Exclusion of biglycan mutations in a cohort of patients with neuromuscular disorders

Abstract

Biglycan has been considered a good candidate for neuromuscular disease based on direct interactions with collagen VI and α-dystroglycan, both of which are linked with congenital muscular dystrophy (CMD). We screened 83 patients with CMD and other neuromuscular disorders and six controls for mutations and variations in the biglycan sequence. We identified a number of novel sequence variations. After family analysis and control screening we found that none of these polymorphisms were disease-causing mutations. Thus mutations in biglycan are not a common cause of neuromuscular disorders in our cohort.

Introduction

Congenital muscular dystrophy (CMD) is characterized by muscle weakness at birth or in the first few months of life. We have recently screened two large Australasian cohorts of CMD and limb girdle muscular dystrophy (LGMD) patients for known genetic causes. Despite recent diagnostic advances, a definite diagnosis was reached in only 24% of CMD patients [1] and 22% of LGMD patients [2], thus the underlying cause of disease in the majority of CMD and LGMD cases remains unknown. Most of the known CMDs and LGMDs are due to deficiencies in proteins which are localized at the membrane or in the extracellular matrix of muscle. Therefore, proteins that interact with these members or have a similar function are excellent candidates for muscular dystrophy.

Biglycan is a small leucine rich repeat proteoglycan [3], [4] widely distributed in connective tissues, including the extracellular matrix of muscle [5]. Biglycan is considered a good candidate for muscular dystrophy because it interacts with α-dystroglycan [6], α- and γ-sarcoglycan [7] and collagen VI [8], which are all associated with muscular dystrophy [9]. Biglycan also interacts with fibrillar collagens and is an important determinant of fibrillar collagen assembly and structure in vivo[10], [11]. In addition, biglycan is able to organize collagen VI microfibrils into hexagonal-like networks in vitro via its core protein [8]. It has therefore been postulated that biglycan could mediate interaction between collagen VI and the fibrillar collagens to aid in the formation of the structurally critical fibrillar collagen networks [12]. These multiple interactions of biglycan raise the possibility that biglycan may mediate linkage of the cell surface molecules α-dystroglycan and α- and γ-sarcoglycans to collagen VI in muscle and/or collagen VI to the fibrillar extracellular matrix surrounding each muscle fibre, thus contributing to muscle integrity.

The most compelling direct evidence for a role for biglycan in human muscular dystrophies comes from the phenotype of the biglycan null mouse. Biglycan deficient mice display a mild muscular dystrophy phenotype characterized by membrane disruption, cell death of a subpopulation of myofibres and central nuclei characteristic of regenerating fibres. These mice also have mild osteoporosis which is only evident from 3 months of age [13], [14], [15].

On this basis, we sought to determine if biglycan is a primary cause of neuromuscular disease by directly sequencing biglycan in muscular dystrophy patients in whom the genetic diagnosis is unknown.