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.
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), Δ GJB6 –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 Δ GJB6 –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.
In two large Turkish consanguineous families, a locus for autosomal recessive nonsyndromic hearing loss (ARNSHL) was mapped to chromosome 6p21.3 by genome-wide linkage analysis in an interval overlapping with the loci DFNB53 (COL11A2), DFNB66, and DFNB67. Fine mapping excluded DFNB53 and subsequently homozygous mutations were identified in the lipoma HMGIC fusion partner-like 5 (LHFPL5) gene, also named tetraspan membrane protein of hair cell stereocilia (TMHS) gene, which was recently shown to be mutated in the "hurry scurry" mouse and in two DFNB67-linked families from Pakistan. In one family, we found a homozygous one-base pair deletion, c.649delG (p.Glu216ArgfsX26) and in the other family we identified a homozygous transition c.494C>T (p.Thr165Met). Further screening of index patients from 96 Turkish ARNSHL families and 90 Dutch ARNSHL patients identified one additional Turkish family carrying the c.649delG mutation. Haplotype analysis revealed that the c.649delG mutation was located on a common haplotype in both families. Mutation screening of the LHFPL5 homologs LHFPL3 and LHFPL4 did not reveal any disease causing mutation. Our findings indicate that LHFPL5 is essential for normal function of the human cochlea.
Ataxia with vitamin E deficiency (AVED) is a rare autosomal recessive disorder, usually with a phenotype resembling Friedreich ataxia, caused by selective impairment of gastrointestinal vitamin E absorption. Vitamin E supplementation improves symptoms and prevents disease progress. In North Africa and Southern Europe, AVED is as common as Friedreich ataxia. There are no reported cases from Turkey. We herein report a 16-year-old Turkish girl with AVED, who was found to have total deletion of the TTPA gene as well as sensorineural deafness, and we present her follow-up data after vitamin E therapy.
