A mutation in myotilin causes spheroid body myopathy.

BACKGROUND: Spheroid body myopathy (SBM) is a rare, autosomal dominant, neuromuscular disorder, which has only been previously reported in a single large kindred. Identification of the mutated gene in this disorder may provide insight regarding abnormal neuromuscular function. METHODS: The authors completed a detailed clinical evaluation on an extensive kindred diagnosed with SBM. Genome-wide linkage analysis was performed to localize the disease gene to a specific chromosomal region. Further marker genotyping and screening of a positional, functional candidate gene were completed to detect the disease-causing mutation. Pathologic analysis of muscle biopsy was performed on three individuals. Biochemical studies were performed on one muscle biopsy specimen from an affected individual. RESULTS: Linkage to chromosome 5q23-5q31 was detected with a lod score of 2.9. Genotyping of additional markers in a larger sample of family members produced a maximum lod score of 6.1 and narrowed the critical interval to 12.2 cM. Screening of the candidate gene titin immunoglobulin domain protein (TTID, also known as MYOT) detected a cytosine-to-thymine mutation in exon 2 of all clinically affected family members. Similar pathologic changes were present in all muscle biopsy specimens. Immunohistologic and biochemical analysis revealed that the TTID protein, also known as myotilin, is a component of the insoluble protein aggregate. CONCLUSIONS: A novel mutation in the TTID gene results in the clinical and pathologic phenotype termed "spheroid body myopathy." Mutations in this gene also cause limb-girdle muscular dystrophy 1A and are associated with myofibrillar myopathy.

Hypomorphic phenotype in mice with pituitary-specific knockout of steroidogenic factor 1.

The bacteriophage Cre recombinase provides a powerful approach for tissue-specific gene inactivation. Using a Cre transgene driven by the common alpha subunit of glycoprotein hormones (alphaGSU-Cre), we have previously inactivated steroidogenic factor 1 (SF-1) in the anterior pituitary, causing hypogonadotropic hypogonadism with sexual infantilism, sterility, and severe gonadal hypoplasia. We now explore the molecular mechanisms underlying a hypomorphic gonadal phenotype in mice carrying two floxed SF-1 alleles (F/F) relative to mice carrying one recombined and one floxed allele (F/R). Because their Cre-mediated disruption of the locus encoding SF-1 was less efficient, alphaGSU-Cre, F/F mice retained some gonadotropin-expressing cells in the anterior pituitary, thereby stimulating some gonadal function. This novel in vivo model for exploring the effects of differing levels of gonadotropins on gonadal development highlights the need for careful genotype-phenotype comparisons in studies using Cre recombinase to produce tissue-specific knockouts.

Persistent Prop1 expression delays gonadotrope differentiation and enhances pituitary tumor susceptibility.

The 'paired'-like homeodomain transcription factor Prop1 is essential for the expansion of the pituitary primordia and for the differentiation and/or function of the hormone-producing cells of the anterior pituitary gland. Prop1 expression is normally extinguished before transcription of most differentiation markers is initiated. We report that constitutive expression of Prop1 interferes with anterior pituitary cell differentiation and increases the susceptibility for pituitary tumors. The terminal differentiation of pituitary gonadotropes is delayed, resulting in transient hypogonadism and a delay in the onset of puberty. Thyrotrope differentiation occurs normally, but thyrotrope function is impaired resulting in mild hypothyroidism. Aged mice exhibit defects consistent with misregulation of pituitary cell proliferation, including adenomatous hyperplasia with the formation of Rathke's cleft cysts and tumors. Thus, silencing Prop1 is important for normal pituitary development and function. These data suggest that gain-of-function mutations in PROP1 could contribute to the most common human pituitary endocrinopathies and tumors.

Steroidogenic factor 1 (SF1) is essential for pituitary gonadotrope function.

Knockout mice lacking the orphan nuclear receptor steroidogenic factor 1 (SF1) exhibit a complex endocrine phenotype that includes adrenal and gonadal agenesis, impaired expression of pituitary gonadotropins, and absence of the ventromedial hypothalamic nucleus (VMH). These multiple defects complicate efforts to delineate primary versus secondary effects of SF1 deficiency in different tissues, such that its direct role in gonadotropes remains uncertain. To define this role, we have expressed Cre recombinase driven by the promoter region of the common alpha subunit of glycoprotein hormones (alpha GSU), thereby inactivating a loxP-modified SF1 locus in the anterior pituitary gland. Although pituitary-specific SF1 knockout mice were fully viable, they were sterile and failed to develop normal secondary sexual characteristics. Their adrenal glands and VMH appeared normal histologically, but their testes and ovaries were severely hypoplastic. alpha GSU-Cre, loxP mice had normal levels of most pituitary hormones, but had markedly decreased expression of LH and FSH. Treatment with exogenous gonadotropins stimulated gonadal steroidogenesis, inducing germ cell maturation in males and follicular and uterine maturation in females--establishing that the gonads can respond to gonadotropins. The pituitary-specific SF1 knockout mice are a novel genetic model of hypogonadotropic hypogonadism that establishes essential role(s) of SF1 in pituitary gonadotropes.

Cre-mediated recombination in the pituitary gland.

Organ-specific expression of a cre recombinase transgene allows for the analysis of gene function in a particular tissue or cell type. Using a 4.6 kb promoter from the mouse glycoprotein hormone alpha-subunit (alphaGSU or Cga) gene, we have generated and characterized a line of transgenic mice that express cre recombinase in the anterior and intermediate lobes of the pituitary gland. Utilizing a cre-responsive reporter transgene, alphaGSU-cre transgene expression was detected in the pituitary primordium and in all five cell types of the adult anterior pituitary. alphaGSU-cre transgene activity was also detected in the cardiac and skeletal muscle. Little or no activity was evident in the gonads, adrenal glands, brain, ventromedial hypothalamus, or kidneys. The alphaGSU-cre transgenic mice characterized here will be a valuable tool for examining gene function in the pituitary gland.