Biomedical Engineering Reference
In-Depth Information
4.2
Congenital Disorders of Glycosylation
CDG are a group of disorders, which have a variable presentation including dys-
morphic features, developmental delay, ataxia, seizures, liver fi brosis, retinopathy,
cardiac dysfunction, and coagulopathies (Jones et al.
2011
). About 50 % of proteins
in the body are N-glycosylated, which is refl ected in the clinical presentation of
these disorders. There is major overlap with other disorders in terms of presentation
which increases the chance of them being missed during the diagnostic work-up.
Their estimated prevalence is about 1 in 20,000; however, this may be higher as
these disorders are underdiagnosed.
Patients are classifi ed as having either type 1 or 2 disorder based on the pattern
of serum transferrin on electrophoresis. This is, however, a screening test and car-
ries a high rate of false positives and negatives. In addition, a number of disorders
may be missed, as they do not have abnormal transferrin. Approximately, greater
than 30 different types of CDG have been described to be caused by defects in dif-
ferent gene products. Additional methodologies include using HPLC techniques to
evaluate linked oligosaccharide levels as well as analysis of the glycan structure by
MALDI-TOF-MS. An initial biochemical diagnosis may lead to clear diagnosis and
subsequent relevant Sanger sequencing of the gene; however, in about 40 % of cases
this might not be the case. It is here that NGS panel testing can offer a tremendous
advantage over the gold standard of Sanger sequencing.
Jones et al. reported the validation studies of an NGS CDG panel using 12
blinded positive control samples (Jones et al.
2011
). Technically, both RainDance
and Fluidigm platforms were used for sequence enrichment and targeted amplifi ca-
tion and the SOLiD platform was the sequencer of choice. The disease-causing
mutations were identifi ed by NGS for all 12 positive controls. Moreover, they pro-
pose an algorithmic process to improve the diagnosis, based on a combination of
biochemical and molecular tests. If clear diagnosis can be established based on
clinical features and biochemical profi le, Sanger sequencing for the relevant gene
should be performed. However, if clinical suspicion remains without any clear bio-
chemical diagnosis, NGS panel testing should be pursued. The panel consists of 25
genes (
ALG2
,
ALG3
,
ALG6
,
ALG8
,
ALG9
,
ATP6V0A2
,
B4GALT1
,
COG1
,
COG7
,
COG8
,
DOLK
,
DPAGT1
,
DPM1
,
GNE
,
MGAT2
,
MOGS
,
MPDUI
,
MPI
,
PMM2
,
RFT1
,
SLC35A1
,
SLC35C1
, and
TUSC3
).
ALG1
was not included because it has a
pseudogene and was analyzed separately as part of the panel. Potentially, two muta-
tions should be identifi ed by the panel for autosomal recessive genes and a diagno-
sis reached. However, if only variants of unknown clinical signifi cance are identifi ed,
biochemical testing, if available, needs to be performed to confi rm the functional
consequence of those changes. NGS offers a way to diagnose patients whose phe-
notypes may not be very clear that fall under the spectrum of CDG. It offers an
effi cient and a relatively faster way to reach a diagnosis rather than sequentially
sequencing each gene by the Sanger method.
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