title: Mutations affecting the BHLHA9 DNA-binding domain cause MSSD, mesoaxial synostotic syndactyly with phalangeal reduction, Malik-Percin type.
abstract: Mesoaxial synostotic syndactyly, Malik-Percin type (MSSD) (syndactyly type IX) is a rare autosomal-recessive nonsyndromic digit anomaly with only two affected families reported so far. We previously showed that the trait is genetically distinct from other syndactyly types, and through autozygosity mapping we had identified a locus on chromosome 17p13.3 for this unique limb malformation. Here, we extend the number of independent pedigrees from various geographic regions segregating MSSD to a total of six. We demonstrate that three neighboring missense mutations affecting the highly conserved DNA-binding region of the basic helix-loop-helix A9 transcription factor (BHLHA9) are associated with this phenotype. Recombinant BHLHA9 generated by transient gene expression is shown to be located in the cytoplasm and the cell nucleus. Transcription factors 3, 4, and 12, members of the E protein (class I) family of helix-loop-helix transcription factors, are highlighted in yeast two-hybrid analysis as potential dimerization partners for BHLHA9. In the presence of BHLHA9, the potential of these three proteins to activate expression of an E-box-regulated target gene is reduced considerably. BHLHA9 harboring one of the three substitutions detected in MSSD-affected individuals eliminates entirely the transcription activation by these class I bHLH proteins. We conclude that by dimerizing with other bHLH protein monomers, BHLHA9 could fine tune the expression of regulatory factors governing determination of central limb mesenchyme cells, a function made impossible by altering critical amino acids in the DNA binding domain. These findings identify BHLHA9 as an essential player in the regulatory network governing limb morphogenesis in humans.
authors.affiliation: Zentrum fuer Humangenetik, Philipps-Universitaet Marburg, 35033 Marburg, Germany; Department of Animal Sciences, Human Genetics Program, Faculty of Biological Sciences, Quaid-i-Azam University, 45320 Islamabad, Pakistan.
authors.forename: Sajid
authors.initials: S
authors.lastname: Malik
authors.affiliation: Department of Medical Genetics, Faculty of Medicine, Gazi University, Besevler, 06500 Ankara, Turkey.
authors.forename: Ferda E
authors.initials: FE
authors.lastname: Percin
authors.affiliation: Zentrum fuer Humangenetik, Philipps-Universitaet Marburg, 35033 Marburg, Germany.
authors.forename: Dorothea
authors.initials: D
authors.lastname: Bornholdt
authors.affiliation: Institut fuer Humangenetik, Universitaetsklinikum Essen, Universitaet Duisburg-Essen, 45147 Essen, Germany.
authors.forename: Beate
authors.initials: B
authors.lastname: Albrecht
authors.affiliation: Department of Mother and Child, Medical Genetics Unit, University Hospital of Modena, 41124 Modena, Italy.
authors.forename: Antonio
authors.initials: A
authors.lastname: Percesepe
authors.affiliation: Zentrum fuer Humangenetik, Philipps-Universitaet Marburg, 35033 Marburg, Germany.
authors.forename: Manuela C
authors.initials: MC
authors.lastname: Koch
authors.affiliation: Department of Hand Surgery and Microsurgery, University Hospital of Modena, 41100 Modena, Italy.
authors.forename: Antonio
authors.initials: A
authors.lastname: Landi
authors.affiliation: Zentrum fuer Humangenetik, Philipps-Universitaet Marburg, 35033 Marburg, Germany.
authors.forename: Barbara
authors.initials: B
authors.lastname: Fritz
authors.affiliation: Department of Animal Sciences, Human Genetics Program, Faculty of Biological Sciences, Quaid-i-Azam University, 45320 Islamabad, Pakistan.
authors.forename: Rizwan
authors.initials: R
authors.lastname: Khan
authors.affiliation: Department of Animal Sciences, Human Genetics Program, Faculty of Biological Sciences, Quaid-i-Azam University, 45320 Islamabad, Pakistan.
authors.forename: Sara
authors.initials: S
authors.lastname: Mumtaz
authors.affiliation: Department of Medical Genetics, Gene Mapping Laboratory, Hacettepe University Medical Faculty, Sihhiye, 06100 Ankara, Turkey.
authors.forename: Nurten A
authors.initials: NA
authors.lastname: Akarsu
authors.affiliation: Zentrum fuer Humangenetik, Philipps-Universitaet Marburg, 35033 Marburg, Germany. Electronic address: grzeschi@staff.uni-marburg.de.
authors.forename: Karl-Heinz
authors.initials: KH
authors.lastname: Grzeschik
mesh_terms: D000595:Amino Acid Sequence; D051792:Basic Helix-Loop-Helix Proteins / Q000235:genetics*; D001665:Binding Sites; D004252:DNA Mutational Analysis; D019281:Dimerization; D005260:Female; D005385:Fingers / Q000002:abnormalities*; D017930:Genes, Reporter; D005838:Genotype; D006239:Haplotypes; D006801:Humans; D007558:Italy; D008297:Male; D008875:Middle Aged; D020125:Mutation, Missense; D010154:Pakistan; D010375:Pedigree; D010641:Phenotype; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment; D013576:Syndactyly / Q000235:genetics*; D014034:Toes / Q000002:abnormalities*; D014421:Turkey; D055815:Young Adult
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; C571985:BHLHA9 protein, human