Introduction
Mucopolysaccharidoses (MPSs) are a group of lysosomal storage disorders
(LSDs) caused by deficiency in the activity of each of 11 lysosomal
enzymes required for degradation of glycosaminoglycans (GAGs). The 11
genes coding for the lysosomal enzymes are IDUA, IDS, SGSH, NAGLU,
HGSNAT, GNS, GALNS, GLB1, ARSB, GUSB , and HYAL1 . Deficiency in
these enzymes results in abnormal accumulation of GAGs in lysosomes of
most cells leading to progressive cellular damage and multiple organ
failure, which is the phenotypic hallmark of most MPS types. There is a
high degree of phenotypic overlap between the seven types of MPSs which
are inherited in either autosomal recessive (MPS types I, III, IV, VI,
VII, and IX) or X-linked (MPS type II) (Brusius-Facchin et al. 2019).
MPSs are diagnosed through a combination of clinical phenotype and
biochemical testing, such as quantitation of urinary GAGs and
measurement of enzyme activity in leukocytes or fibroblasts. Measurement
of GAGs using tandem mass spectrometry is a plausible way to screen for
MPS patients, but it cannot provide a definitive differential diagnosis
for the disease type. Biochemical characterization of the GAGs using
two-dimensional electrophoresis can help with achieving a more specific
differential diagnosis, but it requires enzymatic confirmation. However,
performing several enzymatic assays in screen-positive patients could be
time-consuming and not cost-effective. Molecular characterization of MPS
genes using Sanger or next-generation sequencing is another diagnostic
method that is considered as accurate as of the enzymatic assay,
especially when the assignment of patients to the costly enzyme
replacement therapies is taken into account. Such molecular approaches
could also be beneficial for predicting the prognosis and providing
genetic counseling to the family and their at-risk relatives about
carrier screening and prenatal or preimplantation genetic diagnosis
(Brusius-Facchin et al. 2019; Filocamo et al. 2018; Kadali et al. 2019).
Sanger sequencing of the MPS genes has been widely used for molecular
diagnosis in clinically and/or biochemically diagnosed patients over the
last two decades. This individualized approach, which requires
exon-by-exon sequencing of the corresponding genes, might be
time-consuming and less cost-effective. Furthermore, when it comes to
the differential diagnoses of each MPS type, which encompass a wide
range of other types of MPSs and the other LSDs, Sanger sequencing
cannot be considered an effective tool. In a handful of recent research
articles, next-generation sequencing (NGS) has therefore been suggested
as a quick, efficient, and reliable alternative to Sanger sequencing.
Next-Generation
Sequencing allows achieving a comprehensive molecular investigation of
all MPS types as well as their differential diagnosis in a single test,
which could be of paramount importance, particularly in patients with
non-specific overlapping phenotypes. Whole exome sequencing (WES) and
targeted capture NGS are two NGS-based methods that have been applied
for molecular diagnosis of genetic disorders, and each has its own
capabilities and limitations. While the low sensitivity of the
Whole-Exome Sequencing (WES) resulting from its limited coverage could
be considered a drawback, employing targeted capture NGS approach makes
the technology much less error-prone due to the higher coverage and mean
depth achievable in targeted regions. Additionally, the application of
target panels with a larger number of genes encompassing MPS genes and
their differential diagnosis can facilitate reaching a conclusive
diagnosis (Cobos et al. 2015; Costa-Motta et al. 2014; Filocamo et al.
2018; Kadali et al. 2019; Wood et al. 2013).
Here, we report on the genetic investigation of a cohort of 302 patients
from 289 unrelated families with the clinical and/or biochemical
diagnosis of various types of MPS which is the largest cohort of Iranian
patients reported so far. This report is part of a national project on
“Iran MucoPolysaccharidosis REdiagnosis Study (IMPRESsion)”, which has
been carried out to determine the genetic profile of MPSs in Iran. The
recruited patients have been investigated using either Sanger sequencing
or NGS panel, targeting 594 genes associated with inborn errors of
metabolism. Moreover, in search of possible founder mutations and/or
region-specific pathogenic variants, the geographical origin of the
studied patients has also been collected. Such data can facilitate
mutation detection of patients originating from the designated areas,
and propose a stepwise diagnostic approach starting from testing
region-specific pathogenic variants, followed by more comprehensive
testing by NGS-based methods.