Limb-girdle muscular dystrophy type 1D (LGMD1D) was linked to 7q36 over a decade ago1, but its genetic cause has remained elusive. We have studied nine LGMD families from Finland, the U.S., and Italy, and identified four dominant missense mutations leading to p.Phe93Leu or p.Phe89Ile changes in the ubiquitously expressed co-chaperone DNAJB6. Functional testing in vivo showed that the mutations have a dominant toxic effect mediated specifically by the cytoplasmic isoform of DNAJB6. In vitro studies demonstrated that the mutations increase the half-life of DNAJB6, extending this effect to the wild-type protein, and reduce its protective anti-aggregation effect. Further, we show that DNAJB6 interacts with members of the CASA complex, including the myofibrillar-myopathy-causing protein BAG3. Our data provide the genetic cause of LGMD1D, suggest that the pathogenesis is mediated by defective chaperone function, and highlight how mutations expressed ubiquitously can exert their effect in a tissue-, cellular compartment-, and isoform-specific manner.

In previous studies 1-3 % of ALS patients have TARDBP mutations as the cause of the disease. TARDBP mutations have been reported in ALS patients in different populations but so far there are no studies on the frequency of TARDBP mutations in Finnish ALS patients. A cohort of 50 Finnish patients, 44 SALS and 6 FALS patients, were included in the study. Genomic DNA was extracted from venous blood or muscle tissue and a mutation analysis of TARDBP was performed. No definitely pathogenic mutations could be identified in TARDBP in our patient cohort. However, two previously unknown variations were found: one silent mutation in exon 2 and one relatively deep intronic single nucleotide insertion in intron 5. In addition, two previously known non-pathogenic polymorphisms in intron 5 were detected. The size of our cohort is obviously not large enough to conclusively exclude TARDBP mutations as a very rare cause of ALS in Finland. However, based on our results TARDBP mutations do not appear to be a frequent cause of familial or sporadic ALS in Finland.

Myotonic dystrophy (DM) is the most common adult-onset muscular dystrophy with an estimated prevalence of 1/8000. There are two genetically distinct types, DM1 and DM2. DM2 is generally milder with more phenotypic variability than the classic DM1. Our previous data on co-segregation of heterozygous recessive CLCN1 mutations in DM2 patients indicated a higher than expected DM2 prevalence. The aim of this study was to determine the DM2 and DM1 frequency in the general population, and to explore whether the DM2 mutation functions as a modifier in other neuromuscular diseases (NMD) to account for unexplained phenotypic variability. We genotyped 5535 Finnish individuals: 4532 normal blood donors, 606 patients with various non-myotonic NMD, 221 tibial muscular dystrophy patients and their 176 healthy relatives for the DM2 and DM1 mutations. We also genotyped an Italian idiopathic non-myotonic proximal myopathy cohort (n=93) for the DM2 mutation. In 5496 samples analyzed for DM2, we found three DM2 mutations and two premutations. In 5511 samples analyzed for DM1, we found two DM1 mutations and two premutations. In the Italian cohort, we identified one patient with a DM2 mutation. We conclude that the DM2 mutation frequency is significantly higher in the general population (1/1830; P-value=0.0326) than previously estimated. The identification of DM2 mutations in NMD patients with clinical phenotypes not previously associated with DM2 is of particular interest and is in accord with the high overall prevalence. On the basis of our results, DM2 appears more frequent than DM1, with most DM2 patients currently undiagnosed with symptoms frequently occurring in the elderly population.

Myotonic dystrophy types 1 and 2 (DM1 and DM2) are forms of muscular dystrophy that share similar clinical and molecular manifestations, such as myotonia, muscle weakness, cardiac anomalies, cataracts, and the presence of defined RNA-containing foci in muscle nuclei. DM2 is caused by an expansion of the tetranucleotide CCTG repeat within the first intron of ZNF9, although the mechanism by which the expanded nucleotide repeat causes the debilitating symptoms of DM2 is unclear. Conflicting studies have led to two models for the mechanisms leading to the problems associated with DM2. First, a gain-of-function disease model hypothesizes that the repeat expansions in the transcribed RNA do not directly affect ZNF9 function. Instead repeat-containing RNAs are thought to sequester proteins in the nucleus, causing misregulation of normal cellular processes. In the alternative model, the repeat expansions impair ZNF9 function and lead to a decrease in the level of translation. Here we examine the normal in vivo function of ZNF9. We report that ZNF9 associates with actively translating ribosomes and functions as an activator of cap-independent translation of the human ODC mRNA. This activity is mediated by direct binding of ZNF9 to the internal ribosome entry site sequence (IRES) within the 5′UTR of ODC mRNA. ZNF9 can activate IRES-mediated translation of ODC within primary human myoblasts, and this activity is reduced in myoblasts derived from a DM2 patient. These data identify ZNF9 as a regulator of cap-independent translation and indicate that ZNF9 activity may contribute mechanistically to the myotonic dystrophy type 2 phenotype.

Because of its high prevalence, fibromyalgia (FM) is a major general health issue. Myotonic dystrophy type 2 (DM2) is a recently described autosomal-dominant multisystem disorder. Besides variable proximal muscle weakness, myotonia, and precocious cataracts, muscle pain and stiffness are prominent presenting features of DM2. After noting that several of our mutation-positive DM2 patients had a previous diagnosis of FM, suggesting that DM2 may be misdiagnosed as FM, we invited 90 randomly selected patients diagnosed as having FM to undergo genetic testing for DM2. Of the 63 patients who agreed to participate, 2 (3.2%) tested positive for the DM2 mutation. Their cases are described herein. DM2 was not found in any of 200 asymptomatic controls. We therefore suggest that the presence of DM2 should be investigated in a large sample of subjects diagnosed as having FM, and clinicians should be aware of overlap in the clinical presentation of these 2 distinct disorders.