Electrophysiological and histological characterizations of retinal changes in a mouse model of Sanfilippo Syndrome

Sanfilippo syndrome or Mucopolysaccharidosis III (MPS-III) is a neurodegenerative autosomal recessive lysosomal storage disorder in which patients suffer progressive vision loss. Here we sought to study the underlying functional and morphological changes. B6.129S6-Naglutm1Efn/J, the mouse model of the MPSIIIB, and age-matched wildtype (WT) mice were purchased from the Jackson lab. Scotopic flash ERG and paired flash ERG were recorded bilaterally from 8 knockout (KO) and 7 WT mice when they were 28 and 46-week-old. Rod a-wave leading edges were modeled as described by Cideciyan and Jacobson (1996). Rod b-wave was modeled using the Naka-Rushton equation. Mice (4/group) were sacrificed at the 46th week for immunohistochemistry, in which staining was performed using double-labeling procedures on vertical vibratome retinal sections with antibodies for PKCα (rod bipolar cell), GNAT2 (cones) and the fluorescent nuclear dye TO-PRO3 (photoreceptor somas). Segments of the section taken from designated central and peripheral regions temporal to the optic disc were imaged under confocal microscope for cell counting. At the 28th week, rod a- and b-wave were found significantly diminished in the KO compared to the WT (a-wave Rmax± SE: -125±6 vs -316±8μV; b-wave Rmax: 276±13 vs 357±27μV; unpaired t-test, p<0.05). The cone a- and b-waves of the KO were not significantly different from those of the control at the 28th week, but were significant diminished at the 46th week (Mean a-wave amplitude± S.E: -27.0±2.7 vs -42.1±3.9μV. Mean b-wave± S.E: 204.8±17.2 vs 252.9±11.8μV). KO mice have a reduced mean ONL thickness (number of cells) in central and peripheral retina (5.75±0.36 vs 10.5±0.001 cells; and 5±0.31 vs 8.75±0.30 cells; p<0.05). They also have a reduced number of rod bipolar cells (per 200μm horizontally) in both regions (5.5±0.36 vs 16.75±0.0001; and 7±0.31 vs 15.5±0.31; p<0.05). There was no difference in the number of cones in either region. In an early stage (28th week) of the MPSIIIB model, function of rods was first affected and was suppressed by about 60%. Cones became dysfunctional only in a later stage (46th week) and was suppressed by 36% compared to the control. Deaths of rods and rod bipolar cell, but not cones, were evident at the 46th week. The relationship between the rods and the cones pathways during the degeneration is under investigation.