Triglyceride clearance in subjects with heterozygous EXT1 or EXT2 Mutations

Hypertriglyceridemia is an independent risk factor for cardiovascular disease
and can be caused by decreased clearance of triglyceride-rich
lipoproteins(TRLs) in the liver. In this protocol we will focus on a recently
discovered pathway in which TRLs are cleared through heparan sulfate
proteoglycans (HSPGs)-assisted endocytosis in the liver. Animal research has
elucidated many of the steps in this process but human relevance still remains
to be determined. Our hypothesis is that HSPGs are instrumental in the removal
of TRLs from the circulation in humans. An important gene in this process, EXT
codes for exostosin, which is involved in the elongation of heparan sulphate, a
glycosaminoglycan present throughout the human body. Patients with EXT-1 and
EXT-2 mutations have a defect in their heparin sulfate synthesis and could
therefore help elucidate the role of HSPGs in triglyceride clearance. The
patients are characterized by the HMO (hereditary multiple osteochondroma)
syndrome; they develop multipe benign epiphysial bone tumors during (pre-)
puberty due to 50% reduction in heparansulfate synthesis. Further assessment of
their metabolism has thus far never been published. Knowing that heparin
sulfate in mice are important in triglyceride metabolism, we are interested
whether human subjects with mutations in the gene coding for HS are also
characterized by impaired triglyceride clearance. We will therefore ask
patients with HME/MO to participate in an Oral Fat Loading Test (OFLT) combined
with an LPL test (Lipoprotein Lipase) to elucidate the human relevance of
heparan sulfate in triglyceride metabolism.

In the current study we want to evaluate the triglyceride clearance in a
population with EXT-1 and EXT-2 mutations (n=15) and their unaffected family
members (n=15). We will perform:

* Oral fat loading test (OFLT) to investigate lipid clearance
* LPL test to determine LPL binding
* Standard CV risk biochemistry panel(incl. Cholesterol/crp/hba1c). Also
plasma/urine samples for determination of total glycosaminoglycan
concentration, hyaluronan with degrading enzyme hyaluronidase, heparin sulfate
with degrading enzyme heparanase.

Subjects will be recruited from department of Orthopedics, OLVGC which is a
national referral center for hereditary multiple osteochondromas. All
studies/measurements will be performed at the AMC.

This is a pilot case control study, in which we will invite 15 subjects with a
known EXT-1 or EXT-2 mutation and their unaffected family members to
participate in this study. We will perform all measurements on one day. In the
morning, after an overnight fast subjects will visit the AMC. There, a catheter
will be inserted in the cubital vein and blood will be drawn. Moreover,
patients will be asked to collect a morning urine sample for further analyses.
Then, the Fat Loading Test will be initiated and subsequent hourly blood
drawing is performed from a venous catheter. Finally, 8 hours after the start
of the OFLT a bolus of heparin is infused (LPL test) and blood is drawn after
2, 5 10 and 15 min. Hereafter, the catheter will be removed and subjects are
discharged.

1. Fat loading test with subsequent blooddrawing
2. LPL-test using heparin

Fat loading test is a routine test in our department which requires repetitive
blood sampling from an indwelling venous catheter after the consumption of
cream and Vitamin A. The low dose of heparin used for the LPL test will not
cause any side effects. We believe the information gathered from this study
outweighs the burden of these interventions.