Alessandra d’Azzo, PhD
St. Jude Children’s Research Hospital, Memphis TN USA
SIALIDOSIS (MUCOLIPIDOSIS I)

Sialidosis, also known as mucolipidosis I, is a neurosomatic lysosomal storage disease
belonging to the group of the glycoproteinoses. The disease is inherited as anautosomal recessive trait and is caused by genetic lesions at the NEU1 gene encoding
the lysosomal sialidase, neuraminidase 1 or NEU1. This enzyme is essential for initiating
the degradation of glycoproteins containing the sugar sialic acid (SIA) at the end of
their glycan chains. Thus, defective or deficient activity of NEU1 leads to the buildup ofsialylated glycoproteins in lysosomes and other subcellular sides. The excessive andprogressive accumulation of non-digested or only partially digested SIA-containing
metabolites in many cells of the body is pathognomonic of the disease, and at thebasis of the disease pathogenesis.
Sialidosis affects primarily cells of the reticuloendothelial system, so tissues and organs ofepithelial origin show the first and most acute signs of the disease.
The patients present with two distinct clinical conditions:
Sialidosis type I (normomorphic) is also referred to as “cherry red spot - myoclonus
syndrome” because gradually reduced visual acuity and twitching of the muscles areoften the only discernible symptoms at least initially. Normal intellectual capacity, latepresentation and long survival are characteristics of these forms and explain whypatients with type I sialidosis go undiagnosed for many years or are misdiagnosed.
However, the occurrence of the aforementioned features combined with abnormal
secretion of high molecular weight sialyloligosaccharides in urine and low NEU1 activity
in white blood cells or fibroblast could help in making the diagnosis of most of thesecases. Thus, this type of sialidosis patients might be more numerous than previously
thought.
Sialidosis type II (dysmorphic) is the severe, neuropathic form of the disease. Threesubtypes are recognized within this group, depending on the time of onset and severityof the symptoms: congenital or hydropic with onset in utero, infantile with onset
between birth and 12 months of age, juvenile with onset past 2 years of age. The
clinical presentation includes coarse face, enlargement of spleen and liver, vertebral
deformity, dysostosis multiplex, neurological involvement.
In general, the severity and penetrance of specific phenotypic abnormalities correlatewith the type and combination of NEU1 mutations and the levels of residual enzymeactivity. More than 40 NEU1 disease-causing mutations have been identified in type I
and type II sialidosis patients. They are mostly missense mutations leading to singleamino acid substitutions.
(Seyrantepe et al 2003 https://www.ncbi.nlm.nih.gov/pubmed/14517945;
Caciotti et al 2009 https://www.ncbi.nlm.nih.gov/pubmed/19568825;
d’Azzo et al 2015 http://www.ncbi.nlm.nih.gov/pubmed/26949572;
Franceschetti et al 2016 https://www.ncbi.nlm.nih.gov/pubmed/27621198)

Alessandra d’Azzo, PhD
St. Jude Children’s Research Hospital, Memphis TN USA
Lysosomal NEU1 and its chaperone PPCA
NEU1 is ubiquitously but differentially expressed in different tissues and cell types. The
enzyme is catalytically active and stable in lysosomes when in complex with two otherenzymes: the protective protein/cathepsin A (PPCA) and β-galactosidase. NEU1
interaction with PPCA is essential for maintaining its activity; hence, PPCA functions as a
canonical chaperone/auxiliary protein for NEU1. Extensive biochemical studies havebeen done on complex formation and mode of assembly of NEU1 and PPCA; althougha full understanding of the way these proteins interact awaits the resolution of the 3
dimensional structure of the complex. Detailed information about these studies can befound in the following recent papers:(Bonten et al 2009 http://www.ncbi.nlm.nih.gov/pubmed/1966647;
Bonten et al 2014 http://www.ncbi.nlm.nih.gov/pubmed/24337808)

Mouse models and the identification of pathogenic processes caused by Neu1
deficiency
A Neu1 KO mouse was generated and characterized in early 2000. Total loss of Neu1
activity in these mice is compatible with life, but they develop a severe and progressive
condition closely similar to type II sialidosis, and have a short lifespan (5-7 months)
compared to wild type mice. KO mice present with hallmarks of the disease already atbirth, including oligosacchariduria and extensive lysosomal vacuolation of cells in
virtually all systemic organs and the nervous system. Epithelia, reticulo-endothelia andhistiocytes/macrophages are the first cell types to get affected. The main pathologic
features that recapitulate those seen in patients are growth retardation, timedependent enlargement of the spleen, diffuse edema (buildup of fluid), spine
abnormalities and neurodegeneration. The sialidosis mouse model has proven valuable
to study the basis of disease pathogenesis downstream of Neu1 deficiency andconsequent impaired processing or degradation of substrates that are target of theenzyme in vivo.
(de Geest et al 2002 http://www.ncbi.nlm.nih.gov/pubmed/12023988)
These studies have uncovered new functions of lysosomal NEU1 in basic physiologicalprocesses that go beyond its degradative capacity, and, when deregulated,
contribute to the expression and severity of specific phenotypic alterations known tooccur in sialidosis. For examples, we now can explain, at least in part, the mechanism,
controlled by Neu1, that underlie progressive spleen enlargement, hearing loss, muscle
atrophy and neurodegeneration in the Neu1 KO mouse and likely in patients. Some ofthe cellular and extracellular changes have been attributed to the capacity, and loss
thereof, of NEU1 to keep in check the process called lysosomal exocytosis. The way
NEU1 does it is through the processing of the SIAs on one of the enzyme’ substrates, thelysosomal membrane protein LAMP1. When NEU1 is deficient, LAMP1 remains
unprocessed and this causes an abnormal number of lysosomes to move to and dockat the membrane surrounding the cells and eventually to fuse with such membraneand discharge their contents extracellularly. This continuous, excessive discharge ofpotentially harmful products from the lysosomes eventually affects the balance
between the intracellular and extracellular composite of molecules that maintain tissue

Alessandra d’Azzo, PhD
St. Jude Children’s Research Hospital, Memphis TN USA
and organ physiology. Detailed information about this research and its implication forsialidosis can be found in:(Yogalingam et al 2008 http://www.ncbi.nlm.nih.gov/pubmed/18606142;
Zanoteli et al 2010 http://www.ncbi.nlm.nih.gov/pubmed/20388541;
Wu et al 2010 http://www.ncbi.nlm.nih.gov/pubmed/19857571;
Annunziata et al 2013 http://www.ncbi.nlm.nih.gov/pubmed/24225533)
These findings were also presented at several conferences during the last years
including the ISMRD family conferences in July 2015 and November 2017.

Therapy for sialidosis
The nature and biochemical characteristics of NEU1 have hindered the developmentof suitable therapies for sialidosis. The enzyme by itself tends to aggregate, it is highly
immunogenic and it is catalytically inactive unless bound to its auxiliary chaperone
PPCA. Nonetheless, the strict dependence of NEU1 on PPCA for catalytic activationand stability may turn advantageous for therapeutic purposes. This is because NEU1
residual activity in fibroblasts from several type I patients could be increased byincreasing the levels of available PPCA. This indirect therapeutic approach may beconceivable for patients carrying NEU1 missense mutations that permit its interaction
with PPCA. In this regard, both AAV-PPCA–mediated gene therapy and PPCA-ERT that
hopefully will become available for the treatment of patients with galactosialidosis in
the near future, could in principle be extended to the treatment of at least somepatients with type I sialidosis. If this will become an optional therapy for these patients,
its successful implementation and the identification of eligible patients will require thecoordinated effort of expert clinicians, basic scientists, pharmaceutical companies, andmost importantly family organizations like ISMRD and patient advocacy groups. (Hu etal 2012 http://www.ncbi.nlm.nih.gov/pubmed/22008912 and Bonten et al 2013
http://www.ncbi.nlm.nih.gov/pubmed/23770387)
In conclusion, despite the rarity and complexity of silaidosis, remarkable progress has
been made over the last decade towards the understanding of the underlying
pathogenesis in this disease. If the bench work will continue at this pace, I am inclined
to predict that additional new functions for NEU1 will be uncovered that might open
new possibilities for alternative therapies. In this respect I believe that the most helpful
hints on the roads to such discoveries often come from the sialidosis patients themselves
and their families. Their insightful and courageous drive is a daily encouragement for us
scientists to move ahead and keep looking forward.