Studies of hereditary cancer syndromes have contributed greatly to our understanding of molecular events involved in tumorigenesis. Here we investigate the molecular background of the Peutz-Jeghers syndrome (PJS), a rare hereditary disease in which there is predisposition to benign and malignant tumours of many organ systems. A locus for this condition was recently assigned to chromosome 19p. We have identified truncating germline mutations in a gene residing on chromosome 19p in multiple individuals affected by PJS. This previously identified but unmapped gene, LKB1, has strong homology to a cytoplasmic Xenopus serine/threonine protein kinase XEEK1, and weaker similarity to many other protein kinases. Peutz-Jeghers syndrome is therefore the first cancer-susceptibility syndrome to be identified that is due to inactivating mutations in a protein kinase.

No abstract

The recessively inherited developmental disorder, cartilage-hair hypoplasia (CHH) is highly pleiotropic with manifestations including short stature, defective cellular immunity, and predisposition to several cancers. The endoribonuclease RNase MRP consists of an RNA molecule bound to several proteins. It has at least two functions, namely, cleavage of RNA in mitochondrial DNA synthesis and nucleolar cleaving of pre-rRNA. We describe numerous mutations in the untranslated RMRP gene that cosegregate with the CHH phenotype. Insertion mutations immediately upstream of the coding sequence silence transcription while mutations in the transcribed region do not. The association of protein subunits with RNA appears unaltered. We conclude that mutations in RMRP cause CHH by disrupting a function of RNase MRP RNA that affects multiple organ systems.

Muscle contraction results from the force generated between the thin filament protein actin and the thick filament protein myosin, which causes the thick and thin muscle filaments to slide past each other. There are skeletal muscle, cardiac muscle, smooth muscle and non-muscle isoforms of both actin and myosin. Inherited diseases in humans have been associated with defects in cardiac actin (dilated cardiomyopathy and hypertrophic cardiomyopathy), cardiac myosin (hypertrophic cardiomyopathy) and non-muscle myosin (deafness). Here we report that mutations in the human skeletal muscle alpha-actin gene (ACTA1) are associated with two different muscle diseases, 'congenital myopathy with excess of thin myofilaments' (actin myopathy) and nemaline myopathy. Both diseases are characterized by structural abnormalities of the muscle fibres and variable degrees of muscle weakness. We have identified 15 different missense mutations resulting in 14 different amino acid changes. The missense mutations in ACTA1 are distributed throughout all six coding exons, and some involve known functional domains of actin. Approximately half of the patients died within their first year, but two female patients have survived into their thirties and have children. We identified dominant mutations in all but 1 of 14 families, with the missense mutations being single and heterozygous. The only family showing dominant inheritance comprised a 33-year-old affected mother and her two affected and two unaffected children. In another family, the clinically unaffected father is a somatic mosaic for the mutation seen in both of his affected children. We identified recessive mutations in one family in which the two affected siblings had heterozygous mutations in two different exons, one paternally and the other maternally inherited. We also identified de novo mutations in seven sporadic probands for which it was possible to analyse parental DNA.

Over the past decade there have been major advances in defining the genetic basis of the majority of congenital myopathy subtypes. However the relationship between each congenital myopathy, defined on histological grounds, and the genetic cause is complex. Many of the congenital myopathies are due to mutations in more than one gene, and mutations in the same gene can cause different muscle pathologies. The International Standard of Care Committee for Congenital Myopathies performed a literature review and consulted a group of experts in the field to develop a summary of (1) the key features common to all forms of congenital myopathy and (2) the specific features that help to discriminate between the different genetic subtypes. The consensus statement was refined by two rounds of on-line survey, and a three-day workshop. This consensus statement provides guidelines to the physician assessing the infant or child with hypotonia and weakness. We summarise the clinical features that are most suggestive of a congenital myopathy, the major differential diagnoses and the features on clinical examination, investigations, muscle pathology and muscle imaging that are suggestive of a specific genetic diagnosis to assist in prioritisation of genetic testing of known genes. As next generation sequencing becomes increasingly used as a diagnostic tool in clinical practise, these guidelines will assist in determining which sequence variations are likely to be pathogenic.

Nemaline myopathy (NM) is a common form of congenital myopathy, affecting approximately 1 in 50,000 individuals, and is defined by the presence of nonprogressive generalized muscle weakness and numerous electron-dense protein inclusions (nemaline bodies or rods) in skeletal myofibers (1). The most severely affected Nemaline myopathy (NM) is a genetic muscle disorder characterized by muscle dysfunction and electron-dense protein accumulations (nemaline bodies) in myofibers. Pathogenic mutations have been described in 9 genes to date, but the genetic basis remains unknown in many cases. Here, using an approach that combined whole-exome sequencing (WES) and Sanger sequencing, we identified homozygous or compound heterozygous variants in LMOD3 in 21 patients from 14 families with severe, usually lethal, NM. LMOD3 encodes leiomodin-3 (LMOD3), a 65-kDa protein expressed in skeletal and cardiac muscle. LMOD3 was expressed from early stages of muscle differentiation; localized to actin thin filaments, with enrichment near the pointed ends; and had strong actin filament-nucleating activity. Loss of LMOD3 in patient muscle resulted in shortening and disorganization of thin filaments. Knockdown of lmod3 in zebrafish replicated NM-associated functional and pathological phenotypes. Together, these findings indicate that mutations in the gene encoding LMOD3 underlie congenital myopathy and demonstrate that LMOD3 is essential for the organization of sarcomeric thin filaments in skeletal muscle.

The congenital nemaline myopathies are rare hereditary muscle disorders characterized by the presence in the muscle fibers of nemaline bodies consisting of proteins derived from the Z disc and thin filament. In a single large Australian family with an autosomal dominant form of nemaline myopathy, the disease is caused by a mutation in the ␣-tropomyosin gene TPM3. The typical form of nemaline myopathy is inherited as an autosomal recessive trait, the locus of which we previously assigned to chromosome 2q21.2-q22. We show here that mutations in the nebulin gene located within this region are associated with the disease. The nebulin protein is a giant protein found in the thin filaments of striated muscle. A variety of nebulin isoforms are thought to contribute to the molecular diversity of Z discs. We have studied the 3 end of the 20.8-kb cDNA encoding the Z disc part of the 800-kDa protein and describe six diseaseassociated mutations in patients from five families of different ethnic origins. In two families with consanguineous parents, the patients were homozygous for point mutations. In one family with nonconsanguineous parents, the affected siblings were compound heterozygotes for two different mutations, and in two further families with one detected mutation each, haplotypes are compatible with compound heterozygosity. Immunofluorescence studies with antibodies specific to the C-terminal region of nebulin indicate that the mutations may cause protein truncation possibly associated with loss of fiber-type diversity, which may be relevant to disease pathogenesis.

No abstract