Residual galactose oxidation capacity and galactosylation abnormalities as possible predictors for galactose tolerance and outcome in patients with galactose-1-phosphate uridyltransferase deficiency

Classical galactosemia (CG) is an inborn error of galactose metabolism, caused
by a profound deficiency of the enzyme galactose-1-phosphate uridyltransferase
(GALT). Infants with CG develop a life-threatening disease in the first week of
life after ingestion of galactose from breastmilk or infant formula. The only
available treatment is a life-long galactose restricted diet. Although this
dietary treatment is life-saving in the newborn period, it does not prevent
long-term complications such as cognitive impairment, neurological
complications, and ovarian failure in females. The advised restriction of
galactose varies worldwide, but most patients have a severely restricted diet.
The clinical outcome spectrum of CG is highly heterogeneous, with patient IQs
varying from 44 to 120, which is poorly understood, and valid prognostic
biomarkers are lacking.

Furthermore, since introduction of CG into the Dutch newborn screening program
in 2007, individuals are identified with previously unknown clinical and
biochemical phenotypes and genotypes. Therefore, outcome of individual patients
cannot be predicted. Currently, all individuals with GALT activity <15% are
treated. Recently, it was demonstrated that galactose over-restriction may
cause galactosylation abnormalities comparable to galactose intoxication. Thus,
the strict diet may well be harmful to some. There is an urgent need for
individualized prognostication and treatment.

We hypothesize that the long-term complications in CG are caused by defective
galactosylation of proteins and lipids. We propose that galactosylation
abnormalities in CG are modified by individual residual galactose oxidation
capacity. Measurement of residual galactose oxidation capacity and evaluation
of galactosylation abnormalities may ultimately enable us to predict dietary
galactose tolerance and outcome.

The objective of this study is to investigate the association of the residual
galactose oxidation capacity (measured in vivo and in vitro) and of
galactosylation abnormalities with outcome in patients with galactosemia.

Patients with GALT activity <15% who are currently treated for CG will be
categorized in two subgroups: severe phenotype, or mild phenotype, based on
intelligence/developmental quotient and educational outcome.

Part A: Galactosylation abnormalities in serum (taken previously and stored in
biobank) will be measured in all patients.
Part B: Residual galactose oxidation capacity will be determined with two
methods:
B1: In vivo measurement (galactose breath test, in both children and adult
patients)
B2: In vitro galactose oxidation in fibroblasts (skin biopsy will only be
performed in competent adults).

Part B1:
In vivo galactose oxidation measurement (galactose breath test)
The subject is not allowed to eat or drink 2 hours before the test. Drinking
water is allowed during the test. Adults will be resting in a chair and can
watch television or use their computer. Children may also walk around but must
avoid running and exercise. An oral dose of 7 mg/kg [1-13C] galactose will be
given to each patient once as an aqueous solution. Breath samples will be taken
after 60,90 and 120 minutes by collecting air exhaled into a vacuum tube.

Part B2:
A skin biopsy for in vitro galactose oxidation in fibroblasts will be performed
in competent adults only. If fibroblasts of patients are already available, as
they were sampled as part of patient care, they will be used in this part of
the study if patient/parents have given written informed consent.

Nature and extent of the burden and risks associated with participation,
benefit and group relatedness:

The risks associated with participation in any part of this study can be
considered negligible.

Part A: No risk. Serum samples which have been previously collected in the
Biobank Stofwisselingsziekten will be used.

Part B1: Negligible because it concerns administration of a stable isotope by
mouth. A dose of 7 mg/kg is a safe dose in CG, as it is far below the body*s
daily endogenous production of galactose. There are no known risks of oral
administration of [1-13C] galactose, other than small risk of aspiration. The
burden can be considered minimal, because 1) no invasive procedures will be
performed, 2) effort needed from patients is small since they only need to
drink the in water dissolved [1-13C] galactose and breathe in a tube three
times if possible, 3) patients can rest and watch television or play on their
computer if they want, 4) a relatively small amount of time has to be invested
(procedures are performed once).

Part B2: Negligible, because a skin biopsy is a generally safe procedure, but
complications can occur, including bleeding, bruising, scarring, and infection.
The burden of the procedure is considered minimal for competent adults, as the
procedure is performed while the skin in anesthetized, the procedure is very
brief, and it is performed only once.