Dietmar Müller

Mutations in bone morphogenetic protein receptor 1B cause brachydactyly type A2

Brachydactyly (BD) type A2 is an autosomal dominant hand malformation characterized by shortening and lateral deviation of the index fingers and, to a variable degree, shortening and deviation of the first and second toes. We performed linkage analysis in two unrelated German families and mapped a locus for BD type A2 to 4q21-q25. This interval includes the gene bone morphogenetic protein receptor 1B (BMPR1B), a type I transmembrane serinethreonine kinase. In one family, we identified a T599 → A mutation changing an isoleucine into a lysine residue (I200K) within the glycine/serine (GS) domain of BMPR1B, a region involved in phosphorylation of the receptor. In the other family we identified a C1456 → T mutation leading to an arginine-to-tryptophan amino acid change (R486W) in a highly conserved region C-terminal of the BMPR1B kinase domain. An in vitro kinase assay showed that the I200K mutation is kinase-deficient, whereas the R486W mutation has normal kinase activity, indicating a different pathogenic mechanism. Functional analyses with a micromass culture system revealed a strong inhibition of chondrogenesis by both mutant receptors. Overexpression of mutant chBmpR1b in vivo in chick embryos by using a retroviral system resulted either in a BD phenotype with shortening and/or missing phalanges similar to the human phenotype or in severe hypoplasia of the entire limb. These findings imply that both mutations identified in human BMPR1B affect cartilage formation in a dominant-negative manner.

Gerodermia osteodysplastica is caused by mutations in SCYL1BP1, a Rab-6 interacting golgin

Gerodermia osteodysplastica is an autosomal recessive disorder characterized by wrinkly skin and osteoporosis. Here we demonstrate that gerodermia osteodysplastica is caused by loss-of-function mutations in SCYL1BP1, which is highly expressed in skin and osteoblasts. The protein localizes to the Golgi apparatus and interacts with Rab6, identifying SCYL1BP1 as a golgin. These results associate abnormalities of the secretory pathway with age-related changes in connective tissues.

Severely Incapacitating Mutations in Patients with Extreme Short Stature Identify RNA-Processing Endoribonuclease RMRP as an Essential Cell Growth Regulator

The growth of an individual is deeply influenced by the regulation of cell growth and division, both of which also contribute to a wide variety of pathological conditions, including cancer, diabetes, and inflammation. To identify a major regulator of human growth, we performed positional cloning in an autosomal recessive type of profound short stature, anauxetic dysplasia. Homozygosity mapping led to the identification of novel mutations in the RMRP gene, which was previously known to cause two milder types of short stature with susceptibility to cancer, cartilage hair hypoplasia, and metaphyseal dysplasia without hypotrichosis. We show that different RMRP gene mutations lead to decreased cell growth by impairing ribosomal assembly and by altering cyclin-dependent cell cycle regulation. Clinical heterogeneity is explained by a correlation between the level and type of functional impairment in vitro and the severity of short stature or predisposition to cancer. Whereas the cartilage hair hypoplasia founder mutation affects both pathways intermediately, anauxetic dysplasia mutations do not affect B-cyclin messenger RNA (mRNA) levels but do severely incapacitate ribosomal assembly via defective endonucleolytic cleavage. Anauxetic dysplasia mutations thus lead to poor processing of ribosomal RNA while allowing normal mRNA processing and, therefore, genetically separate the different functions of RNase MRP.

Impaired epidermal ceramide synthesis causes autosomal recessive congenital ichthyosis and reveals the importance of ceramide acyl chain length.

The barrier function of the human epidermis is supposed to be governed by lipid composition and organization in the stratum corneum. Disorders of keratinization, namely ichthyoses, are typically associated with disturbed barrier activity. Using autozygosity mapping and exome sequencing, we have identified a homozygous missense mutation in CERS3 in patients with congenital ichthyosis characterized by collodion membranes at birth, generalized scaling of the skin, and mild erythroderma. We demonstrate that the mutation inactivates ceramide synthase 3 (CerS3), which is synthesized in skin and testis, in an assay of N-acylation with C26-CoA, both in patient keratinocytes and using recombinant mutant proteins. Moreover, we show a specific loss of ceramides with very long acyl chains from C26 up to C34 in terminally differentiating patient keratinocytes, which is in line with findings from a recent CerS3-deficient mouse model. Analysis of reconstructed patient skin reveals disturbance of epidermal differentiation with an earlier maturation and an impairment of epidermal barrier function. Our findings demonstrate that synthesis of very long chain ceramides by CerS3 is a crucial early step for the skin barrier formation and link disorders presenting with congenital ichthyosis to defects in sphingolipid metabolism and the epidermal lipid architecture.

Shprintzen-Goldberg syndrome: fourteen new patients and a clinical analysis.

The Shprintzen–Goldberg syndrome (SGS) is a disorder of unknown cause comprising craniosynostosis, a marfanoid habitus and skeletal, neurological, cardiovascular, and connective‐tissue anomalies. There are no pathognomonic signs of SGS and diagnosis depends on recognition of a characteristic combination of anomalies. Here, we describe 14 persons with SGS and compare their clinical findings with those of 23 previously reported individuals, including two families with more than one affected individual. Our analysis suggests that there is a characteristic facial appearance, with more than two thirds of all individuals having hypertelorism, down‐slanting palpebral fissures, a high‐arched palate, micrognathia, and apparently low‐set and posteriorly rotated ears. Other commonly reported manifestations include hypotonia in at least the neonatal period, developmental delay, and inguinal or umbilical hernia. The degree of reported intellectual impairment ranges from mild to severe. The most common skeletal manifestations in SGS were arachnodactyly, pectus deformity, camptodactyly, scoliosis, and joint hypermobility. None of the skeletal signs alone is specific for SGS. Our study includes 14 mainly German individuals with SGS evaluated over a period of 10 years. Given that only 23 other persons with SGS have been reported to date worldwide, we suggest that SGS may be more common than previously assumed.

Exclusion mapping of the X-linked dominant chondrodysplasia punctata/ichthyosis/cataract/short stature (Happle) syndrome: possible involvement of an unstable pre-mutation

SummaryHomology with the mouse bare patches mutant suggests that the gene for the X-linked dominant chondrodysplasia punctata / ichthyosis / cataract / short stature syndrome (Happle syndrome) is located in the human Xq28 region. To test this hypothesis, we performed a linkage study in three families comprising a total of 12 informative meioses. Multiple recombinations appear to exclude the Xq28 region as the site of the gene. Surprisingly, multiple crossovers were also found with 26 other markers spread along the rest of the X chromosome. Two-point linkage analysis and analysis of recombination chromosomes seem to exclude the gene from the entire X chromosome. Three different mechanisms are discussed that could explain the apparent exclusion of an X-linked gene from the X chromosome by linkage analysis: (a) different mutations on the X chromosome disturbing X inactivation, (b) metabolic interference, i.e. allele incompatibility of an X-linked gene, and (c) an unstable pre-mutation that can become silent in males. We favour the last explanation, as it would account for the unexpected sex ratio (M∶F) of 1.2∶1 among surviving siblings, and for the striking clinical variability of the phenotype, including stepwise increases in disease expression in successive generations.

Goltz–Gorlin (focal dermal hypoplasia) and the microphthalmia with linear skin defects (MLS) syndrome: no evidence of genetic overlap

Focal dermal hypoplasia (FDH) is an X-linked developmental disorder with male lethality characterized by patchy dermal hypoplasia, skeletal and dental malformations, and microphthalmia or anophthalmia. Recently, heterozygous loss-of-function mutations in the PORCN gene have been described to cause FDH. FDH shows some clinical overlap with the microphthalmia with linear skin defects (MLS) syndrome, another X-linked male lethal condition, associated with mutations of HCCS in the majority of cases. We performed DNA sequencing of PORCN in 13 female patients with the clinical diagnosis of FDH as well as four female patients with MLS syndrome and no mutation in HCCS. We identified PORCN mutations in all female patients with FDH. Eleven patients seem to have constitutional PORCN alterations in the heterozygous state and two individuals are mosaic for the heterozygous sequence change in PORCN. No PORCN mutation was identified in the MLS-affected patients, providing further evidence that FDH and MLS do not overlap genetically. X chromosome inactivation (XCI) analysis revealed a random or slightly skewed XCI pattern in leukocytes of individuals with intragenic PORCN mutation suggesting that defective PORCN does not lead to selective growth disadvantage, at least in leukocytes. We conclude that the PORCN mutation detection rate is high in individuals with a clear-cut FDH phenotype and somatic mosaicism can be present in a significant proportion of patients with mild or classic FDH.

Branchio-oculo-facial syndrome and branchio-otic/branchio-oto-renal syndromes are distinct entities

Branchio-oculo-facial syndrome (BOF, MIM 1136201) is a rare autosomal dominant disorder. The symptoms of this disorder include bilateral postauricular cervical branchial sinus defects with haemangiomatous, scarred skin, cleft lip with or without cleft palate, pseudocleft of the upper lip, nasolacrimal duct obstruction, low set ears with posterior rotation, a malformed, asymmetrical nose with a broad bridge and flattened tip, and, occasionally, prematurely grey hair. Father to son transmission of this disorder has been observed,2 which indicate autosomal dominant inheritance. Another disorder with hearing loss resulting from bilateral branchial cleft fistulas is branchio-oto-renal syndrome (BOR, MIM 113650). Common features of both syndrome are summarised in table 1. Some characteristics of both BOR and BOF syndromes have been reported in a father (BOF) and his son (BOR), but the constant features of BOF syndrome were not observed in either of them. This observation led to the conclusion that BOF and BOR might be allelic variants of the same gene.3 It was suggested that, in both syndromes, penetrance and expression could be variable, and it was concluded that BOF and BOR syndromes are the variable results of mutations in the same autosomal gene.3 However, it was pointed out later that both subjects in fact should be considered as BOF syndrome rather than BOF and BOR syndrome, and that these syndromes are distinct entities and may not be allelic.4 Another related disorder is branchio-otic syndrome (BO, MIM 602588), which comprises branchial fistulas, preauricular pits, and hearing impairment, but lacks renal anomalies (table 1). View this table: Table 1 Branchiogenic disorders The first candidate gene for BOR has been mapped. This gene, EYA1 (“eyes absent-like”, a human homologue of the Drosophila eyes absent gene), was found by positional cloning5 and maps to chromosome 8q13.3. Mutations in EYA1 have been described,6–8 which made …

DOCK6 mutations are responsible for a distinct autosomal-recessive variant of Adams-Oliver syndrome associated with brain and eye anomalies.

Adams–Oliver syndrome (AOS) is characterized by the association of aplasia cutis congenita with terminal transverse limb defects, often accompanied by additional cardiovascular or neurological features. Both autosomal‐dominant and autosomal‐recessive disease transmission have been observed, with recent gene discoveries indicating extensive genetic heterogeneity. Mutations of the DOCK6 gene were first described in autosomal‐recessive cases of AOS and only five DOCK6‐related families have been reported to date. Recently, a second type of autosomal‐recessive AOS has been attributed to EOGT mutations in three consanguineous families. Here, we describe the identification of 13 DOCK6 mutations, the majority of which are novel, across 10 unrelated individuals from a large cohort comprising 47 sporadic cases and 31 AOS pedigrees suggestive of autosomal‐recessive inheritance. DOCK6 mutations were strongly associated with structural brain abnormalities, ocular anomalies, and intellectual disability, thus suggesting that DOCK6‐linked disease represents a variant of AOS with a particularly poor prognosis.

Additional Clinical and Molecular Analyses of TFAP2A in Patients With the Branchio-Oculo-Facial Syndrome

The branchio‐oculo‐facial syndrome (BOFS) is a rare disorder with approximately 50 sporadic and familial cases in the literature. We report on the clinical and molecular analyses of five additional patients with BOFS (two familial and three sporadic). DNA analysis of the TFAP2A gene associated with BOFS using DNA sequencing detected a mutation [c.763A>G (p.Arg255Gly)] in two unrelated patients. This mutation had been reported in another patient and indicates a probable mutational hotspot in the TFAP2A gene. We also detected three new mutations which are restricted to exons 4–6. These gene regions are almost free of any single nucleotide polymorphisms. An evolutionary sequence comparison showed a high degree of sequence conservation from humans to the honey bee (Apis mellifera) in exon 6 showing that this part of the protein is probably essential. Our study represents the second group of BOFS patients with molecular confirmation, expanding the phenotype and spectrum of mutations and limiting it to a restricted part of the gene.