Localization of merosin-negative congenital muscular dystrophy to chromosome 6q2 by homozygosity mapping

Congenital muscular dystrophies (CMD) are autosomalrecessive, heterogeneous disorders. The commonestforms are the Fukuyama CMD (FCMD), associated withmental retardation and structural brain anomalies, andclassical (occidental) CMD, with pure muscle expres-sion. FCMD has been localized to chromosome 9q31 -q33. Following the discovery of merosin deficiency insome CMD cases, we have localized, by homozygositymapping and linkage analysis (Zmax = 5.6; 0 = 0.0 formarker AFM127xb2) in four merosln-negatlve familiesa CMD gene In a 16 cM region of chromosome 6q2 inthe region of the laminin M chain gene. In threeconsanguineous, merosin-positive, CMD families therewas no linkage to either chromosome 6q2 or9q31 - q33.INTRODUCTIONCongenital muscular dystrophies (CMDs), are autosomalrecessive severe muscle diseases of early onset and the mostfrequent cause of severe neonatal hypotonia of muscular origin(1 -3). Clinical manifestations occur at birth or in the first monthsof life and consist of muscle hypotonia and weakness, markedlydelayed motor milestones, severe and early contractures, oftenassociated with joint deformities. Serum creatine kinase isvariably raised, up 10 times normal, but may be normal. Thehistological changes in muscle biopsies consist of markedvariation in muscle fibre size, a few necrotic and regeneratingfibres, marked increase in endomysial collagen tissue, variableadipous tissue and no specific ultrastructural features. Untilrecently the diagnosis of CMD has been based on the clinicalpicture and the morphological changes in the muscle biopsy.Within the group of diseases classified as CMDs, variousdistinct phenotypes have been defined. The two more commonforms are the classical CMD, without clinical involvement ofthe central nervous system, although imaging changes in the whitematter may be found on magnetic resonance imaging (MRI), andthe Fukuyama type (FCMD) prevalent in Japan, associated withsevere mental retardation and major structural brain abnormalities(4,5). The association in one patient from a consanguineousfamily of both FCMD and group A xeroderma pigmentosum,led to the localization of FCMD gene to chromosome 9q31 —q33 (6).The marked increase in connective tissue in muscle hassuggested that an abnormality of one of the components of theextracellular matrix could be involved in the pathogenesis of thisdisease (7). However, our initial studies (8,9) failed to detectspecific changes in extracellular matrix proteins. As it wasdemonstrated that a large oligomeric complex of sarcolemmalglycoproteins associated with dystrophin provides a link betweenthe subsarcolemmal cytoskeleton and laminin (10,11), a majorcomponent of the extracellular matrix, we have investigatedwhether one of the laminin subunits (12,13) could be involvedin CMD. Furthermore, a partial merosin reduction was observedin some cases of FCMD (14). This showed a specific absenceof merosin (laminin M chain) in thirteen patients out of twentyaffected by classical, non-Fukuyama, form of CMD (15). Similarmerosin negative results were obtained by C.A.Sewry andV.Dubowitz in ten out of twenty two cases, and by K.P.Campbellin three patients (personal communications). As the laminin Mchain gene (LAMM) has been mapped to chromosome 6q22—q23(16) and the FCMD was localized in chromosome 9q31-q32(6), we focused our analysis in CMD consanguineous families(either with or without merosin deficiency) on these twochromosomes. The high consanguinity led us to use homozygositymapping (17), with highly polymorphic markers consistingexclusively of (CA)