Are MYO1C and MYO1F associated with hearing loss?

The role of myosins in the pathogenesis of hearing loss is well established: five genes encoding unconventional myosins and two genes encoding nonmuscle conventional myosins have so far been described to be essential for normal auditory function and mutations in these genes associated with hearing impairment. To better understand the role of this gene family we performed a mutational screening on two candidate genes, MYO1C and MYO1F, analyzing hundreds of patients, affected by bilateral sensorineural hearing loss and coming from different European countries. This research activity led to the identification of 6 heterozygous missense mutations in MYO1C and additional 5 heterozygous missense mutations in MYO1F. Homology modelling suggests that some of these mutations could have a potential influence on the structure of the ATP binding site and could probably affect the ATPase activity or the actin binding process of both myosins. This study suggests a role of the above mentioned myosin genes in the pathogenesis of hearing loss.

The pharmacological chaperone 1-deoxynojirimycin increases the activity and lysosomal trafficking of multiple mutant forms of acid alpha-glucosidase.

Pompe disease is a lysosomal storage disorder (LSD) caused by mutations in the gene that encodes acid alpha-glucosidase (GAA). Recently, small molecule pharmacological chaperones have been shown to increase protein stability and cellular levels for mutant lysosomal enzymes and have emerged as a new therapeutic strategy for the treatment of LSDs. In this study, we characterized the pharmacological chaperone 1-deoxynojirimycin (DNJ) on 76 different mutant forms of GAA identified in Pompe disease. DNJ significantly increased enzyme activity and protein levels for 16 different GAA mutants in patient-derived fibroblasts and in transiently transfected COS-7 cells. Additionally, DNJ increased the processing of these GAA mutants to their mature lysosomal forms, suggesting facilitated trafficking through the secretory pathway. Immunofluorescence microscopy studies showed increased colocalization of GAA with the lysosomal marker LAMP2 after incubation with DNJ, confirming increased lysosomal trafficking. Lastly, a GAA structural model was constructed based on the related eukaryotic glucosidase maltase-glucoamylase. The mutated residues identified in responsive forms of GAA are located throughout most of the structural domains, with half of these residues located in two short regions within the catalytic domain. Taken together, these data support further evaluation of DNJ as a potential treatment for Pompe disease in patients that express responsive forms of GAA.

Pharmacological Enhancement of Mutated α-Glucosidase Activity in Fibroblasts from Patients with Pompe Disease.

We investigated the use of pharmacological chaperones for the therapy of Pompe disease, a metabolic myopathy due to mutations of the gene encoding the lysosomal hydrolase α-glucosidase (GAA) and characterized by generalized glycogen storage in cardiac and skeletal muscle. We studied the effects of two imino sugars, deoxynojirimycin (DNJ) and N-butyldeoxynojirimycin (NB-DNJ), on residual GAA activity in fibroblasts from eight patients with different forms of Pompe disease (two classic infantile, two non-classic infantile onset, four late-onset forms), and with different mutations of the GAA gene. We demonstrated a significant increase of GAA activity (1.3-7.5-fold) after imino sugar treatment in fibroblasts from patients carrying the mutations L552P (three patients) and G549R (one patient). GAA enhancement was confirmed in HEK293T cells where the same mutations were overexpressed. No increase of GAA activity was observed for the other mutations. Western blot analysis showed that imino sugars increase the amount of mature GAA molecular forms. Immunofluorescence studies in HEK293T cells overexpressing the L552P mutation showed an improved trafficking of the mutant enzyme to lysosomes after imino sugar treatment. These results provide a rationale for an alternative treatment, other than enzyme replacement, to Pompe disease.

Pharmacological enhancement of mutated alpha-glucosidase activity in fibroblasts from patients with Pompe disease.

We investigated the use of pharmacological chaperones for the therapy of Pompe disease, a metabolic myopathy due to mutations of the gene encoding the lysosomal hydrolase alpha-glucosidase (GAA) and characterized by generalized glycogen storage in cardiac and skeletal muscle. We studied the effects of two imino sugars, deoxynojirimycin (DNJ) and N-butyldeoxynojirimycin (NB-DNJ), on residual GAA activity in fibroblasts from eight patients with different forms of Pompe disease (two classic infantile, two non-classic infantile onset, four late-onset forms), and with different mutations of the GAA gene. We demonstrated a significant increase of GAA activity (1.3-7.5-fold) after imino sugar treatment in fibroblasts from patients carrying the mutations L552P (three patients) and G549R (one patient). GAA enhancement was confirmed in HEK293T cells where the same mutations were overexpressed. No increase of GAA activity was observed for the other mutations. Western blot analysis showed that imino sugars increase the amount of mature GAA molecular forms. Immunofluorescence studies in HEK293T cells overexpressing the L552P mutation showed an improved trafficking of the mutant enzyme to lysosomes after imino sugar treatment. These results provide a rationale for an alternative treatment, other than enzyme replacement, to Pompe disease.

A new locus (DFNA47) for autosomal dominant non-syndromic inherited hearing loss maps to 9p21-22 in a large Italian family.

Hearing loss is the most common sensory disorder in humans, and genetic factors are a major cause. Approximately 15-20% of genetic cases exhibit an autosomal dominant pattern of transmission. So far, 41 autosomal dominant loci have been mapped and 17 genes have been identified. Here we report the mapping of a novel locus for autosomal dominant non-syndromic hearing loss, DFNA47, to chromosome 9p21-22 in a large multigenerational Italian family with progressive hearing impairment. Most affected individuals noticed hearing impairment after their teens with subsequent gradual progression to a moderate-severe loss. There were no obvious vestibular dysfunction and other associated abnormalities. A maximum lod score of 3.14 was obtained with marker D9S157 (at theta=0) after a genome wide search. The study of additional markers allowed us to confirm this region with positive lod scores of 3.58 (at theta=0 from D9S285) and of 3.67 (at theta=0 from D9S162). Recombinants define a region of approximately 9 cM flanked by markers D9S268 and D9S942. Multipoint linkage analysis showed a Lod score of 4.26. Few known genes map to the region, and those possibly related by function to hearing are being screened for disease-causing mutations.

A gene network regulating lysosomal biogenesis and function.

Lysosomes are organelles central to degradation and recycling processes in animal cells. Whether lysosomal activity is coordinated to respond to cellular needs remains unclear. We found that most lysosomal genes exhibit coordinated transcriptional behavior and are regulated by the transcription factor EB (TFEB). Under aberrant lysosomal storage conditions, TFEB translocated from the cytoplasm to the nucleus, resulting in the activation of its target genes. TFEB overexpression in cultured cells induced lysosomal biogenesis and increased the degradation of complex molecules, such as glycosaminoglycans and the pathogenic protein that causes Huntington's disease. Thus, a genetic program controls lysosomal biogenesis and function, providing a potential therapeutic target to enhance cellular clearing in lysosomal storage disorders and neurodegenerative diseases.

Abnormal mannose-6-phosphate receptor trafficking impairs recombinant alpha-glucosidase uptake in Pompe disease fibroblasts.

BACKGROUND: Pompe disease (PD) is a metabolic myopathy caused by alpha-glucosidase (GAA) deficiency and characterized by generalized glycogen storage. Heterogeneous GAA gene mutations result in wide phenotypic variability, ranging from the severe classic infantile presentation to the milder intermediate and late-onset forms. Enzyme replacement therapy (ERT) with recombinant human GAA (rhGAA), the only treatment available for PD, intriguingly shows variable efficacy in different PD patients. To investigate the mechanisms underlying the variable response to ERT, we studied cell morphology of PD fibroblasts, the distribution and trafficking of the cation-independent mannose-6-phosphate receptor (CI-MPR) that mediates rhGAA uptake, and rhGAA uptake itself. RESULTS: We observed abnormalities of cell morphology in PD cells. Electron microscopy analysis showed accumulation of multivesicular bodies and expansion of the Golgi apparatus, and immunolocalization and western blot analysis of LC3 showed activation of autophagy. Immunofluorescence analysis showed abnormal intracellular distribution of CI-MPR in PD fibroblasts, increased co-localization with LC3 and reduced availability of the receptor at the plasma membrane. The recycling of CI-MPR from the plasma membrane to the trans-Golgi network was also impaired. All these abnormalities were more prominent in severe and intermediate PD fibroblasts, correlating with disease severity. In severe and intermediate PD cells rhGAA uptake and processing were less efficient and correction of GAA activity was reduced. CONCLUSION: These results indicate a role for disrupted CI-MPR trafficking in the variable response to ERT in PD and have implications for ERT efficacy and optimization of treatment protocols.

Identification of a novel mutation in the myosin VIIA motor domain in a family with autosomal dominant hearing loss (DFNA11).

We ascertained a large Italian family with an autosomal dominant form of non-syndromic sensorineural hearing loss with vestibular involvement. A genome-wide scan found linkage to locus DFNA11. Sequencing of the MYO7A gene in the linked region identified a new missense mutation resulting in an Ala230Val change in the motor domain of the myosin VIIA. Myosin VIIA has already been implicated in several forms of deafness, but this is the third mutation causing a dominant form of deafness, located in the myosin VIIA motor domain in a region never involved in hearing loss until now. A modelled protein structure of myosin VII motor domain provides evidence for a significant functional effect of this missense mutation.

Multiple mutations of MYO1A, a cochlear-expressed gene, in sensorineural hearing loss.

Myosin I isozymes have been implicated in various motile processes, including organelle translocation, ion-channel gating, and cytoskeleton reorganization. Unconventional myosins were among the first family of proteins found to be associated with hearing loss in both humans and mice. Here, we report the identification of a nonsense mutation, of a trinucleotide insertion leading to an addition of an amino acid, and of six missense mutations in MYO1A cDNA sequence in a group of hearing-impaired patients from Italy. MYO1A, which is located within the DFNA48 locus, is the first myosin I family member found to be involved in causing deafness and may be a major contributor to autosomal dominant-hearing loss.

Nonmuscle myosin heavy-chain gene MYH14 is expressed in cochlea and mutated in patients affected by autosomal dominant hearing impairment (DFNA4).

Myosins have been implicated in various motile processes, including organelle translocation, ion-channel gating, and cytoskeleton reorganization. Different members of the myosin superfamily are responsible for syndromic and nonsyndromic hearing impairment in both humans and mice. MYH14 encodes one of the heavy chains of the class II nonmuscle myosins, and it is localized within the autosomal dominant hearing impairment (DFNA4) critical region. After demonstrating that MYH14 is highly expressed in mouse cochlea, we performed a mutational screening in a large series of 300 hearing-impaired patients from Italy, Spain, and Belgium and in a German kindred linked to DFNA4. This study allowed us to identify a nonsense and two missense mutations in large pedigrees, linked to DFNA4, as well as a de novo allele in a sporadic case. Absence of these mutations in healthy individuals was tested in 200 control individuals. These findings clearly demonstrate the role of MYH14 in causing autosomal dominant hearing loss and further confirm the crucial role of the myosin superfamily in auditive functions.