4 Mutations in the heparan sulfate backbone elongating enzymes EXT1 and EXT2 have no major effect on endothelial glycocalyx and the glomerular filtration barrier 69 collected. Participants were asked to perform one site visit, during which a glycocalyx measurement was performed and urine samples were collected. Glycocalyx measurements Sublingual microvasculature was visualised non-invasively using a sidestream–darkfield MicroScan Video Microscope (MicroVision Medical, Inc., Wallingford, PA), connected to GlycocheckTM acquisition and analysis software (Microvascular Health Solutions Inc., Salt Lake City, UT, USA) to automatically acquire microvascular video recordings after predefined image quality criteria (motion, intensity, and focus) are fulfilled. Each complete measurement consists of at least ten 2-second videos (40 frames/video), containing a total of about 3000 vascular segments of 10μm length each. During the measurement, the camera is moved to different positions under the tongue to obtain different sublingual microvascular sites. The vascular segments are automatically subjected to a quality check by the GlycoCheck™ software, discarding invalid segments. Next, the software obtains up to 840 radial intensity profiles for each valid vascular segment and, based on the RBC column width (RBCW) it automatically groups vessels from 4 to 25 μm diameter in 22 separate diameter classes (1μm each). Data from two complete measurements were extracted, analyzed and averaged offline to avoid sampling error and to counterbalance spatial-temporal heterogeneity of the sublingual microcirculation. These analyses resulted in the following parameters: perfused capillary density; capillary blood volume; dynamic perfused boundary region (PBR). PBR is a measure for glycocalyx quality by measuring dynamic lateral RBC movement into the glycocalyx: a larger PBR reflects a disturbance of the glycocalyx (5, 68). Detailed information about the video acquisition and software used is described elsewhere (68). Urinary albumin / protein excretion Spot urine samples were collected during the site visit and analysed for albumin, total protein, and creatinine at the LUMC department of Clinical Chemistry. Albumincreatinin ratio (ACR) and protein-creatinin ratio (PCR) were used to identify the presence of (micro)albuminuria and (micro)proteinuria according to the KDIGO CKD guideline (2). Historic PALGA cohort selection The Dutch pathology national automated archive (PALGA) was searched for cases containing both the search terms ‘kidney and biopsy’ and ‘osteochondroma and multiple osteochondromas’, to identify potential MO patients who have undergone a renal biopsy
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