Chemistry Reference
In-Depth Information
Figure 22.1
Detection of
N
- glycosylation defects
by isoelectric focusing of serum transferrin.
Transferrin carries two
N
- glycans, which are
normally terminated by Sia, thereby introducing
four to six negative charges (left panel). Isoelec-
tric focusing separates the transferrin glyco-
forms according to their amount of negative
charges. Normal control (NC) transferrin
mainly shows a glycoform carrying four nega-
tive charges (right panel). Transferrin from
CDG samples accumulates glycoforms with 3,
2, 1 or even 0 negative charges, which indicates
a loss of glycan structures. The double-band
patterns seen in lanes 1, 2 and 7 are caused by
an amino acid polymorphism often found in
transferrin.
Info Box 1
In 1976, the Swedish neurologist Helena Stibler described structural abnor-
malities in the serum glycoprotein transferrin in cases of alcoholism. The cause
of the anomaly was shown to be related to the absence of glycan chains on
transferrin, which normally carries two
N
-glycan chains. The decreased glyco-
sylation, as evidenced by isoelectric focusing analysis, matched with episodes
of alcohol abuse since transferrin glycosylation normalized in periods of absti-
nence. Because of its high sensitivity and specifi city, carbohydrate - defi cient
transferrin (CDT) became a standard diagnostic marker of alcohol abuse. As
the CDT test became broadly adopted, a few cases of false-positivity were
noticed in patients affected by a rare inherited disorder of protein glycosylation,
which was then called carbohydrate- defi cient glycoprotein syndrome. This syn-
drome was later found to represent several forms of CDG-I and - II. The pioneer
work of Helena Stibler and of the Belgian pediatrician Jaak Jaeken was instru-
mental in the fi rst description of diseases of
N
- glycosylation. The isoelectric
focusing test of serum transferrin has paved the way for the identifi cation of
many types of
N
-glycosylation disorders. Whereas the importance of the test in
the diagnosis of glycosylation disorders is undisputed, the biochemical mecha-
nisms underlying the effect of ethanol exposure on protein glycosylation are
still enigmatic.
Dol-P-Man, in the assembly of LLO in the ER and in the transfer of oligosaccha-
rides to proteins (Figure 22.2 ). The fi rst gene associated with CDG-I, and by far
the most frequent defective one, is the phosphomannomutase- 2 (
PMM2
, OMIM
212065) gene. The estimated incidence of
PMM2
defi ciency averages 1 : 50 000,
meaning that, fortunately, it is a rare disease. Remarkably, the most common
