Chloride channel myotonia: exon 8 hot-spot for dominant-negative interactions.

Myotonia congenita (MC) is the commonest genetic skeletal muscle ion channelopathy. It is caused by mutations in CLCN1 on chromosome 7q35, which alter the function of the major skeletal muscle voltage-gated chloride channel. Dominant and recessive forms of the disease exist. We have undertaken a clinical, genetic and molecular expression study based upon a large cohort of over 300 UK patients. In an initial cohort of 22 families, we sequenced the DNA of the entire coding region of CLCN1 and identified 11 novel and 11 known mutations allowing us to undertake a detailed genotype-phenotype correlation study. Generalized muscle hypertrophy, transient weakness and depressed tendon reflexes occurred more frequently in recessive than dominant MC. Mild cold exacerbation and significant muscle pain were equally common features in dominant and recessive cases. Dominant MC occurred in eight families. We noted that four newly identified dominant mutations clustered in exon 8, which codes for a highly conserved region of predicted interaction between the CLC-1 monomers. Expressed in Xenopus oocytes these mutations showed clear evidence of a dominant-negative effect. Based upon the analysis of mutations in this initial cohort as well as a review of published CLCN1 mutations, we devised an exon hierarchy analysis strategy for genetic screening. We applied this strategy to a second cohort of 303 UK cases with a suspected diagnosis of MC. In 23 individuals, we found two mutations and in 86 individuals we identified a single mutation. Interestingly, 40 of the cases with a single mutation had dominant exon 8 mutations. In total 48 individuals (from 34 families) in cohort 1 and 2 were found to harbour dominant mutations (37% of mutation positive individuals, 30% of mutation positive families). In total, we have identified 23 new disease causing mutations in MC, confirming the high degree of genetic heterogeneity associated with this disease. The DNA-based strategy we have devised achieved a genetic diagnosis in 36% of individuals referred to our centre. Based on these results, we propose that exon 8 of CLCN1 is a hot-spot for dominant mutations. Our molecular expression studies of the new exon 8 mutations indicate that this region of the chloride channel has an important role in dominant negative interactions between the two chloride channel monomers. Accurate genetic counselling in MC should be based not only upon clinical features and the inheritance pattern but also on molecular genetic analysis and ideally functional expression data.

A cloned, immortal line of murine melanoblasts inducible to differentiate to melanocytes.

Cultures of differentiated melanocytes can readily be grown from the dissociated epidermis of neonatal mice, and immortal cell lines often develop from these. However, the first cells that grow and transiently dominate the cultures, while similar to melanocytes, are unpigmented. These have been shown to be precursors of melanocytes and may be termed melanoblasts. Under our previous standard culture conditions, involving the use of keratinocyte feeder cells, foetal calf serum, the phorbol ester 12-O-tetradecanoyl phorbol acetate (TPA) and cholera toxin, all the melanoblasts spontaneously differentiated to pigmented melanocytes within about 3 weeks. We now describe some factors affecting the proliferation and differentiation of diploid murine melanoblasts in the presence of serum. Murine stem cell factor/steel factor (SCF), basic fibroblast growth factor (bFGF) and murine leukaemia inhibitory factor/differentiation-inhibiting activity (LIF/DIA) all increased melanoblast numbers. SCF and LIF also slightly inhibited melanoblast differentiation, while cholera toxin and TPA promoted differentiation. Using some of these findings, and by regular replacement of keratinocyte or fibroblastoid feeder cells, we have established a clonal line of immortal murine melanoblasts, 'melb-a'. These cells express tyrosinase-related protein-2 but not, in general, tyrosinase. They can be induced to differentiate irreversibly to functional melanocytes (also proliferative and immortal) by plating in the absence of feeder cells. Thus a new immortal melanocyte line, 'melan-a2', has also been produced.

Detection of a human DNA sequence correlated with melanocyte-like differentiation and tumour suppression after transfection into murine melanoma cells.

Since there is much indirect evidence for dominant suppressor genes for melanoma, we sought to isolate such a gene. Metastatic B16-F10 murine melanoma cells were lipofected with a normal human genomic library in a cosmid vector that also confers resistance to the drug G418. A few of the G418-resistant colonies acquired combinations of properties resembling those of normal melanocytes, including differentiated features (pigmentation, dendricity), slower growth, flat shape, monolayered colony form, stimulation of proliferation by a phorbol ester, and anchorage dependence. Four out of eight also showed reduced tumorigenicity in mice. Southern blotting indicated the presence of numerous cosmids in the melanocyte-like transfectants. DNA from one such line was used for secondary transfection. One secondary G418-resistant line showed pronounced melanocytic properties and marked tumour suppression in syngeneic and nude mice. A human repetitive sequence detected in this line was used in the polymerase chain reaction (PCR) to isolate intervening unique DNA sequences. One unique human sequence was attenuated in all tumors arising from both primary and secondary transfectants, suggesting close linkage with the sequence responsible for tumour suppression.

Suppression of properties associated with malignancy in murine melanoma-melanocyte hybrid cells.

Murine and human melanoma cells differ relatively reliably from non-tumorigenic melanocytes in certain biological properties. When cultured at low pH, melanocytes tend to be pigmented and melanoma cells unpigmented. The growth of virtually all metastatic melanoma cells is inhibited by phorbol esters such as TPA (12-O-tetradecanoyl phorbol-13-acetate), which stimulate melanocyte growth. Melanocytes fail to grow in suspension culture or produce tumours when implanted in animals, while many melanoma lines can do both. Here we studied which of these properties were dominant in hybrid cells formed by fusion of drug-resistant murine B16-F10RR melanoma cells to melanocytes of the albino and brown lines, melan-c and melan-b. The albino melanocytes are unpigmented but well-differentiated, the brown melanocytes produce pale brown pigment and the melanoma cells are unpigmented under the conditions used. All hybrid colonies observed produced black pigment, except some melan-b/melanoma hybrids when growing sparsely with TPA. Thus pigmentation was generally dominant. 14/15 hybrid lines showed stimulation of proliferation by TPA, as do melanocytes. Most hybrid lines showed no or reduced capacity for growth in suspension, though some grew better in suspension when TPA was present. There was marked suppression of the tumorigenicity of the parental melanoma cells in 4/8 hybrids examined, and tumorigenicity was reduced in the others, despite considerable chromosome loss by the passage level tested. Thus most properties of the non-tumorigenic pigment cells were dominant, as often observed for other cell lineages, and providing further evidence for gene loss in the genesis of malignant melanoma.

Regional localization of polymorphic markers on chromosome 10 by physical and genetic mapping.

The human vimentin gene and a random DNA segment (D10S39) were mapped to the short arm of human chromosome 10 by linkage analysis. A panel of somatic cell hybrids and monosomy cell-lines, which divide chromosome 10 into seven regions, was used to localize 10 polymorphic markers on this chromosome. The physical map locations obtained correlate well with linkage maps of chromosome 10. Two markers which have been shown to be closely linked to the gene for multiple endocrine neoplasia type 2A map distal to a translocation breakpoint in band 10q11.2.

Chimerism in skin of bone marrow transplant recipients.

Skin biopsies from 3 patients receiving one-haplotype-matched bone marrow grafts have provided a unique opportunity to demonstrate the presence of donor cells in situ using immunohistological techniques and a monoclonal antibody directed against an epitope common to HLA-A2 and HLA-A28 antigens. The infiltrating cells were also analyzed in consecutive tissue sections with a panel of monoclonal antibodies to human leukocyte antigens, T cells, and epidermal Langerhans cells. Most of the infiltrating cells were shown to be T lymphocytes of donor origin, regardless of whether the histological changes were consistant with graft-versus-host disease (GVHD) or were eczematous. Donor T cells were also shown to colonize histologically normal skin soon after transplantation. Epidermal keratinocytes, dermal endothelium, and adnexal structures did not express the donor HLA type (i.e., were host derived) but the origin of the epidermal Langerhans cells could not clearly be established. The data show that donor cells preferentially migrate to certain sites in skin after transplantation and are not always associated with GVHD.

Studies on the purification and properties of a 6.8-S DNA polymerase activity found in calf-thymus DNA polymerase-alpha fraction.

The heterogeneity of calf thymus DNA polymerase-alpha has been further investigated. In particular, an enzyme (enzyme D) which exhibits higher activity on poly(dA) . (dT)10 (A:T = 20:1) compared with that on activated DNA, has been further purified and its properties compared with two other activities of the DNA polymerase-alpha fraction (enzymes A1 and C) which do not show a preference for poly(dA) . (dT)10 over activated DNA. As with A1 and C, enzyme D was shown to have many of the characteristic properties of DNA polymerase-alpha in that it is an acidic protein as judged by its binding to DEAE-cellulose, has a molecular weight of about 140000, does not use a poly (A) . (dT)10 template-initiator complex and is inhibited by N-ethylmaleimide. It exhibits anomalous gel filtration behaviour on Sepharose 6B and it binds relatively weakly to DNA-cellulose compared with DNA polymerase-beta. The extreme sensitivity of enzyme D to inhibtion by N-ethylmaleimide distinguishes it from A1 and C, as does its elution position from a DEAE-cellulose column. On the other hand enzymes C and D are readily inactivated by heating at 45 degrees C unlike enzyme A1. The possible interrelationships of the multiple activities of calf thymus DNA polymerase-alpha are discussed.