Mutations in LAMB1 cause cobblestone brain malformation without muscular or ocular abnormalities

Abstract

Cobblestone brain malformation (COB) is a neuronal migration disorder characterized by protrusions of neurons beyond the first cortical layer at the pial surface of the brain. It is usually seen in association with dystroglycanopathy types of congenital muscular dystrophies (CMDs) and ocular abnormalities termed muscle-eye-brain disease. Here we report homozygous deleterious mutations in LAMB1, encoding laminin subunit beta-1, in two families with autosomal-recessive COB. Affected individuals displayed a constellation of brain malformations including cortical gyral and white-matter signal abnormalities, severe cerebellar dysplasia, brainstem hypoplasia, and occipital encephalocele, but they had less apparent ocular or muscular abnormalities than are typically observed in COB. LAMB1 is localized to the pial basement membrane, suggesting that defective connection between radial glial cells and the pial surface mediated by LAMB1 leads to this malformation.

Figure 1

Mutations in LAMB1 Lead to Cobblestone Brain Malformation (A and C) Family pedigrees showing first-cousin marriages and a total of four affected children. Filled boxes represent affected individuals. (B) Whole-genome linkage analysis of family 520. Chromosomal position (x axis) and multipoint LOD scores (y axis) including a positive LOD score (∼2.3) on chromosome 7 (red arrow) are shown. (D) Homozygosity plot for family 1257. Red bars represent stretches of homozygosity above a threshold of 3 cM. x axis: chromosomal position. y axis: degree of homozygosity. An arrow points to the homozygous interval containing LAMB1. (E and F) Schematic structure of LAMB1, the encoded protein, position of mutations, and corresponding family number in parentheses. Abbreviations are as follows: LamNT, Laminin N-terminal; LAM IV, Laminin IV type B; EGFLAM, Laminin Epidermal Growth Factor-like domain.

Figure 2

Brain MRI in Individuals with LAMB1 Mutations Brain MRI revealing (A–D) cobblestone malformation (COB), white-matter signal abnormality (arrows), an irregular layer of ectopic neurons (arrowheads), (E–H) cerebellar hemisphere and vermal hypoplasia (arrows), brain-stem hypoplasia (asterisk), (I–K) brain-stem hypoplasia (asterisk), and corpus-callosum hypoplasia (arrow).

Figure 3

LAMB1 Expression and Genetic Network (A) RT-PCR of each known laminin gene in mouse tissue across development. Abbreviations are as follows: fb, forebrain; cb, cerebellum; ms, muscle; men, meninges; and H20, water control for PCR. Arrows indicate primer dimers. (B) Immunohistochemical staining of P7 wild-type mouse cerebellum with anti-doublecortin (DCX, green), anti-LAMB1 (red), and DAPI (blue). Arrows point to blood vessels over the pial BM. Arrowheads show Purkinje cells. Abbreviations are as follows: EGL, external granular layer; PCL, Purkinje cell layer; and IGL, internal granular layer. The scale bar represents 50 μm. (C) Coexpression network analysis showing genes that are highly correlated (green circles). LAMB1, LAMC3, ZIC1, ZIC2, and FLNA are the brain malformation seed genes (red circles).