Homozygous XYLT2 variants as a cause of spondyloocular syndrome.

TLDR: The phenotypic description of SOS is extended by adding follow‐up data from 5 affected individuals in one of the two families, presented here, and the body of evidence that SOS is caused by homozygous variants in the XYLT2 gene is extended.

Abstract: Spondyloocular syndrome (SOS) is a rare autosomal recessive skeletal disorder. Two recent studies have shown that it is the result of biallelic sequence variants in the XYLT2 gene with pleiotropic effects in multiple organs including retina, heart muscle, inner ear, cartilage, and bone. The XYLT2 gene encodes xylosyltransferase 2, which catalyzes the transfer of xylose (monosaccharide) to the core protein of proteoglycans (PG) leading to initiating the process of proteoglycan assembly. SOS was originally characterized in two families A and B of Iraqi and Turkish origin, respectively. Using DNA from affected members of the same two families we performed whole exome sequencing, which revealed two novel homozygous missense variants (c.1159C>T, p.Arg387Trp) and (c.2548G>C, p.Asp850His). Our findings extend the body of evidence that SOS is caused by homozygous variants in the XYLT2 gene. In addition, this report has extended the phenotypic description of SOS by adding follow-up data from five affected individuals in one of the two families, presented here.

Figures

Table 1. SOS Phenotypic comparison of affected individuals from different reported studies.

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Accepted Article
Homozygous XYLT2 variants as a cause of spondyloocular
syndrome
Muhammad Umair
1,2,6
, Gertrud Eckstein
1
, Günther Rudolph
3
, Tim Strom
1
, Elisabeth
Graf
1
, Doris Hendig
4
, Julie Hoover
5
, Jasemin Alanay
2
, Thomas Meitinger
1,2
, Heinrich
Schmidt
3,*
, Wasim Ahmad
6,*
1
Institute of Human Genetics, Helmholtz Zentrum Munchen, Neuherberg, Germany
2
Institute of Human Genetics, Technische Universitat, Munchen, Germany
3
University Eye Hospital, Ludwig Maximilians University, Munich, Germany
4
Institute for Laboratory and Transfusion Medicine, Heart and Diabetes, Center North Rhine-
Westphalia, University Hospital of the Ruhr University, Germany
5
University Children’s Hospital, Division of Endocrinology and Diabetology, Munich, Germany
6
Department of Biochemistry, Faculty of Biological Sciences, Quaid-i-Azam University,
Islamabad, Pakistan
*
Corresponding Authors
*Wasim Ahmad PhD, Department of Biochemistry, Faculty of Biological Sciences, Quaid-i-
Azam University, Islamabad Pakistan
Tel: +92-51-90643003, E-mail: wahmad@qau.edu.pk
*Heinrich Schmidt, University Eye Hospital, Ludwig Maximilians University, Munich,
Germany
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This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which
may lead to differences between this version and the Version of Record. Please cite this
article as doi: 10.1111/cge.13179
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Accepted Article
E-mail: Heinrich.Schmidt@med.uni-muenchen.de
Conflict of interest: Declared none.
Acknowledgements
Muhammad Umair was supported by HEC (IRSIP and Indigenous PhD fellowship programs),
Pakistan.
ABSTRACT
Spondyloocular syndrome (SOS) is a rare autosomal recessive skeletal disorder. Two recent
studies have shown that it is the result of biallelic sequence variants in the XYLT2 gene with
pleiotropic effects in multiple organs including retina, heart muscle, inner ear, cartilage, and
bone. The XYLT2 gene encodes xylosyltransferase 2, which catalyzes the transfer of xylose
(monosaccharide) to the core protein of proteoglycans (PG) leading to initiating the process of
proteoglycan assembly.
SOS was originally characterized in two families A and B of Iraqi and Turkish origin,
respectively. Using DNA from affected members of the same two families we performed whole
exome sequencing, which revealed two novel homozygous missense variants (c.1159C>T,
p.Arg387Trp) and (c.2548G>C, p.Asp850His). Our findings extend the body of evidence that
SOS is caused by homozygous variants in the XYLT2 gene. In addition, this report has extended
the phenotypic description of SOS by adding follow-up data from five affected individuals in one
of the two families, presented here.
This article is protected by copyright. All rights reserved.

Accepted Article
Keywords: spondyloocular syndrome, SOS, XYLT2, WES, skeletal dysplasia, missense variants
This article is protected by copyright. All rights reserved.

Accepted Article
INTRODUCTION
Proteoglycans (PGs) are a class of extracellular matrix and surface-associated proteins involved
in many physiological processes such as signal transduction, cellular homeostasis, membrane
integrity, co-repressor activity, morphogen gradient formation, lipid catabolism and scaffolding
(1). PGs consist of a core protein, which is linked to glycosaminoglycan (GAG) disaccharide
chains. Structurally, PGs are very diverse having different lengths and composition of
disaccharides, with various types of modifications of phosphorylation, sulfation, and different
combinations of core proteins (2). Assembly of GAGs on the core protein results in different
groups of sulfated PGs such as chondroitin sulfate (CSPGs), heparan sulfate (HSPGs) and
modified form of CSPGs the dermatan sulfate (DSPGs) (3). Synthesis of a common
tetrasaccharide linker chain is the initial step in heterogeneous CSPGs, HSPGs and DSPGs
formation.
Xylosyltransferases (XylTs) catalyzes the transfer of xylose to the core protein serine. Two
enzymes XYLT1 (MIM 608124) and XYLT2 (MIM 608125) with xylosyltransferase activity
have been described to catalyze this reaction in humans (4). Additional sugar residues are added
by specific enzymes including B4GALT7 (GalT-I), B3GALT6 (GalT-II) and B3GAT3 (GlcAT-
1). A defect in any of these steps causes severe autosomal recessive disorders (MIM 604327,
615291, 606374) (5).
Spondyloocular syndrome (SOS; MIM 605822) is caused by biallelic sequence variants in the
XYLT2 gene (6). Here, we have reported the clinical follow-up of two families originally used to
describe SOS. In addition, using WES we have identified two homozygous missense variants in
the XYLT2 in the same two families.
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Accepted Article
MATERIALS AND METHODS
Study Approval
The present study was performed according to the declaration of Helsinki protocols and
approved by Institutional Review Board (Technical University, Munich, Germany). Written
informed consent for publication of images was obtained from all members.
Research Subjects
A consanguineous family A, of Iraqi origin, was clinically described previously by Schmidt et al.
(7) and Rudolph et al. (8). Similarly, family B, of Turkish origin, was described previously by
Alanay et al. (9).
Genetic Analysis
In family A, genome-wide linkage analysis was performed using the Affymetrix 10K Mapping
Array. The statistical analysis was carried out with Allegro (10). Whole exome sequencing was
performed in both the families (A and B) using HiSeq 2500 systems (Illumina, San Diego, CA,
USA) (11). All the filtered variants were inserted into an in-house database. Subsequently, we
queried the database to identify only rare homozygous variants, thus two homozygous variants
were identified in the families, which were further validated by Sanger sequencing.
RESULTS
Clinical description
Family A, of Iraqi origin, has been extensively described previously in 2001 and 2003 (7, 8).
Characteristic features such as facial hypotonia, facial dysmorphism, short trunk, reduced lumbal
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Frequently asked questions

Q1. What are the contributions mentioned in the paper "Homozygous xylt2 variants as a cause of spondyloocular syndrome: " ?

Using DNA from affected members of the same two families the authors performed whole exome sequencing, which revealed two novel homozygous missense variants ( c. 1159C > T, p. Arg387Trp ) and ( c. 2548G > C, p. Asp850His ). In addition, this report has extended the phenotypic description of SOS by adding follow-up data from five affected individuals in one of the two families, presented here. This article is protected by copyright. 

Q2. What is the role of xylosyltransferase 2?

Since xylosyltransferase 2 catalyzes the initiation of GAG assembly to the PG core protein serine (12, 13), it is crucial to PG function. 

Q3. What is the role of xylosylated macromolecules in cellular processes?

Proteoglycans play a key role in multiple cellular processes and represent a group of glycosylated macromolecules, mostly expressed on the cell surface and in the extracellular matrix. 

Q4. What are the clinical features of the two families?

features including learning difficulties, fractures, bilateral cataract, osteopenia, platyspondyly, and retinal detachment reported by Munns et al. (6) and Taylan et al. (18) were observed in patients of their two families. 

Q5. What enzymes have been described to catalyze this reaction in humans?

Two enzymes XYLT1 (MIM 608124) and XYLT2 (MIM 608125) with xylosyltransferase activity have been described to catalyze this reaction in humans (4). 

Q6. How many homozygous variants were identified in the families?

the authors queried the database to identify only rare homozygous variants, thus two homozygous variants were identified in the families, which were further validated by Sanger sequencing. 

Q7. What is the definition of a PG?

PGs are very diverse having different lengths and composition of disaccharides, with various types of modifications of phosphorylation, sulfation, and different combinations of core proteins (2). 

Q8. What was the phenotype of the affected member?

In addition, the affected member showed unique features including short stature, hyperextensible joints, hyperelastic skin and thoracic kyphosis. 

Q9. What is the key to initiation of GAGs?

it is certain that an addition of xylose to the serine residue at the core protein of PGs is the crucial first step in GAGs initiation, which is catalyzed by xylosyltransferase (16). 

Q10. What is the role of XylTs in PGs?

Assembly of GAGs on the core protein results in different groups of sulfated PGs such as chondroitin sulfate (CSPGs), heparan sulfate (HSPGs) and modified form of CSPGs the dermatan sulfate (DSPGs) (3). 

Q11. What are the two mutations in XYLT2?

The identified two mutations (p.Arg387Trp, p.Asp850His), are highly conserved (Fig. 2c) and might possibly affect secondary structure leading to non-functional XYLT2 protein. 

Q12. What are the clinical features of the affected individuals?

Clinical features reported in affected individuals by Munns et al. (6) such as ureter dilatation and hearing impairment were not observed in affected individuals of the two families presented here.