Nephrosialidosis: ultrastructural and lectin histochemical study

Severe kidney dysfunction in sialidosis mice reveals an essential role for neuraminidase 1 in reabsorption

Sialidosis is an ultra-rare multisystemic lysosomal disease caused by mutations in the neuraminidase 1 (NEU1) gene. The severe type II form of the disease manifests with a prenatal/infantile or juvenile onset, bone abnormalities, severe neuropathology, and visceromegaly. A subset of these patients present with nephrosialidosis, characterized by abrupt onset of fulminant glomerular nephropathy. We studied the pathophysiological mechanism of the disease in 2 NEU1-deficient mouse models, a constitutive Neu1-knockout, Neu1ΔEx3, and a conditional phagocyte-specific knockout, Neu1Cx3cr1ΔEx3. Mice of both strains exhibited terminal urinary retention and severe kidney damage with elevated urinary albumin levels, loss of nephrons, renal fibrosis, presence of storage vacuoles, and dysmorphic mitochondria in the intraglomerular and tubular cells. Glycoprotein sialylation in glomeruli, proximal distal tubules, and distal tubules was drastically increased, including that of an endocytic reabsorption receptor megalin. The pool of megalin bearing O-linked glycans with terminal galactose residues, essential for protein targeting and activity, was reduced to below detection levels. Megalin levels were severely reduced, and the protein was directed to lysosomes instead of the apical membrane. Together, our results demonstrated that desialylation by NEU1 plays a crucial role in processing and cellular trafficking of megalin and that NEU1 deficiency in sialidosis impairs megalin-mediated protein reabsorption.

Parental Whole-Exome Sequencing Enables Sialidosis Type II Diagnosis due to an NEU1 Missense Mutation as an Underlying Cause of Nephrotic Syndrome in the Child

Introduction

Monogenetic renal diseases, including recessively inherited nephrotic syndromes, represent a significant health burden despite being rare conditions. Precise diagnosis, including identification of the underlying molecular cause, is especially difficult in low-income countries and/or if affected individuals are unavailable for biochemical testing. Whole-exome sequencing (WES) has opened up novel diagnostic perspectives for these settings. However, sometimes the DNA of affected individuals is not suitable for WES due to low amounts or degradation.

Methods

We report on the use of parental WES with implementation of specific stepwise variant filtering to identify the underlying molecular cause of the childhood-onset nephrotic syndrome as nephrosialidosis resulting from a mutation in NEU1.

Results

Sequencing both parents enabled a nephrosialidosis diagnosis in the deceased child. To date, only 16 other cases of nephrosialidosis have been reported in the literature, with only 1 genetically confirmed case. After we reviewed the clinical information of all reported cases, we found that most patients presented with proteinuria, which started at between 2 and 3 years of age. Renal pathology showed mainly focal segmental glomerulosclerosis (FSGS)with vacuolated cells, and steroid treatment was always unsuccessful. Hepatomegaly was present in nearly all cases, whereas corneal clouding and a cherry red spot on the macula was observed in only approximately 50% of cases. Fourteen of 16 previously reported cases were no longer alive at the time of reporting.

Conclusions

Our findings demonstrate the power of parental WES to diagnose rare genetic diseases, such as childhood-onset nephrotic syndrome. We further provide a comprehensive overview of the clinical course of nephrosialidosis and raise awareness of this ultra-rare condition as an underlying cause of FSGS.

Histological studies of renal biopsy in a boy with nephrosialidosis

Nephrosialidosis is a rare subgroup of sialidosis characterized by a progressive and fatal course of nephropathy. The authors report a 2-year-old boy who had suffered from steroid-resistant nephrotic syndrome. Renal biopsy showed diffuse and severe vacuolization of glomerular and tubular epithelial cells. The vacuoles were ultrastructurally membrane bound, most of which were not empty but contained electron-dense material lining the inner surface of the membrane. The pathologic changes were consistent with neuraminidase-deficient disorders, which was later confirmed by biochemical analysis. Although rare, nephrosialidosis should be considered in children with steroid-resistant nephrotic syndrome if renal biopsies exhibit severe vacuolar degeneration of renal epithelial cells.

Perineuronal oligodendrocytes protect against neuronal apoptosis through the production of lipocalin-type prostaglandin D synthase in a genetic demyelinating model

The genetic demyelinating mouse "twitcher" is a model of the human globoid cell leukodystrophy, caused by galactosylceramidase (GALC) deficiency. Demyelination in the twitcher brain is secondary to apoptotic death of oligodendrocytes (OLs). Lipocalin-type prostaglandin (PG) D synthase (L-PGDS), a protein expressed in mature OLs, was progressively upregulated in twitcher OLs; whereas expression of OL-associated proteins such as carbonic anhydrase II, myelin basic protein, and myelin-associated glycoprotein was downregulated during demyelination in twitcher brains. The upregulation of L-PGDS was more remarkable in perineuronal OLs than in interfascicular OLs. A larger number of L-PGDS-positive OLs was found in selected fiber tracts of twitcher brains where fewer apoptotic cells were detected. The distribution of L-PGDS-positive OLs was inversely related to the severity of demyelination, as assessed by accumulation of scavenger macrophages. Mice doubly deficient for L-PGDS and GALC disclosed a large number of apoptotic neurons, which were never seen in twitcher brains, in addition to an increased number of apoptotic OLs. A linear positive correlation was observed between the population of L-PGDS-positive OLs in the twitcher brain and the ratio of apoptotic nuclei in the double mutant versus those in the twitcher, suggesting a dose-dependent effect of L-PGDS against apoptosis. These lines of evidence suggest that L-PGDS is an anti-apoptotic molecule protecting neurons and OLs from apoptosis in the twitcher mouse. This is a novel example of OL-neuronal interaction.

Familial amyloid polyneuropathy associated with transthyretin Gly42 mutation: a quantitative light and electron microscopic study of the peripheral nervous system

We performed extensive quantitative analyses of the peripheral nervous system (PNS) of two siblings with familial amyloid polyneuropathy (FAP) caused by a transthyretin (TTR) Gly42 mutation. Pronounced amyloid deposition was found in the sympathetic ganglia (SyG), dorsal root ganglia (DRG) and throughout the length of the peripheral nerve fibers with some accumulation in the more proximal portion. There was severe neuronal loss in the SyG and DRG together with nerve fiber depletion in the nerve trunk, while only a small amount of amyloid deposition with mild fiber loss was seen in the spinal roots. Sprouts of regenerating axons were very scanty even in the spinal nerves or roots. A teased fiber study mainly showed demyelinating fibers, but axonal degeneration was also present throughout peripheral nerves. An electron microscopic study showed fine amyloid fibrils in direct contact with the axoplasmic membrane of demyelinated axons and destruction of axons in some areas. Amyloid deposition within the PINS in this type of FAP resembled that in type I FAP (TTR Met30). However, direct axonal damage by amyloid fibrils appeared to be more prominent in our cases than in type I FAP. Lectin histochemistry using Ulex europaeus agglutinin I demonstrated preferential depletion of small neurons in the DRG and their primary afferent fibers in the spinal dorsal horn. Primary axonal degeneration and ganglionopathy due to amyloid deposition appear to be the pathogenetic mechanisms for peripheral neuropathy in this type of FAP.