Non-selective isolation of human somatic cell hybrids by unit-gravity sedimentation

Complementation studies with clinical and biochemical characterizations of a new variant of multiple sulphatase deficiency

A patient with a new variant of multiple sulphatase deficiency (MSDv) is reported. Unlike the usual type, onset was late and progress was slow. The phenotypic changes were those usually seen in multiple sulphatase deficiency but much milder. Cytoplasmic accumulations were found in skin fibroblasts, and urinary mucopolysaccharides and sulphatides were high. Arylsulphatases A, B and C (ASA, B and C), heparan N-sulphatase sulphoiduronate sulphatase, and N-acetylgalactosamine 6-sulphatase all had low activity in lymphocytes and cultured skin fibroblasts. Complementation for ASA activity was found in hybrids between MSDv and metachromatic leukodystrophy (MLD) as well as between multiple sulphatase deficiency (MSD) and MLD. Complementation for ASC activity was also seen in hybrids between MSDv and X-linked ichthyosis (XLI), and between MSD and XLI. However, neither ASA nor ASC activity increased in hybrid cells of MSDv and MSD. These results suggested that the mutations of MSDv and of MSD were allelic, although of different phenotypes.

Transformation of human cultured fibroblasts with plasmids carrying dominant selection markers and immortalizing potential

The disadvantages of using human cultured cells for biochemical and genetic studies are their limited lifespan in vitro and their lack of chemical selection markers. These problems are now overcome by transfecting human cultured fibroblasts with the pSV3-gpt and pSV3-neo plasmid DNA which carry genes coding for the immortalizing SV40 large T-antigen and dominant selection markers. Transformed human fibroblasts were obtained at a frequency of about 10(-5) with both selection systems. These transformed cells showed a twofold increase in growth rate and three to tenfold increase in cell number at confluence. The improved growth characteristics were associated with the expression of the SV40 T-antigen detected with immunoprecipitation. These cell lines also changed from their usual spindle shapes to an epithelioid morphology characteristic of transformed cells. From 60 to 100% of the cells transfected with pSV3 plasmid DNA demonstrated numerical and structural abnormalities in their karyotypes. Cells transfected with DNA from a similar plasmid, pSV2-neo, which differed from the pSV3-neo plasmid only by missing the sequence encoding the complete early region of SV40, neither expressed T-antigen nor showed any change in morphology, improvement in growth characteristics or abnormalities in karyotype. However, they were still selectable with the aminoglycoside G-418. Therefore, by appropriate choice of vector plasmids, dominant selection markers and improved growth characteristics can be imparted separately or simultaneously to human fibroblasts. The morphological, biochemical and chromosomal changes resulting from such transformations must be recognized in using this approach for biochemical and genetic studies.

Pseudo arylsulfatase-A deficiency in healthy individuals: genetic and biochemical relationship to metachromatic leukodystrophy

Metachromatic leukodystrophy is a hereditary neurodegenerative disorder in man associated with deficient arylsulfatase-A activity (aryl-sulfate sulfohydrolase, EC 3.1.6.1). The same enzyme deficiency has been noted in clinically normal individuals, a condition known as pseudo arylsulfatase-A deficiency. With a nonselective method, somatic cell hybrids were obtained from cultured fibroblasts of these two types of individuals; the hybrids showed no restoration of arylsulfatase-A activity. Thus, metachromatic leukodystrophy and pseudo arylsulfatase-A deficiency are allelic conditions. Although these conditions cannot be distinguished by simple quantitative arylsulfatase-A activity assays, they can be differentiated with sucrose density gradient centrifugation, Cellogel electrophoresis, or isoelectric focusing in polyacrylamide gels. In each case, a small amount of activity with characteristics of arylsulfatase-A was found only from fibroblasts of pseudo arylsulfatase-A-deficient individuals and not from those of metachromatic leukodystrophy patients. This residual enzyme has the same pH optimum, heat stability, inhibitor sensitivity, and Km as the normal enzyme but slightly different isoelectric points. In conclusion, although pseudo arylsulfatase-A deficiency and metachromatic leukodystrophy have very different clinical outcomes, they are due to mutations of the same structural gene, coding for arylsulfatase-A. These two conditions can be differentiated now by simple electrophoretic analysis of the residual arylsulfatase-A activity.

Gene dosage effects in human diploid and tetraploid fibroblasts

The effects of cell ploidy on the biochemical characteristics of cultured cells were compared using human diploid vs tetraploid fibroblasts isolated with a non-selective method. Their DNA replication was compared by thymidine incorporation, and DNA content by Feulgen staining and quantitative analysis. Their RNA and protein content, cell sizes and the specific activities of glucose-6-phosphate dehydrogenase (G-6-PD) and 6-phosphogluconate dehydrogenase (6-PGD) were assayed quantitatively. With the exception of RNA content, all other parameters demonstrated a 2-fold increase reflecting the increase in cell ploidy. These direct gene dosage effects on the genetic material and functional expression of the human genome were in contrast to previous observations in other species and validate the use of human intraspecific euploid hybrids for biochemical and genetic studies.

Somatic cell hybridization studies on the genetic regulation and allelic mutations in metachromatic leukodystrophy

Metachromatic leukodystrophy is a hereditary neurodegenerative disease associated with deficient arylsulfatase A activity. Clinical variants differ in onset times and severity of the disease but each breeds true within families. Somatic cell hybridization techniques were used to clarify the genetic relationship among these mutants. Hybrid clones isolated with a nonselective method from fusing fibroblasts of an infantile and a juvenile variant did not show complementation of arylsulfatase A activity. Hence, these clinical variants are allelic mutants. Previous somatic cell hybridization studies suggested that "arylsulfatase A-deficiency" is a dominant phenotype, in contrast to its apparent recessive mode of inheritance. To resolve this discrepancy, hybrid clones from fusing normal and arylsulfatase A-deficient fibroblasts were isolated nonselectively. They continued to express arylsulfatase A activity. Hence, even in vitro, "arylsulfatase A-deficiency" remains as a recessive phenotype.

Complementation of arylsulfatase A in somatic hybrids of metachromatic leukodystrophy and multiple sulfatase deficiency disorder fibroblasts

Metachromatic leukodystrophy and multiple sulfatase deficiency disorder are severe neurodegenerative diseases inherited as separate autosomal recessive traits. Arylsulfatase A (aryl-sulfate sulfohydrolase, EC 3.1.6.1) activity is deficient in both diseases but in multiple sulfatase deficiency disorder, activities of arylsulfatases B and C and other sulfatases are also reported to be reduced. Somatic hybrid cell clones produced by fusing cultured fibroblasts from patients with these diseases were isolated by a nonselective technique based on unit-gravity sedimentation. Arylsulfatase A activity was restored in these hybrids. The complemented enzyme resembled the normal arylsulfatase A in heat stability, pH optimum, Km, electrophoretic mobility, and immunologic reactivity. Because a structurally normal enzyme can be restored in a hybrid only though intergenic complementation, these results indicate that the mutations responsible for the deficiency of arylsulfatase A activity in metachromatic leukodystrophy and multiple sulfatase deficiency disorder are nonallelic and that at least two genetic loci control the expression of arylsulfatase A activity in the human genome. Furthermore, arylsulfatase C activity was also restored to normal in the hybrids, indicating that a common sulfatase inhibitor is not the cause of the multiple sulfatse deficiency.

Complementation of genetic disease: a velocity sedimentation procedure for the enrichment of heterokaryons

Methodology is described to enrich for heterokaryons after mammalian cell fusion. A heterogeneous cell mixture can be separated on a Sta-Put apparatus into fractions of uniform size cells by sedimentation through a 1% bovine serum albumin-5% Ficoll gradient. Unfused RAG and LM/TK- cells, differing by 10% in diameter, have been sorted by size; following fusion, larger and faster sedimenting cells were shown to be hybrids. This methodology can be utilized in genetic complementation studies of human genetic diseases where selection procedures for proliferating hybrids do not exist. When fibroblasts from individuals with Tay-Sachs disease [deficient in hexosaminidase A (HEX A-)] and Sandhoff-Jatzkewitz disease (HEX A- and HEX B-) are fused, HEX A is generated, demonstrating complementation of two different mutations. After Sta-Put fractionation, the HEX A complementation product was associated with the faster sedimenting multinuclear cells and not with the mononuclear parental cells. This methodology will facilitate detection of genetic differences in fibroblasts from related inherited disorders.

Enrichment of human heterokaryons by Ficoll gradient for complementation analysis of iduronate sulfatase deficiency

Ficoll gradients have been used to enrich for heterokaryons in cultures of human skin fibroblasts following polyethylene glycol (PEG) induced fusion. These gradients provide a simple and consistent method for obtaining populations of multinucleated cells, at least twofold greater than those resulting from fusion alone. Formation of glucose-6-phosphate dehydrogenase (G6PD) heteropolymers has been used as a functional assay for the presence of heterokaryons. Analysis of cell populations enriched for multinucleated cells has revealed complementation leading to iduronate sulfatase activity in heterokaryons derived from iduronate sulfatase-deficient fibroblasts expressing the Hunter and multiple sulfatase-deficiency mutations.