Reduced expression of fukutin related protein in mice results in a model for fukutin related protein associated muscular dystrophies.

Mutations in fukutin related protein (FKRP) are responsible for a common group of muscular dystrophies ranging from adult onset limb girdle muscular dystrophies to severe congenital forms with associated structural brain involvement, including Muscle Eye Brain disease. A common feature of these disorders is the variable reduction in the glycosylation of skeletal muscle alpha-dystroglycan. In order to gain insight into the pathogenesis and clinical variability, we have generated two lines of mice, the first containing a missense mutation and a neomycin cassette, FKRP-Neo(Tyr307Asn) and the second containing the FKRP(Tyr307Asn) mutation alone. We have previously associated this missense mutation with a severe muscle-eye-brain phenotype in several families. Homozygote Fkrp-Neo(Tyr307Asn) mice die soon after birth and show a reduction in the laminin-binding epitope of alpha-dystroglycan in muscle, eye and brain, and have reduced levels of FKRP transcript. Homozygous Fkrp(Tyr307Asn) mice showed no discernible phenotype up to 6 months of age, contrary to the severe clinical course observed in patients with the same mutation. These results suggest the generation of a mouse model for FKRP related muscular dystrophy requires a knock-down rather than a knock-in strategy in order to give rise to a disease phenotype.

Muscular dystrophies due to defective glycosylation of dystroglycan.

Muscular dystrophies are a clinically and genetically heterogeneous group of disorders. Until recently most of the proteins associated with muscular dystrophies were believed to be proteins of the sarcolemma associated with reinforcing the plasma membrane or in facilitating its re-sealing following injury. In the last few years a novel and frequent pathogenic mechanism has been identified that involves the abnormal glycosylation of alpha-dystroglycan (ADG). This peripheral membrane protein undergoes complex and crucial glycosylation steps that enable it to interact with LG domain containing extracellular matrix proteins such as laminins, agrin and perlecan. Mutations in six genes (POMT1, POMT2, POMGnT1, fukutin, FKRP and LARGE) have been identified in patients with reduced glycosylation of ADG. While initially a clear correlation between gene defect and phenotype was observed for each of these 6 genes (for example, Walker Warburg syndrome was associated with mutations in POMT1 and POMT2, Fukuyama congenital muscular dystrophy associated with fukutin mutations, and Muscle Eye Brain disease associated with POMGnT1 mutations), we have recently demonstrated that allelic mutations in each of these 6 genes can result in a much wider spectrum of clinical conditions. Thus, the crucial aspect in determining the phenotypic severity is not which gene is primarily mutated, but how severely the mutation affects the glycosylation of ADG. Systematic mutation analysis of these 6 glycosyltransferases in patients with a dystroglycan glycosylation disorder identifies mutations in approximately 65% suggesting that more genes have yet to be identified.

The identification of circular extrachromosomal DNA in the nuclear genome of Trypanosoma brucei.

Nuclear extrachromosomal DNA elements have been identified in several kinetoplastids such as Leishmania and Trypanosoma cruzi, but never in Trypanosoma brucei. They can occur naturally or arise spontaneously as the result of sublethal drug exposure of parasites. In most cases, they are represented as circular elements and are mitotically unstable. In this study we describe the presence of circular DNA in the nucleus of Trypanosoma brucei. This novel type of DNA was termed NR-element (NlaIII repeat element). In contrast to drug-induced episomes in other kinetoplastids, the T. brucei extrachromosomal NR-element is not generated by drug selection. Furthermore, the element is stable during mitosis over many generations. Restriction analysis of tagged NR-element DNA, unusual migration patterns during pulsed field gel electrophoresis (PFGE) and CsCl/ethidium bromide equilibrium centrifugation demonstrates that the NR-element represents circular DNA. Whereas it has been found in all field isolates of the parasites we analysed, it is not detectable in some laboratory strains notably the genome reference strain 927. The DNA sequence of this element is related to a 29 bp repeat present in the subtelomeric region of VSG-bearing chromosomes of T. brucei. It has been suggested that this subtelomeric region is part of a transition zone on chromosomes separating the relatively stable telomeric repeats from the recombinationaly active region downstream of VSG genes. Therefore, we discuss a functional connection between the occurrence of this circular DNA and subtelomeric recombination events in T. brucei.