Keith J. Johnson

Drosophila homolog of the myotonic dystrophy-associated gene, SIX5, is required for muscle and gonad development

SIX5 belongs to a family of highly conserved homeodomain transcription factors implicated in development and disease. The mammalian SIX5/SIX4 gene pair is likely to be involved in the development of mesodermal structures. Moreover, a variety of data have implicated human SIX5 dysfunction as a contributor to myotonic dystrophy type 1 (DM1), a condition characterized by a number of pathologies including muscle defects and testicular atrophy. However, this link remains controversial. Here, we investigate the Drosophila gene, D-Six4, which is the closest homolog to SIX5 of the three Drosophila Six family members. We show by mutant analysis that D-Six4 is required for the normal development of muscle and the mesodermal component of the gonad. Moreover, adult males with defective D-Six4 genes exhibit testicular reduction. We propose that D-Six4 directly or indirectly regulates genes involved in the cell recognition events required for myoblast fusion and the germline:soma interaction. While the exact phenotypic relationship between D-Six4 and SIX4/5 remains to be elucidated, the defects in D-Six4 mutant flies suggest that human SIX5 should be more strongly considered as being responsible for the muscle wasting and testicular atrophy phenotypes in DM1.

Genetics (molecular biology) and Meniere's disease.

COCH is not the FMD gene detected in our linkage study; furthermore, COCH and FMD are not allelic. The indications are that FMD is heterogenetic. The linkage analysis points to the possibility of one FMD mutation in one of the neighboring candidate genes on chromosome 14, and, with anticipation, possibly a triple repeat amplification. Recently, the myotonic dystrophy type 2 locus has been shown to contain an expanded tetranucleotide repeat [46], so the search for a similar repeat on 14q is indicated.

Genetic linkage and radiation hybrid mapping of the three human GABAC receptor ρ subunit genes: GABRR1, GABRR2 and GABRR3

GABA(C) receptors mediate rapid inhibitory neurotransmission in retina. We have mapped, in detail, the human genes which encode the three polypeptides that comprise this receptor: rho1 (GABRR1), rho2 (GABRR2) and rho3 (GABRR3). We show that GABRR1 and GABRR2 are located close together, in a region of chromosome 6q that contains loci for inherited disorders of the eye, but that GABRR3 maps to chromosome 3q11-q13.3. Our mapping data suggest that the rho polypeptide genes, which are thought to share a common ancestor with GABA(A) receptor subunit genes, diverged at an early stage in the evolution of this gene family.

Genomic mapping and evolution of human GABAA receptor subunit gene clusters

Division of Molecular Genetics, Institute of Biomedical and Life Sciences, University of Glasgow, Anderson College, 56 Dumbarton Road, Glasgow G11 6NU, UK Department of Biochemistry and Molecular Genetics, Imperial College School of Medicine at St Mary’s Hospital, London W2 1PG, UK Institut für Zellbiochemie und klinische Neurobiologie, Universita ̈ts-Krankenhaus Eppendorf, Universita ̈t Hamburg, 20246 Hamburg, Germany Institut für Humangenetik, Justus-Liebig-Universita ̈t Giessen, 35392 Giessen, Germany Laboratoire de Genetique Moleculaire de la Neurotransmission et des Processus Neurodegeneratifs, Centre National de la Recherche Scientifique, 91198 Gif-sur-yvette, France Department of Cardiothoracic Surgery, Hammersmith Hospital, London W12 0HS, UK

Structure, mapping and expression of the human gene encoding the homeodomain protein, SIX2 ☆

Vertebrate genes with sequence similarity to the Drosophila homeobox gene, sine oculis (so), constitute the SIX family. There is notable expression of members of this family in anterior neural structures, and several SIX genes have been shown to play roles in vertebrate and insect development, or have been implicated in maintenance of the differentiated state of tissues. Mutations in three of these genes in man (SIX5, SIX6 and SIX3) are associated with severe phenotypes, and therefore, the cloning of other human genes from this family is of interest. We have cloned and characterised the gene that encodes human SIX2, elucidated its gene structure and conducted expression studies in a range of tissues. SIX2 is widely expressed in the late first-trimester fetus, but has a limited range of expression sites in the adult. The expression pattern of SIX2 and its localisation to chromosome 2p15-p16 will be of use in assessing its candidacy in human developmental disorders.

Association of CTG repeats and the 1-kb Alu insertion/deletion polymorphism at the myotonin protein kinase gene in the Japanese population suggests a common Eurasian origin of the myotonic dystrophy mutation.

We have studied linkage disequilibrium between CTG repeats and anAlu insertion/deletion polymorphism at the myotonin protein kinase gene (DMPK) in 102 Japanese families, of which 93 were affected with myotonic dystrophy (DM). All of the affected chromosomes are in complete linkage disequilibrium with theAlu insertion allele. Among the normal chromosomes, alleles of CTG repeats 5 and ⩾ 17 are exclusively associated with the insertion allele. On the other hand, intermediate alleles of 11-6 repeats show a significantly greater association with the deletion allele. A strikingly similar pattern of linkage disequilibrium observed in European populations suggests a common origin of the DM mutation in the Japanese and European populations.

Further evidence for a major ancient mutation underlying myotonic dystrophy from linkage disequilibrium studies in the Japanese population

AbstractThe myotonic dystrophy (DM) mutation is an unstable (CTG)n repeat, present at a copy number of 5–37 repeats on normal chromosomes but amplified to 50–3000 copies on DM chromosomes. Previous findings in Caucasian populations of a DM founder chromosome raise a question about the molecular events involved in the expansion mutation. To investigate whether a founder chromosome for the DM mutation exists in the Japanese population, we genotyped families using polymorphic markers near the (CTG)n repeat region and constructed haplotypes. Six different haplotypes were found and DM alleles were always haplotype A. To find an origin of the (CTG)n repeat mutation and to investigate the mechanism of the expansion mutation in the Japanese population we have studied 90 Japanese DM families comprising 190 affected and 130 unaffected members. The results suggest that a few common ancestral mutations in both Caucasian and Japanese populations have originated by expansion of an ancestral n = 5 repeat to n = 19−37 copies. These data support multistep models of triplet repeat expansion that have been proposed for both DM and Friedreichs ataxia.

Characterisation of the transcription factor, SIX5, using a new panel of monoclonal antibodies

SIX5 is a member of the human SIX family of transcription factors, many of which are involved in eye development. However, SIX5 transcripts are known to be present at very low levels in cells and no study has yet convincingly demonstrated detection of endogenous SIX5 protein by Western blotting or immunolocalisation. We have produced a new panel of 18 monoclonal antibodies (mAbs) that recognise at least four different epitopes in order to identify authentic human SIX5 protein in cells and tissues. Phage‐displayed peptide libraries were used to identify individual amino‐acids important for antibody binding within each epitope. Endogenous SIX5 migrated in SDS–PAGE with an apparent Mr of 100 kDa and was present at similar levels in all foetal tissues and cell lines tested. In HeLa cells, it was located in the nucleoplasm with a granular distribution. An mRNA for a shorter splicing isoform of SIX5, with an altered carboxy‐terminus, has been described, but further mAbs specific for this isoform did not detect any endogenous protein. We conclude that the full‐length isoform is the major functional protein in vivo while the putative shorter protein is undetectable and may not be expressed at all.

Linkage disequilibrium detected between dystrophia myotonica and APOC2 locus in the Finnish population

SummaryThree polymorphic loci APOC2, CKMM and p134C were used to haplotype 15 Finnish dystrophia myotonica (DM) families representing about one third of all DM patients in this isolated population. Compound APOC2 and CKMM haplotypes reveal linkage disequilibrium: 90% of DM chromosomes co-occur with the haplotypes that occur in 31% of normal chromosomes only. The same disequilibrium is present when only polymorphisms occurring at the APOC2 locus are used. Surprisingly, no statistically significant linkage disequilibrium was discovered at the CKMM locus alone. Of the meiotic events, 84% were informative when both APO2 and CKMM loci were used. When studied selectively, 60% of meiotic events were informative at the APOC2 locus, whereas CKMM alone resulted in 65% meiotic informativeness. The distal marker p134C was found to have an unfortunately low information content in our population.