High frequency of unequal recombination in pseudoautosomal region shown by proviral insertion in transgenic mouse

The mammalian X and Y chromosomes, in contrast to the autosomes, pair during male meiosis only near the telomeres. Alleles localized in this region can undergo reciprocal exchange during meiosis. Because such sequences do not show strict sex-linked inheritance, they have been termed pseudoautosomal. In man, several DNA sequences have been described which show pseudoautosomal transmission and which are localized in the pairing region at the ends of the short arms of both the X and Y chromosomes (refs 6-9, and D. Page, unpublished results). We now show that the transgenic mouse strain, Mov-15, contains a single Moloney murine leukaemia virus (M-MuLV) genome in its germline, and genetic evidence indicates that the provirus is integrated into the pseudoautosomal region of the sex chromosome. Proviral copies are lost or gained in 7% of male meioses in this strain, and mouse sequences flanking the provirus are tandemly repeated and highly variable. We conclude that unequal recombination events occur with high frequency in the pairing region, possibly because of the presence of repeated sequences.     

Structure of pre-pro-von Willebrand factor and its expression in heterologous cells

Von Willebrand factor (vWF), a multifunctional haemostatic glycoprotein derived from endothelial cells and megakaryocytes, mediates platelet adhesion to injured subendothelium and binds coagulation factor VIII in the circulation. Native vWF is a disulphide-bonded homopolymer; the monomeric subunits, of apparent relative molecular mass (Mr) 220,000 (220K) are derived from an intracellular precursor estimated at 260-275K. Multimer assembly is preceded by the formation of dimers, linked near their C-termini, which then assemble into filamentous polymers. The importance of the removal of the large vWF pro-polypeptide during multimer assembly, and whether this or other stages of the complex post-translational processing require components specific to endothelial cells or megakaryocytes, is unknown. Here we report an analysis of the complete sequence of pre-pro-vWF and expression of the molecule in heterologous cells. The vWF precursor is composed of several repeated subdomains. When expressed in COS and CHO cells, it is cleaved and assembled into biologically active high relative molecular mass disulphide bonded multimers. This suggests that the information for assembly of this complex molecule resides largely within its primary structure.     

Long-range colour-generating interactions across the retina

The existence of colour-generating interactions across the corpus callosum has recently been suggested from observations with a 'split-brain' patient, thus indicating long-range colour computations at the cortical level. Observations on induced colours described here suggest long-range colour computations at the retinal level. If a white surface surrounded by a particular colour is fixated for some time, the resulting after-image has two colours: the surround appears in complementary colour, whereas the white centre takes on the colour of the surround. The question of whether such colour induction is located in the retina or more centrally was tested in a brain-injured patient with hemianopia. It could be demonstrated that areas of the visual field that are no longer represented in the geniculo-striatal pathway still contribute to colour induction, suggesting that colour induction is a retinal phenomenon.     

Implications of fragile X expression in normal males for the nature of the mutation

The fragile site at Xq27, associated with a common form of X-linked mental retardation (XLMR), is expressed in a variable proportion of the peripheral lymphocytes of affected males when the cells are cultured under thymidylate stress (Td stress) produced by folate or thymidylate deprivation. Some clinically normal males--transmitting males--are known to carry and transmit the fragile X mutation and yet show no cytogenetic expression in lymphocytes. Normal males with no family history of X-linked mental retardation express the site only rarely. When the fragile X chromosome from affected males is isolated in a rodent genetic background by somatic cell hybridization, the level of expression is similar to that seen in lymphocytes under Td stress. Here we show that X chromosomes from two transmitting males and two normal control males, all of which were fragile X negative in lymphocytes or lymphoblasts, could be made to express the fragile site in hybrids, although at levels that were below those seen in hybrids from affected males. Furthermore, transmitting males could be differentiated from normal males by their significantly higher expression rates when hybrids were exposed to caffeine before cytogenetic harvest. One male chimpanzee also showed low level expression in hybrid cells. These data suggest that the hybrid system lowers the threshold for fragile X expression, a fragile site at Xq27 may be present on all human and chimpanzee X chromosomes and constitutes a previously unrecognized common fragile site and the hybrid system with caffeine post-treatment can distinguish between the common Xq27 fragile site of control males, the occult mutant fragile site of a transmitting male, and the fully expressed fragile site of an affected male with XLMR. Thus the mutation producing XLMR may represent a multi-step alteration of a naturally occurring DNA sequence producing a continuum of cytogenetic expression and a threshold for clinical manifestation.     

Immunoquantitation of some cytochrome P-450 isozymes in liver microsomes from streptozotocin-diabetic rats

Streptozotocin-diabetes in rats leads to a decrease of cytochrome P-450 UT-A (the major form in control rats) and an increase of cytochrome P-450 PB-B (the major one induced by phenobarbital treatment) in liver microsomes. The increased benzphetamine-N-demethylase activity can be related to the induction of cytochrome P-450 PB-B.     

HLA-D region beta-chain DNA endonuclease fragments differ between HLA-DR identical healthy and insulin-dependent diabetic individuals

The human HLA-D histocompatibility region encodes class II antigens each of which consists of two polypeptide chains (alpha and beta) inserted in the plasma membrane. These molecules are implicated in the regulation of the immune response but several human diseases are also found to be associated with certain HLA-DR antigens. The occurrence of insulin-dependent (type I) diabetes (IDDM) is strongly associated with HLA-DR3 and/or 4 (ref. 5). The class II antigens, however, show a marked genetic polymorphism associated with the beta-chains which seem, from hybridization studies, to be encoded by several genes. We have therefore used the beta-chain cDNA probe, pDR-beta-1 (refs 8, 10) to test whether there are differences in hybridization pattern between DNA from healthy individuals and diabetic patients, after digestion with restriction endonucleases. Among the HLA-DR 4 and 3/4 individuals, the IDDM patients showed an increased frequency of a PstI 18 kilobase (kb) fragment. A BamHI 3.7 kb fragment, frequent among controls (30-40%), was rarely detected in the IDDM patients (0-2%). These differences may be related to susceptibility to develop the disease.