig in Triticum monococcum L. spans the Lr10 resistance locus in hexaploid wheat (Triticum aestivum L.)

For many agronomically important plant genes, only their position on a genetic map is known. In the absence of an efficient transposon tagging system, such genes have to be isolated by map-based cloning. In bread wheat Triticum aestivum, the genome is hexaploid, has a size of 1.6 × 1010 bp, and contains more than 80% of repetitive sequences. So far, this genome complexity has not allowed chromosome walking and positional cloning. Here, we demonstrate that chromosome walking using bacterial artificial chromosome (BAC) clones is possible in the diploid wheat Triticum monococcum (Am genome). BAC end sequences were mostly repetitive and could not be used for the first walking step. New probes corresponding to rare low-copy sequences were efficiently identified by low-pass DNA sequencing of the BACs. Two walking steps resulted in a physical contig of 450 kb on chromosome 1AmS. Genetic mapping of the probes derived from the BAC contig demonstrated perfect colinearity between the physical map of T. monococcum and the genetic map of bread wheat on chromosome 1AS. The contig genetically spans the Lr10 leaf rust disease resistance locus in bread wheat, with 0.13 centimorgans corresponding to 300 kb between the closest flanking markers. Comparison of the genetic to physical distances has shown large variations within 350 kb of the contig. The physical contig can now be used for the isolation of the orthologous regions in bread wheat. Thus, subgenome chromosome walking in wheat can produce large physical contigs and saturate genomic regions to support positional cloning.

DNase I-hypersensitive sites I and II of the human growth hormone locus control region are a major developmental activator of somatotrope gene expression

High-level expression of the human growth hormone (hGH) gene is limited to somatotrope and lactosomatotrope cells of the anterior pituitary. We previously identified a locus control region (LCR) for the hGH gene composed of four tissue-specific DNase I-hypersensitive sites (HS) located between −14.6 kb and −32 kb 5′ to the hGH transcription start site that is responsible for establishing a physiologically regulated chromatin domain for hGH transgene expression in mouse pituitary. In the present study we demonstrated that the LCR mediates somatotrope and lactosomatotrope restriction on an otherwise weakly and diffusely expressed hGH transgene. The subregion of the LCR containing the two pituitary-specific HS, HSI and HSII (−14.6 to −16.2 kb relative to the hGH promoter and denoted HSI,II), was found to be sufficient for mediating somatotrope and lactosomatotrope restriction, for appropriately timed induction of hGH transgene expression between embryonic days 15.5 and 16.5, and for selective extinction of hGH expression in mature lactotropes. When studied by cell transfection, the HSI,II fragment selectively enhanced transcription in a presomatotrope-derived cell line, although at levels (2- to 3-fold) well below that seen in vivo. The LCR activity of the HSI,II element was therefore localized by scoring transgene expression in fetal founder pituitaries at embryonic day 18.5. The data from these studies indicated that a 404-bp segment of the HSI,II region encodes a critical subset of LCR functions, including the establishment of a productive chromatin environment, cell-specific restriction and enhancement of expression, and appropriately timed induction of the hGH transgene during embryonic development.

New susceptibility locus for rheumatoid arthritis suggested by a genome-wide linkage study

Rheumatoid arthritis (RA), the most common autoimmune disease, is associated in families with other autoimmune diseases, including insulin-dependent diabetes mellitus (IDDM). Its genetic component has been suggested by familial aggregation (λs = 5), twin studies, and segregation analysis. HLA, which is the only susceptibility locus known, has been estimated to account for one-third of this component. The aim of this paper was to identify new RA loci. A genome scan was performed with 114 European Caucasian RA sib pairs from 97 nuclear families. Linkage was significant only for HLA (P < 2.5⋅10−5) and nominal for 19 markers in 14 other regions (P < 0.05). Four of the loci implicated in IDDM potentially overlap with these regions: the putative IDDM6, IDDM9, IDDM13, and DXS998 loci. The first two of these candidate regions, defined in the RA genome scan by the markers D18S68-D18S61-D18S469 (18q22–23) and D3S1267 (3q13), respectively, were studied in 194 additional RA sib pairs from 164 nuclear families. Support for linkage to chromosome 3 only was extended significantly (P = 0.002). The analysis of all 261 families provided a linkage evidence of P = 0.001 and suggested an interaction between this putative RA locus and HLA. This locus could account for 16% of the genetic component of RA. Candidate genes include those coding for CD80 and CD86, molecules involved in antigen-specific T cell recognition. In conclusion, this first genome scan in RA Caucasian families revealed 14 candidate regions, one of which was supported further by the study of a second set of families.

A cis-acting element that directs the activity of the murine methylation modifier locus Ssm1

Silencing of chromosomal domains has been described in diverse systems such as position effect variegation in insects, silencing near yeast telomeres, and mammalian X chromosome inactivation. In mammals, silencing is associated with methylation at CpG dinucleotides, but little is known about how methylation patterns are established or altered during development. We previously described a strain-specific modifier locus, Ssm1, that controls the methylation of a complex transgene. In this study we address the questions of the nature of Ssm1’s targets and whether its effect extends into adjacent sequences. By examining the inheritance of methylation patterns in a series of mice harboring deletion derivatives of the original transgene, we have identified a discrete segment, derived from the gpt gene of Escherichia coli, that is a major determinant for Ssm1-mediated methylation. Methylation analysis of sequences adjacent to a transgenic target indicates that the influence of this modifier extends into the surrounding chromosome in a strain-dependent fashion. Implications for the mechanism of Ssm1 action are discussed.

Bidirectional binding of the TATA box binding protein to the TATA box

By selective attachment of a DNA cleavage agent to specific residues in the yeast TATA box binding protein (yTBP), we demonstrate that, in solution, yTBP binds to the TATA boxes of both the adenovirus major late promoter and the yeast CYC1 promoter with only a modest preference in orientation and binds well to several overlapping binding sites. The general factors TFIIA and TFIIB each increase the rotational and translational selectivity of yTBP but are not sufficient, at least individually, to confer a unique polarity to the preinitiation complex. We conclude that TBP alone cannot define the productive orientation of general factor assembly on a promoter.

Conserved interaction between distinct Krüppel-associated box domains and the transcriptional intermediary factor 1 β

The Krüppel-associated box (KRAB) domain, originally identified as a 75-aa sequence present in numerous Krüppel-type zinc-finger proteins, is a potent DNA-binding-dependent transcriptional repression domain that is believed to function through interaction with the transcriptional intermediary factor 1 (TIF1) β. On the basis of sequence comparison and phylogenetic analysis, we have recently defined three distinct subfamilies of KRAB domains. In the present study, individual members of each subfamily were tested for transcriptional repression and interaction with TIF1β and two other closely related family members (TIF1α and TIF1γ). All KRAB variants were shown, (i) to repress transcription when targeted to DNA through fusion to a heterologous DNA-binding domain in mammalian cells, and (ii) to interact specifically with TIF1β, but not with TIF1α or TIF1γ. Taken together, these results implicate TIF1β as a common transcriptional corepressor for the three distinct subfamilies of KRAB zinc-finger proteins and suggest a high degree of conservation in the molecular mechanism underlying their transcriptional repression activity.