Alternative splicing of exon 17 and a missense mutation in exon 20 of the insulin receptor gene in two brothers with a novel syndrome of insulin resistance (congenital fiber-type disproportion myopathy)

Insulin Receptor Trafficking: Consequences for Insulin Sensitivity and Diabetes

Insulin receptor (INSR) has been extensively studied in the area of cell proliferation and energy metabolism. Impaired INSR activities lead to insulin resistance, the key factor in the pathology of metabolic disorders including type 2 diabetes mellitus (T2DM). The mainstream opinion is that insulin resistance begins at a post-receptor level. The role of INSR activities and trafficking in insulin resistance pathogenesis has been largely ignored. Ligand-activated INSR is internalized and trafficked to early endosome (EE), where INSR is dephosphorylated and sorted. INSR can be subsequently conducted to lysosome for degradation or recycled back to the plasma membrane. The metabolic fate of INSR in cellular events implies the profound influence of INSR on insulin signaling pathways. Disruption of INSR-coupled activities has been identified in a wide range of insulin resistance-related diseases such as T2DM. Accumulating evidence suggests that alterations in INSR trafficking may lead to severe insulin resistance. However, there is very little understanding of how altered INSR activities undermine complex signaling pathways to the development of insulin resistance and T2DM. Here, we focus this review on summarizing previous findings on the molecular pathways of INSR trafficking in normal and diseased states. Through this review, we provide insights into the mechanistic role of INSR intracellular processes and activities in the development of insulin resistance and diabetes.

Novel mutation in insulin receptor gene identified after muscle biopsy in a Niuean woman with severe insulin resistance

BACKGROUND

Severe, early-onset insulin resistance in the absence of obesity, hepatic steatosis and dyslipidaemia is frequently attributable to a genetic defect affecting the insulin receptor. We describe a patient with severe insulin resistance in whom insulin receptor mutation analysis was mistakenly recorded as normal. Western blot analysis of skeletal muscle showed reduced insulin receptor protein and led to re-evaluation of the insulin receptor and the discovery of a novel mutation.

CASE REPORT

A Niuean women, first evaluated at age 6 years for severe acanthosis nigricans, hirsutism, poor growth and cognitive impairment, had extremely elevated fasting insulin levels of 10740 IU/l (fasting reference range 4-24 IU/l) and a normal glucose concentration (4.9 mmol/l). Diabetes was diagnosed at age 9 years and metformin treatment introduced. By age 14 years, she developed refractory hyperglycaemia despite metformin, rosiglitazone and 240 IU insulin daily. Insulin receptor genetic analysis was documented as normal. At age 23 years, with a blood glucose concentration of 37 mmol/l and an HbA1c concentration of 116 mmol/mol, U500 insulin 2000 IU/day was required for glycaemic control. She developed severe proliferative diabetic retinopathy with neovascular glaucoma leading to blindness. There was no renal dysfunction, dyslipidaemia or hepatic steatosis. A muscle biopsy was performed to evaluate the insulin signalling pathway and showed reduced insulin receptor protein. Sequencing of the insulin receptor showed a homozygous p.Val1010Leu missense mutation.

CONCLUSION

This patient illustrates the use of muscle biopsy in the evaluation of a patient with unexplained severe insulin resistance. This approach may usefully be applied to other cases of severe insulin resistance, where genetic testing for known mutations in the insulin signalling pathway has been negative.

Sequencing analysis of insulin receptor defects and detection of two novel mutations in INSR gene

Mutations in the insulin receptor gene cause the inherited insulin resistant syndromes Leprechaunism and Rabson–Mendenhall syndrome. These recessive conditions are characterized by intrauterine and post-natal growth restrictions, dysmorphic features, altered glucose homeostasis, and early demise. The insulin receptor gene (INSR) maps to the short arm of chromosome 19 and is composed of 22 exons. Here we optimize the conditions for sequencing this gene and report novel mutations in patients with severe insulin resistance.

Methods

PCR amplification of the 22 coding exons of the INSR gene was performed using M13-tailed primers. Bidirectional DNA sequencing was performed with BigDye Terminator chemistry and M13 primers and the product was analyzed on the ABI 3100 genetic analyzer. Data analysis was performed using Mutation Surveyor software comparing the sequence to a reference INSR sequence (Genbank NC_000019).

Results

We sequenced four patients with Leprechaunism or Rabson–Mendenhall syndromes as well as seven samples from normal individuals and confirmed previously identified mutations in the affected patients. Three of the four mutations identified in this group caused premature insertion of a stop codon. In addition, the INSR gene was sequenced in 14 clinical samples from patients with suspected insulin resistance and one novel mutation was found in an infant with a suspected diagnosis of Leprechaunism.

Discussion

Leprechaunism and Rabson–Mendenhall syndrome are very rare and difficult to diagnose. Diagnosis is currently based mostly on clinical criteria. Clinical availability of DNA sequencing can provide an objective way of confirming or excluding the diagnosis.

Autosomal dominant congenital fibre type disproportion: a clinicopathological and imaging study of a large family

Congenital fibre type disproportion (CFTD) is considered a non-progressive or slowly progressive muscle disease with relative smallness of type 1 fibres on pathological examination. Although generally benign, CFTD has a variable natural course and severe progression has been observed in some patients. The pathogenesis of the disorder is unknown and many authors consider CFTD a syndrome with multiple aetiologies rather than a separate clinical entity. A positive family history has been reported in about 40% of cases, but the inheritance pattern is not clear. Both autosomal recessive and dominant modes of inheritance have been suggested. The present paper describes a large, multigenerational kindred that has an inherited myopathy fulfilling the histological criteria of CFTD, with autosomal dominant transmission and high penetrance. The clinical picture, remarkably similar in all affected family members, started in early infancy with mild limb muscle weakness. There was slow progression of symptoms into adulthood, with moderate to severe, mainly proximal, muscle weakness without loss of ambulation. Muscle biopsy from two affected individuals demonstrated predominance of small type 1 muscle fibres without other significant findings. Nerve conduction studies were normal and needle electromyography showed a myopathic pattern. MRI examination performed on three patients from successive generations showed involvement of proximal limb and paraspinal muscles. The clinical and pathological homogeneity in the present family, together with the lack of additional histological abnormalities after decades of disease progression in two affected individuals, supports this being a distinct myopathy with fibre type disproportion. Whether the disease in this family can be regarded as a form of the congenital myopathy known as CFTD or rather a unique condition sharing histological features with CFTD needs further investigation. This is, to our knowledge, the largest kindred with muscle fibre type disproportion reported to date. Our data confirm autosomal dominant inheritance, and this is the first MRI document of this disorder.

The floppy infant: contribution of genetic and metabolic disorders

The floppy infant syndrome is a well-recognized entity for pediatricians and neonatologists. The condition refers to an infant with generalized hypotonia presenting at birth or in early life. The diagnostic work up in many instances is often complex, and requires multidisciplinary assessment. Advances in genetics and neurosciences have lead to recognition of newer diagnostic entities (several congenital myopathies), and rapid molecular diagnosis is now possible for several conditions such as spinal muscular atrophy (SMA), congenital muscular dystrophies (CMD), several forms of congenital myopathies and congenital myotonic dystrophy. The focus of the present review is to describe the advances in our understanding in the genetic, metabolic basis of neurological disorders, as well as the investigative work up of the floppy infant. An algorithm for the systematic evaluation of infants with hypotonia is suggested for the practicing pediatrician/neonatologist.

Congenital myopathies and related disorders

PURPOSE OF REVIEW

Considerable progress has been made in molecular genetic research and in identifying the underlying pathogenesis of congenital myopathies, with implications for genetic counseling. Therefore an overview of such advances in the last two years is most timely and relevant for a more precise delineation of these disorders.

RECENT FINDINGS

New mutations have been described on the ryanodine receptor gene, including the carboxyl-terminus region, and experimental models developed to explain their role in central core disease. Phenotype-genotype correlations for nemaline myopathy have improved our understanding of those related to gene mutations. In multi-minicore disease, collaborative studies support genetic heterogeneity and autosomal-recessive inheritance. Research on X-linked myotubular myopathies has revealed a high percentage of mothers of sporadic cases as carriers. Although not initially included within the congenital myopathies, desmin-related or myofibrillar myopathies are described here because they are closely related to other congenital myopathies with intracytoplasmic inclusions. Western blot for myotubularin and desmin has been proposed as a useful diagnostic test for both X-linked myotubular myopathy and desmin-related myopathy, and in-vitro and mouse models for the latter have provided insights into its pathogenesis. Several entities still await genetic characterization. Here we focus on clinical features, inheritance, and molecular genetics.

SUMMARY

Advances in immunohistochemistry and molecular genetics in congenital muscular dystrophies have enriched our knowledge of this heterogeneous group of disorders, leading to more accurate classification and differentiation between the various congenital myopathies.

Rosiglitazone treatment of patients with extreme insulin resistance and diabetes mellitus due to insulin receptor mutations has no effects on glucose and lipid metabolism

BACKGROUND

Rosiglitazone, a thiazolidinedione (TZD), increases insulin sensitivity by reducing levels of plasma NEFA, triglycerides (TG), glucose and serum insulin. Rosiglitazone treatment decreases insulin resistance in type 2 diabetic patients, but no data exist concerning rosiglitazone treatment of patients with syndromes of extreme insulin resistance.

OBJECTIVES

To evaluate whether hyperglycaemia in two lean patients with primary severe insulin resistance due to insulin receptor (IR) mutations and diabetes mellitus could be reduced by supplement of rosiglitazone for 180 days and secondary, to evaluate the effects on plasma NEFA, TG, Apo B, PAI-1 and serum insulin.

SUBJECTS

Both patients (brothers) have known mutations in the IR gene localized to the tyrosine kinase domain and a deletion of exon 17 in part of their IR mRNA. Prior to the study the HbA1c values were higher than 10% in both patients for more than 12 months during treatment with insulin and metformin.

RESULTS

After 180 days of rosiglitazone supplement (8 mg day(-1)), no changes were observed in fasting plasma glucose and HbA1c. Incremental plasma glucose areas under the curves during a 75-g oral glucose tolerance test (OGTT) were unchanged. Likewise, no improvements were seen in either first or second phase insulin secretion during a 0.3 g kg(-1) intravenous glucose tolerance test (IVGTT). Fasting plasma VLDL and HDL cholesterol, TG and Apo B levels were unchanged, whereas a small increase was seen in total and LDL cholesterol levels. Fasting plasma NEFA increased by 51% in KC after 90 days of treatment, and after 180 days plasma NEFA was still 26% higher, when compared with pretreatment levels. In BC an initial 16% decrease was seen in plasma NEFA after 90 days of treatment. Plasma NEFA was increased 14% after 180 days of treatment, when compared with pretreatment levels, but 35% when compared with day 90. Plasma PAI-1 decreased in both patients after 45 and 90 days of treatment but the decrease was only maintained in KC (47%).

CONCLUSIONS

Rosiglitazone treatment, in combination with insulin and metformin, of patients with severe primary insulin resistance due to IR mutations and diabetes mellitus, had no impact on the measured estimates of glucose and lipid metabolism. These findings may suggest that the effect of rosiglitazone on glucose and lipid metabolism are dependent on the presence of intact IR protein.

Congenital myopathies and congenital muscular dystrophies

Congenital myopathies and congenital myopathic dystrophies are distinct groups of inherited diseases of muscle, genetically heterogeneous, that manifest in early life or infancy. Congenital myopathic dystrophy is characterized by a dystrophic pattern, whereas no necrotic or degenerative changes are present in congenital myopathies. Much progress has been made in recent years in clarifying the classification of the congenital myopathies. This is a clinically and genetically heterogeneous group of conditions originally classified according to unique morphological changes seen in muscle. Not unlike the later-onset muscular dystrophies, the discovery of the genetic aetiology of many of the congenital myopathies has led to a revamping of how these conditions can now be diagnosed and this should enable physicians to give a more accurate prognosis to patients and their families. New mutations in the ryanodine receptor, slow tropomyosin, troponin T1, actin, and nebulin genes have been described in the last 2 years. Clinical and genetic guidelines for conditions like nemaline rod myopathy and central core disease have been suggested. The notion of minus and surplus protein myopathies has been developed. Several groups of congenital myopathic dystrophy have been identified. In the first category, without intellectual impairment or major structural brain abnormalities, half of the cases are merosin deficient due to mutations of the laminin alpha 2 chain gene. If generally the muscular phenotype is severe, mild allelic variants have been reported with early onset dystrophies and partial merosin deficiency. Among other pure congenital myopathic dystrophies unlinked to the laminin alpha 2 gene, one form has been assigned to chromosome 1q42. In the group of congenital myopathic dystrophies associated with mental retardation and structural brain abnormalities, two main entities are genetically characterized: (1) Fukuyama congenital myopathic dystrophy, affecting the Japanese population, is due to fukutin gene mutations, and (2) the muscle eye brain syndrome assigned to chromosome 1p32-34. In several cases, the gene localization remains unknown.