Search

Earlier studies had demonstrated that fusion of normal with immortal human cells yielded hybrids having limited division potential. This indicated that the phenotype of limited proliferation (cellular senescence) is dominant and that immortal cells result from recessive changes in normal growth-regulatory genes. In additional studies, we exploited the fact that the immortal phenotype is recessive and, by fusing various immortal human cell lines with each other, identified four complementation groups for indefinite division. Assignment of cell lines to specific groups allowed us to take a focused approach to identify the chromosomes and genes involved in growth regulation that have been modified in immortal cells. We report here that introduction of a normal human chromosome 4 into three immortal cell lines (HeLa, J82, T98G) assigned to complementation group B resulted in loss of proliferation and reversal of the immortal phenotype. No effect on the proliferation potential of cell lines representative of the other complementation groups was observed. This result suggests that a gene(s) involved in cellular senescence and normal growth regulation resides on chromosome 4.

Human-rodent hybrid cell lines have been analyzed with regard to their human DNA content by using various DNA probe sets, derived from the hybrids, for in situ hybridization to normal human metaphase chromosome spreads. Total genomic hybrid DNA was compared with probe sets of hybrid DNA that were highly enriched in human sequences. The latter probes were obtained by amplification through the polymerase chain reaction (PCR) using oligonucleotide primers directed to human specific subsequences of the interspersed repetitive sequences Alu and L1. Previously unidentified chromosomal material within hybrid lines was characterized with speed and precision. It is demonstrated that the complete human complement of hybrid lines can be rapidly assessed by comparing the data obtained with the Alu-PCR products with the results from the L1-PCR products or from the genomic hybrid DNA. This approach using interspersed repetitive sequence-PCR products is simple and fast and also provides an alternative way of generating complex DNA probe sets for the specific delineation of entire chromosomes or subchromosomal regions by in situ hybridization.

Choroideremia, an X-chromosome linked retinal dystrophy of unknown pathogenesis, causes progressive nightblindness and eventual central blindness in affected males by the third to fourth decade of life. Choroideremia has been mapped to Xq13-21 by tight linkage to restriction fragment length polymorphism loci. We have recently identified two families in which choroideremia is inherited with mental retardation and deafness. In family XL-62, an interstitial deletion in Xq21 is visible by cytogenetic analysis and two linked anonymous DNA markers, DXYS1 and DXS72, are deleted. In the second family, XL-45, an interstitial deletion was suspected on phenotypic grounds but could not be confirmed by high-resolution cytogenetic analysis. We used phenol-enhanced reassociation of 48,XXXX DNA in competition with excess XL-45 DNA to generate a library of cloned DNA enriched for sequences that might be deleted in XL-45. Two of the first 83 sequences characterized from the library were found to be deleted in probands from family XL-45 as well as from family XL-62. Isolation of these sequences proves that XL-45 does contain a submicroscopic deletion and provides a starting point for identifying overlapping genomic sequences that span the XL-45 deletion. Each overlapping sequence will be studied to identify exons from the choroideremia locus.

The Miller-Dieker syndrome (MDS), composed of characteristic facial abnormalities and a severe neuronal migration disorder affecting the cerebral cortex, is caused by visible or submicroscopic deletions of chromosome band 17p13. Twelve anonymous DNA markers were tested against a panel of somatic cell hybrids containing 17p deletions from seven MDS patients. All patients, including three with normal karyotypes, are deleted for a variable set of 5-12 markers. Two highly polymorphic VNTR (variable number of tandem repeats) probes, YNZ22 and YNH37, are codeleted in all patients tested and make molecular diagnosis for this disorder feasible. By pulsed-field gel electrophoresis, YNZ22 and YNH37 were shown to be within 30 kilobases (kb) of each other. Cosmid clones containing both VNTR sequences were identified, and restriction mapping showed them to be less than 15 kb apart. Three overlapping cosmids spanning greater than 100 kb were completely deleted in all patients, providing a minimum estimate of the size of the MDS critical region. A hypomethylated island and evolutionarily conserved sequences were identified within this 100-kb region, indications of the presence of one or more expressed sequences potentially involved in the pathophysiology of this disorder. The conserved sequences were mapped to mouse chromosome 11 by using mouse-rat somatic cell hybrids, extending the remarkable homology between human chromosome 17 and mouse chromosome 11 by 30 centimorgans, into the 17p telomere region.

A cDNA clone to human 4-beta-galactosyltransferase (EC 2.4.1.38) was isolated from a human liver lambda gt11 expression library by using a monospecific polyclonal antiserum to affinity-purified bovine enzyme. The authenticity of this cDNA clone has been demonstrated by several criteria. Under conditions of chronic treatment with the beta-adrenergic receptor agonist isoproterenol, rat parotid glands show an approximately 10-fold increase in 4-beta-galactosyltransferase activity. The increased enzyme activity was reflected in dot-blot analysis of control and isoproterenol-treated rat parotid RNA by using the human cDNA as probe. Hybrid-selection and in vitro translation identified a protein product with a molecular mass of 47 kDa that was immunoprecipitated with the bovine antiserum. The full-length human cDNA clone was then isolated and the DNA sequence for the NH2-terminal portion of the protein was deduced. Comparison of the NH2-terminal protein sequence from the bovine protein with that of the human cDNA clone confirmed its identity. In addition, the human cDNA clone was used to localize the gene for 4-beta-galactosyltransferase to human chromosome 4 by Southern analysis of a somatic cell hybrid panel.

Antibody specific for the native form of Chinese hamster hypoxanthine/guanine phosphoribosyltransferase (HPRT; IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) was used to detect the synthesis of HPRT protein in a rabbit reticulocyte lysate translation system primed with mRNA from Chinese hamster tissues and cultured cells. Electrophoretic analysis of the immunopurified products from the translation of mRNA from wild-type and a series of mutant Chinese hamster cells indicated that HPRT synthesis in vitro qualitatively and quantitatively corresponded to synthesis in vivo. The translation system was used to identify two mRNA sources producing high levels of HPRT protein: Chinese hamster brain and a mouse neuroblastoma HPRT revertant cell line, NBR4. Translation of NBR4 mRNA generated 25-50 times more HPRT protein than mRNA from wild-type cells. The basis for HPRT overproduction is considered in view of an X chromosome alteration found in NBR4 cells.

Current efforts to map the human genome are focused on individual chromosomes or smaller regions and frequently rely on the use of somatic cell hybrids. We report the application of the polymerase chain reaction to direct amplification of human DNA from hybrid cells containing regions of the human genome in rodent cell backgrounds using primers directed to the human Alu repeat element. We demonstrate Alu-directed amplification of a fragment of the human HPRT gene from both hybrid cell and cloned DNA and identify through sequence analysis the Alu repeats involved in this amplification. We also demonstrate the application of this technique to identify the chromosomal locations of large fragments of the human X chromosome cloned in a yeast artificial chromosome and the general applicability of the method to the preparation of DNA probes from cloned human sequences. The technique allows rapid gene mapping and provides a simple method for the isolation and analysis of specific chromosomal regions.