Abdullah Uzumcu

Mutations in different components of FGF signaling in LADD syndrome.

Lacrimo-auriculo-dento-digital (LADD) syndrome is characterized by lacrimal duct aplasia, malformed ears and deafness, small teeth and digital anomalies. We identified heterozygous mutations in the tyrosine kinase domains of the genes encoding fibroblast growth factor receptors 2 and 3 (FGFR2, FGFR3) in LADD families, and in one further LADD family, we detected a mutation in the gene encoding fibroblast growth factor 10 (FGF10), a known FGFR ligand. These findings increase the spectrum of anomalies associated with abnormal FGF signaling.

Loss of desmoplakin isoform I causes early onset cardiomyopathy and heart failure in a Naxos-like syndrome

Background: Desmosomes are cellular junctions important for intercellular adhesion and anchoring the intermediate filament (IF) cytoskeleton to the cell membrane. Desmoplakin (DSP) is the most abundant desmosomal protein with 2 isoforms produced by alternative splicing. Methods: We describe a patient with a recessively inherited arrhythmogenic dilated cardiomyopathy with left and right ventricular involvement, epidermolytic palmoplantar keratoderma, and woolly hair. The patient showed a severe heart phenotype with an early onset and rapid progression to heart failure at 4 years of age. Results: A homozygous nonsense mutation, R1267X, was found in exon 23 of the desmoplakin gene, which results in an isoform specific truncation of the larger DSPI isoform. The loss of most of the DSPI specific rod domain and C-terminal area was confirmed by Western blotting and immunofluorescence. We further showed that the truncated DSPI transcript is unstable, leading to a loss of DSPI. DSPI is reported to be an obligate constituent of desmosomes and the only isoform present in cardiac tissue. To address this, we reviewed the expression of DSP isoforms in the heart. Our data suggest that DSPI is the major cardiac isoform but we also show that specific compartments of the heart have detectable DSPII expression. Conclusions: This is the first description of a phenotype caused by a mutation affecting only one DSP isoform. Our findings emphasise the importance of desmoplakin and desmosomes in epidermal and cardiac function and additionally highlight the possibility that the different isoforms of desmoplakin may have distinct functional properties within the desmosome.

Frequencies of gap- and tight-junction mutations in Turkish families with autosomal-recessive non-syndromic hearing loss

Mutations in genes encoding gap‐ and tight‐junction proteins have been shown to cause distinct forms of hearing loss. We have now determined the GJB2[connexin 26 (Cx26)] mutation spectrum in 60 index patients from mostly large Turkish families with autosomal‐recessive inherited non‐syndromic sensorineural hearing loss (NSSHL). GJB2 mutations were found in 31.7% of the families, and the GJB2–35delG mutation accounted for 73.6% of all GJB2 mutations. The carrier frequency of GJB2–35delG in the normal Turkish population was found to be 1.17% (five in 429). In addition to the described W24X, 233delC, 120delE and R127H mutations, we also identified a novel mutation, Q80R, in the GJB2 gene. Interestingly, the Q80R allele was inherited on the same haplotype as V27I and E114G polymorphisms. As little is known about the mutation frequencies of most other recently identified gap‐ and tight‐junction genes as a cause for hearing loss, we further screened our patients for mutations in GJB3 (Cx31), GJA1 (Cx43), ΔGJB6–D13S1830 (Cx30) and the gene encoding the tight‐junction protein, claudin 14 (CLDN14). Several novel polymorphisms, but no disease‐associated mutations, were identified in the CLND14 and GJA1 genes, and we were unable to detect the ΔGJB6–D13S1830 deletion. A novel putative mutation, P223T, was found in the GJB3 gene in heterozygous form in a family with two affected children. Our data shows that the frequency of GJB2 mutations in Turkish patients with autosomal‐recessive NSSHL and the carrier rate of the GJB2–35delG mutation in the Turkish population, is much lower than described for other Mediterranean countries. Furthermore, mutations in other gap‐ and tight‐junction proteins are not a frequent cause of hearing loss in Turkey.

MYO15A (DFNB3) mutations in Turkish hearing loss families and functional modeling of a novel motor domain mutation.

Myosin XVA is an unconventional myosin which has been implicated in autosomal recessive nonsyndromic hearing impairment (ARNSHI) in humans. In Myo15A mouse models, vestibular dysfunction accompanies the autosomal recessive hearing loss. Genomewide homozygosity mapping and subsequent fine mapping in two Turkish families with ARNSHI revealed significant linkage to a critical interval harboring a known deafness gene MYO15A on chromosome 17p13.1‐17q11.2. Subsequent sequencing of the MYO15A gene led to the identification of a novel missense mutation, c.5492G → T (p.Gly1831Val) and a novel splice site mutation, c.8968 − 1G → C. These mutations were not detected in additional 64 unrelated ARNSHI index patients and in 230 Turkish control chromosomes. Gly1831 is a conserved residue located in the motor domains of the different classes of myosins of different species. Molecular modeling of the motor head domain of the human myosin XVa protein suggests that the Gly1831Val mutation inhibits the powerstroke by reducing backbone flexibility and weakening the hydrophobic interactions necessary for signal transmission to the converter domain.

A mutation in the signal sequence of LRP5 in a family with an osteoporosis-pseudoglioma syndrome (OPPG)-like phenotype indicates a novel disease mechanism for trinucleotide repeats.

We extend the spectrum of phenotypes caused by mutations in the Wnt/Norrin coreceptor low‐density lipoprotein receptor‐related protein 5 (LRP5) by identifying two novel types of mutation in related individuals whose presenting features were profound muscle hypotonia, mild mental retardation, blindness, and growth retardation. One mutation removes 6 out of 9 consecutive leucine residues in the LRP5 signal peptide (c.43_60del or p.Leu15_Leu20del), which impairs polypeptide entry into the endoplasmic reticulum (ER), trafficking to the cell membrane, and signal transduction. The second mutation resulted from nonhomologous recombination between Alu repeat sequences, which deleted exons 14–16 and would produce a nonfunctional, truncated, and frameshifted polypeptide, if expressed [chr11:g.(13871447_1387511)_(13879636_13879700)del (NW_925106.1) or p.Pro1010GlnfsX38]. We confirmed that the length of the LRP5 signal peptide poly‐leucine repeat is polymorphic in the general population, and, importantly, we were able to demonstrate in independent in vitro assays that different allele sizes affect receptor processing and signal transduction. Consequently, this polymorphism may have physiologic effects in vivo. This latter finding is relevant since through a genomewide search we identified nearly 400 human proteins that contain poly‐leucine repeats within their signal peptide. We chose 18 of these proteins and genotyped the underlying trinucleotide repeat in healthy Caucasian individuals. More than one length allele was observed in one‐half of the proteins. We therefore propose that natural variation in poly‐leucine‐stretches within signal peptides constitutes a currently unrecognized source of variability in protein translation and expression. Hum Mutat 0, 1–8, 2009.

A new syndrome, congenital extraocular muscle fibrosis with ulnar hand anomalies, maps to chromosome 21qter.

Background: Congenital fibrosis of the extraocular muscles (CFEOM) is a heterogeneous group of disorders that may be associated with other anomalies. The association of a CFEOM syndrome with ulnar hand abnormalities (CFEOM/U) has not been reported to date. Objective: To describe a new autosomal recessive syndrome of CFEOM and ulnar hand abnormalities, and localise the disease causing gene. Methods: Clinical evaluation of the affected members and positional mapping. Results: Six affected patients with CFEOM/U (aged 2 to 29 years) from a large consanguineous Turkish family were studied. Ophthalmological involvement was characterised by non-progressive restrictive ophthalmoplegia with blepharoptosis of the right eye. The postaxial oligodactyly/oligosyndactyly of the hands was more severe on the right side. A genome-wide scan established linkage of this new autosomal recessive syndrome to a locus on chromosome 21qter. The multipoint LOD score was 4.53 at microsatellite marker D21S1259, and fine mapping defined a ∼1.5 Mb critical region between microsatellite marker D21S1897 and the telomere of the long arm. Conclusions: CFEOM/U maps to a 1.5 Mb region at chromosome 21qter. Future identification of the disease causing gene may provide insights into the development of the extraocular muscles and brain stem α motor neurones, as well as anteroposterior limb development.

Molecular genetic screening of MBS1 locus on chromosome 13 for microdeletions and exclusion of FGF9, GSH1 and CDX2 as causative genes in patients with Moebius syndrome.

Moebius syndrome is a rare disorder primarily characterized by congenital facial palsy, frequently accompanied by ocular abduction anomalies, and occasionally associated with orofacial, limb and musculoskeletal malformations. Abnormal development of cranial nerves V through XII underlines the disease pathogenesis. Although some investigations suggested that a causative gene may lie on 13q12.2-q13, there have been no molecular studies targeting possible microdeletions in this region to date. In the present study, we performed microdeletion analyses on 13q12.11-q13 in nine patients, and sequenced three candidate genes in nineteen patients for functional relevance and further resolution of our screening. We ruled out microdeletions on the critical region as a common cause of Moebius syndrome and excluded FGF9, GSH1 and CDX2 genes.

Mutational screening of BASP1 and transcribed processed pseudogene TPΨg-BASP1 in patients with Möbius syndrome

Möbius syndrome is a rare disorder primarily characterized by congenital facial palsy, frequently accompanied by ocular abduction anomalies and occasionally associated with orofacial, limb and musculoskeletal malformations. Abnormal development of cranial nerves V through XII underlines the disease pathogenesis. Although a genetic etiology for Möbius syndrome was proposed, molecular genetic studies to identify the causative gene(s) are scarce. In this study, we selected two candidate genes. One is BASP1 residing in a human chromosome 5p15.1-p15.2, syntenic to mouse chromosome 15qA2-qB2, to which a mouse model with facial nerve anomalies was mapped. The other is transcribed processed pseudogene TPPsig-BASP1, which is located on chromosome 13q flanking the putative locus for Möbius syndrome and might be involved in the regulation of the transcripts encoded by BASP1. Mutation analyses in nineteen patients excluded these genes as being candidates for Möbius syndrome.