Angela Huebner

Mutations in MRAP, encoding a new interacting partner of the ACTH receptor, cause familial glucocorticoid deficiency type 2

Familial glucocorticoid deficiency (FGD), or hereditary unresponsiveness to adrenocorticotropin (ACTH; OMIM 202200), is an autosomal recessive disorder resulting from resistance to the action of ACTH on the adrenal cortex, which stimulates glucocorticoid production. Affected individuals are deficient in cortisol and, if untreated, are likely to succumb to hypoglycemia or overwhelming infection in infancy or childhood. Mutations of the ACTH receptor (melanocortin 2 receptor, MC2R) account for ∼25% of cases of FGD. FGD without mutations of MC2R is called FGD type 2. Using SNP array genotyping, we mapped a locus involved in FGD type 2 to chromosome 21q22.1. We identified mutations in a gene encoding a 19-kDa single–transmembrane domain protein, now known as melanocortin 2 receptor accessory protein (MRAP). We show that MRAP interacts with MC2R and may have a role in the trafficking of MC2R from the endoplasmic reticulum to the cell surface.

A Mutation in the Dimerization Domain of Filamin C Causes a Novel Type of Autosomal Dominant Myofibrillar Myopathy

Myofibrillar myopathy (MFM) is a human disease that is characterized by focal myofibrillar destruction and pathological cytoplasmic protein aggregations. In an extended German pedigree with a novel form of MFM characterized by clinical features of a limb-girdle myopathy and morphological features of MFM, we identified a co-segregating, heterozygous nonsense mutation (8130G-->A; W2710X) in the filamin c gene (FLNC) on chromosome 7q32.1. The mutation is the first found in FLNC and is localized in the dimerization domain of filamin c. Functional studies showed that, in the truncated mutant protein, this domain has a disturbed secondary structure that leads to the inability to dimerize properly. As a consequence of this malfunction, the muscle fibers of our patients display massive cytoplasmic aggregates containing filamin c and several Z-disk-associated and sarcolemmal proteins.

A founder mutation in Anoctamin 5 is a major cause of limb girdle muscular dystrophy

The limb-girdle muscular dystrophies are a group of disorders with wide genetic and clinical heterogeneity. Recently, mutations in the ANO5 gene, which encodes a putative calcium-activated chloride channel belonging to the Anoctamin family of proteins, were identified in five families with one of two previously identified disorders, limb-girdle muscular dystrophy 2L and non-dysferlin Miyoshi muscular dystrophy. We screened a candidate group of 64 patients from 59 British and German kindreds and found the truncating mutation, c.191dupA in exon 5 of ANO5 in 20 patients, homozygously in 15 and in compound heterozygosity with other ANO5 variants in the rest. An intragenic single nucleotide polymorphism and an extragenic microsatellite marker are in linkage disequilibrium with the mutation, suggesting a founder effect in the Northern European population. We have further defined the clinical phenotype of ANO5-associated muscular dystrophy. Patients show adult onset proximal lower limb weakness with highly raised serum creatine kinase values (average 4500 IU/l) and frequent muscle atrophy and asymmetry of muscle involvement. Onset varies from the early 20 s to 50 s and the weakness is generally slowly progressive, with most patients remaining ambulant for several decades. Distal presentation is much less common but a milder degree of distal lower limb weakness is often observed. Upper limb strength is only mildly affected and cardiac and respiratory function is normal. Females appear less frequently affected. In the North of England population we have identified eight patients with ANO5 mutations, suggesting a minimum prevalence of 0.27/100,000, twice as common as dysferlinopathy. We suggest that mutations in ANO5 represent a relatively common cause of adult onset muscular dystrophy with high serum creatine kinase and that mutation screening, particularly of the common mutation c.191dupA, should be an early step in the diagnostic algorithm of adult limb-girdle muscular dystrophy patients.

Distinct muscle imaging patterns in myofibrillar myopathies

Objective: To compare muscle imaging findings in different subtypes of myofibrillar myopathies (MFM) in order to identify characteristic patterns of muscle alterations that may be helpful to separate these genetic heterogeneous muscular disorders. Methods: Muscle imaging and clinical findings of 46 patients with MFM were evaluated (19 desminopathy, 12 myotilinopathy, 11 filaminopathy, 1 αB-crystallinopathy, and 3 ZASPopathy). The data were collected retrospectively in 43 patients and prospectively in 3 patients. Results: In patients with desminopathy, the semitendinosus was at least equally affected as the biceps femoris, and the peroneal muscles were never less involved than the tibialis anterior (sensitivity of these imaging criteria to detect desminopathy in our cohort 100%, specificity 95%). In most of the patients with myotilinopathy, the adductor magnus showed more alterations than the gracilis muscle, and the sartorius was at least equally affected as the semitendinosus (sensitivity 90%, specificity 93%). In filaminopathy, the biceps femoris and semitendinosus were at least equally affected as the sartorius muscle, and the medial gastrocnemius was more affected than the lateral gastrocnemius. The semimembranosus mostly showed more alterations than the adductor magnus (sensitivity 88%, specificity 96%). Early adult onset and cardiac involvement was most often associated with desminopathy. In patients with filaminopathy, muscle weakness typically beginning in the 5th decade of life was mostly pronounced proximally, while late adult onset (>50 years) with distal weakness was more often present in myotilinopathy. Conclusions: Muscle imaging in combination with clinical data may be helpful for separation of distinct myofibrillar myopathy subtypes and in scheduling of genetic analysis.

Nonclassic lipoid congenital adrenal hyperplasia masquerading as familial glucocorticoid deficiency.

CONTEXT Familial glucocorticoid deficiency (FGD) is an autosomal recessive disorder resulting from resistance to the action of ACTH on the adrenal cortex. Affected individuals are deficient in cortisol and, if untreated, are likely to succumb to hypoglycemia and/or overwhelming infection. Mutations of the ACTH receptor (MC2R) and the melanocortin 2 receptor accessory protein (MRAP), FGD types 1 and 2 respectively, account for approximately 45% of cases. OBJECTIVE A locus on chromosome 8 has previously been linked to the disease in three families, but no underlying gene defect has to date been identified. DESIGN The study design comprised single-nucleotide polymorphism genotyping and mutation detection. SETTING The study was conducted at secondary and tertiary referral centers. PATIENTS Eighty probands from families referred for investigation of the genetic cause of FGD participated in the study. INTERVENTIONS There were no interventions. RESULTS Analysis by single-nucleotide polymorphism array of the genotype of one individual with FGD previously linked to chromosome 8 revealed a large region of homozygosity encompassing the steroidogenic acute regulatory protein gene, STAR. We identified homozygous STAR mutations in this patient and his affected siblings. Screening of our total FGD patient cohort revealed homozygous STAR mutations in a further nine individuals from four other families. CONCLUSIONS Mutations in STAR usually cause lipoid congenital adrenal hyperplasia, a disorder characterized by both gonadal and adrenal steroid deficiency. Our results demonstrate that certain mutations in STAR (R192C and the previously reported R188C) can present with a phenotype indistinguishable from that seen in FGD.

Missense mutations of ACTA1 cause dominant congenital myopathy with cores

ongenital myopathies (CM) are neuromuscular disorders classified by characteristic histopathological findings in muscle fibers. Areas devoid of oxidative enzyme activity (core lesions) are pathological hallmarks of autosomal dominant or recessive central core disease (CCD; MIM 117000) and multiminicore disease (MmD; MIM 255320). While large and solitary cores in the center and along the entire length of muscle fibers are considered typical for CCD and multiple smaller cores within muscle fibers define MmD, this classic histological distinction is complicated by the marked histological variability of core lesions. 1 Minicores and central cores have been detected concomitantly as well as separately in successive muscle biopsy specimens of single patients and in myofibers of different affected family members. 1–4 So far, mutations in

Rapsyn N88K is a frequent cause of congenital myasthenic syndromes in European patients

Background: Mutations in various genes of the neuromuscular junction may cause congenital myasthenic syndromes (CMS). Most mutations identified to date affect the ε-subunit gene of the acetylcholine receptor (AChR), leading to end-plate AChR deficiency. Recently, three different mutations in the RAPSN gene have been identified in four CMS patients with AChR deficiency. Objective: To perform mutation analysis of the RAPSN gene in patients with sporadic or autosomal recessive CMS. Methods: One hundred twenty CMS patients from 110 unrelated families were analyzed for the RAPSN mutation N88K by restriction fragment length polymorphism and sequence analysis. Results: In 12 CMS patients from 10 independent families, RAPSN N88K was identified either homozygous or heteroallelic to another missense mutation. Symptoms usually started perinatally or in the first years of life. However, one patient did not show any myasthenic symptoms before the third decade. Clinical symptoms typically included bilateral ptosis, weakness of facial, bulbar, and limb muscles, and a favorable response to anticholinesterase treatment. Crisis-like exacerbations with respiratory insufficiency provoked by stress, fever, or infections in early childhood were frequent. All RAPSN N88K families originate from Central or Western European countries. Genotype analysis indicated that they derive from a common ancestor (founder). Conclusions: The RAPSN mutation N88K is a frequent cause of rapsyn-related CMS in European patients. In general, patients (RAPSN N88K) were characterized by mild to moderate myasthenic symptoms with favorable response to anticholinesterase treatment. However, severity and onset of symptoms may vary to a great extent.

Triple A syndrome: genotype–phenotype assessment

The triple A or Allgrove syndrome is an autosomal‐recessive disease (MIM*231550) characterized by the triad of achalasia, alacrima and adrenocorticotropic hormone (ACTH)‐resistant adrenal insufficiency. Associated features of the syndrome are neurological and dermatological abnormalities. Until the discovery of the AAAS gene as the responsible gene in triple A syndrome, the diagnosis was based on characteristic clinical features. Here we present the clinical and molecular genetic data which demonstrated the marked phenotypic variability in three unrelated patients with triple A syndrome. The final diagnosis of triple A syndrome was confirmed by molecular analysis. In one patient with isolated achalasia, the diagnosis of triple A syndrome could only be made on the basis of the molecular genetic analysis of the AAAS gene. We therefore suggest that the diagnosis of triple A syndrome should be considered in patients who exhibit only one or two of the main symptoms (i.e. alacrima, achalasia or adrenal insufficiency). These patients require careful neurological investigation, and mutation analysis of the AAAS gene should be performed.

LPIN1 gene mutations: a major cause of severe rhabdomyolysis in early childhood.

Autosomal recessive LPIN1mutations have been recently described as a novel cause of rhabdomyolysis in a few families. The purpose of the study was to evaluate the prevalence of LPIN1mutations in patients exhibiting severe episodes of rhabdomyolysis in infancy. After exclusion of primary fatty acid oxidation disorders, LPIN1 coding sequence was determined in genomic DNA and cDNA. Among the 29 patients studied, 17 (59%) carried recessive nonsense or frameshift mutations, or a large scale intragenic deletion. In these 17 patients, episodes of rhabdomyolysis occurred at a mean age of 21 months. Secondary defect of mitochondrial fatty oxidation or respiratory chain was found in skeletal muscle of two patients. The intragenic deletion, c.2295‐866_2410‐30del, was identified in 8/17 patients (47%), all Caucasians, and occurred on the background of a common haplotype, suggesting a founder effect. This deleted human LPIN1 form was unable to complement Δpah1 yeast for growth on glycerol, in contrast to normal LPIN1. Since more than 50% of our series harboured LPIN1mutations, LPIN1 should be regarded as a major cause of severe myoglobinuria in early childhood. The high frequency of the intragenic LPIN1deletion should provide a valuable criterion for fast diagnosis, prior to muscle biopsy.

Inherited ACTH insensitivity illuminates the mechanisms of ACTH action

Adrenocorticotrophin (ACTH) insensitivity is a potentially lethal inherited disorder of ACTH signalling in the adrenal. Inactivating mutations of the ACTH receptor account for approximately 25% of these cases. A second genetic cause for this syndrome has recently been identified in the MRAP gene. The MRAP protein appears to function in the trafficking and cell surface expression of the ACTH receptor, and might indicate the existence of more widespread G-protein-coupled receptor trafficking mechanisms. Molecular defects underlying other causes of ACTH insensitivity syndromes will probably contribute further to our understanding of these pathways.