Genetics and Therapies for GM2 Gangliosidosis

Abstract

Tay-Sachs disease, caused by impaired β-N-acetylhexosaminidase activity, was the first GM2 gangliosidosis to be studied and one of the most severe and earliest lysosomal diseases to be described. The condition, associated with the pathological build-up of GM2 ganglioside, has acquired almost iconic status and serves as a paradigm in the study of lysosomal storage diseases. Inherited as a classical autosomal recessive disorder, this global disease of the nervous system induces developmental arrest with regression of attained milestones; neurodegeneration progresses rapidly to cause premature death in young children. There is no effective treatment beyond palliative care, and while the genetic basis of GM2 gangliosidosis is well established, the molecular and cellular events, from diseasecausing mutations and glycosphingolipid storage to disease manifestations, remain to be fully delineated. Several therapeutic approaches have been attempted in patients, including enzymatic augmentation, bone marrow transplantation, enzyme enhancement, and substrate reduction therapy. Hitherto, none of these stratagems has materially altered the course of the disease. Authentic animal models of GM2 gangliodidosis have facilitated in-depth evaluation of innovative applications such as gene transfer, which in contrast to other interventions, shows great promise. This review outlines current knowledge pertaining the pathobiology as well as potential innovative treatments for the GM2 gangliosidoses.

Keywords: GM2 gangliosidosis; Lysosomal storage disease; Neurodegeneration; Sandhoff disease; Tay-Sachs disease; Therapies..

Fig. (1)

Catabolism of GM2 ganglioside by the β-hexosaminidase system and transport to the lysosome. GM2 ganglioside is hydrolysed in vivo in the lysosome by the concerted action of the isozyme Hex A and GM2 activator protein. Hex A is a heterodimer of the α- and β-subunits of β-hexosaminidase, and encoded by HEXA and HEXB, which localize to human chromosomes 15 and 5, respectively. Hex B and Hex S are homodimers of the β- and α-subunits. Only dimeric forms of Hex can hydrolyse specific natural and artificial substrates. Mutations in HEXA, HEXB and GM2A cause Tay-Sachs disease, Sandhoff disease and the GM2 activator protein deficiency respectively, also known as variants B, 0, and AB on the basis of the residual β-hexosaminidase isozyme activities in affected individuals. In a similar manner to other lysosomal hydrolases, Hex can travel to the lysosome directly (1, a) or indirectly (2, b). The latter route, known as the secretion/recapture mechanism, can be exploited for therapeutic applications; whereby the enzyme is secreted into the extracellular space and taken up by the same (d) or neighbouring cells, such as the axons of neurones (c). The enzyme is transported in a retrograde manner to other parts of the cell thus correcting the enzymatic defect. Endoplasmic Reticulum (ER); Lysosome (L); Mitochondria (M).

Fig. (2)

Pathologic features of GM2 gangliosidosis. The most notable feature is the presence of grossly enlarged neurones throughout the nervous system, due to the presence of membranous cytoplasmic bodies (MCBs *), which are abnormal lysosomes, easily revealed by electron microscopy (EM). MCBs stain with Periodic acid-Schiff reagent (PAS) and antibodies against GM2 ganglioside. Activation and expansion of microglia is evident in many areas and revealed by staining for cell markers such as CD68, prominent here in the VPM/VPL nuclei of the thalamus. Neurodegeneration is detectable by silver staining, abundant in neuronal cell bodies and axons such as those of the internal capsule (IC). Impaired autophagy in neurones is shown by co-staining proteins p62 and ubiquitin. All sections are from Sandhoff mouse brain. Fields CA1 (CA1) and CA3 (CA3) of hippocampus (Hipp); fimbria (fi); granular layer of the dentate gyrus (GrDG); internal capsule (IC); nucleus (N); ventroposterior medial (VPM) and lateral (VPL or LP) thalamic nuclei; primary somatosensory cortex (S1).

Fig. (3)

Brain parenchymal gene transfer of rAAV monocistronic vectors rescues the Sandhoff mouse phenotype. Infusion of rAAVs coding for human α- (hhex α) and β-subunits (hhex β) of β-hexosaminidase leads to distribution of Hex enzyme (red precipitate) throughout the entire neuraxis, seen here in a mutant animal aged 2 years. Dual mRNA in situ hybridization with probes specific for the α- and β-subunits detects the presence of both mRNAs in transduced neurones, a pre-requisite for the formation of abundant amounts of Hex A. Consequently, storage material is cleared or prevented as seen here in the spinal cord of a 2-year old mutant Sandhoff mouse. Central canal (cc); grey matter (gm); white matter (wm); CAG promoter is a composite promoter of the CMV enhancer, the chicken beta actin promoter, and the rabbit beta globin intron (CAGp); human immunodeficiency virus tat protein transduction domain (tat); woodchuck hepatitis virus posttranscriptional regulatory element (WPRE); inverted terminal repeat (ITR).

Fig. (4)

A single ventricular co-infusion of rAAVs into the Sandhoff brain rescues the mouse phenotype. Four-week old Sandhoff mice were injected into one of the brain lateral ventricles with rAAV2/1 coding for α- and β-subunits of β-hexosaminidase. The intervention increased survival with supranormal formation of all three Hex isozymes, which are distributed throughout the entire central nervous system. Fractionation of the isozymes was carried out on the brain contralateral injection side by ion exchange chromatography. Hex activities were detected with the artificial substrate 4-MUG (4-methylumbelliferyl-2-acetamido-2-deoxy-ß-D-glucopyranoside).