Ornithine transcarbamylase (OTC) deficiency is an X-linked disorder that causes recurrent and life-threatening episodes of hyperammonemia. The clinical picture in heterozygous females is highly diverse and derives from the genotype and the degree of inactivation of the mutated X chromosome in hepatocytes. Here, we describe molecular genetic, biochemical, and histopathological findings in the livers explanted from two female patients with late-onset OTC deficiency. Analysis of X-inactivation ratios by DNA methylation-based assays showed remarkable intra-organ variation ranging from 46:54 to 82:18 (average 70:30, n = 37), in favor of the active X chromosome carrying the mutation c.583G>C (p.G195R), in the first patient and from 75:25 to 90:10 (average 82:18, n = 20) in favor of the active X chromosome carrying the splicing mutation c.663+1G>A in the second patient. The X-inactivation ratios in liver samples correlated highly with the proportions of OTC-positive hepatocytes calculated from high-resolution image analyses of the immunohistochemically detected OTC in frozen sections that was performed on total area > 5 cm2. X-inactivation ratios in blood in both female patients corresponded to the lower limit of the liver values. Our data indicate that the proportion of about 20-30% of hepatocytes expressing the functional OTC protein is not sufficient to maintain metabolic stability. X-inactivation ratios assessed in liver biopsies taken from heterozygous females with X-linked disorders should not be considered representative of the whole liver.

Department of Clinical and Transplant Pathology Institute for Clinical and Experimental Medicine Prague Czech Republic

Department of Pediatrics and Adolescent Medicine 1st Faculty of Medicine Charles University and General University Hospital Prague Prague Czech Republic

Laboratory of Experimental Hepatology Institute for Clinical and Experimental Medicine Prague Czech Republic

Research Unit for Rare Diseases Department of Pediatrics and Adolescent Medicine 1st Faculty of Medicine Charles University and General University Hospital Prague Ke Karlovu 2 128 08 Prague 2 Czech Republic

Research Unit for Rare Diseases Department of Pediatrics and Adolescent Medicine 1st Faculty of Medicine Charles University and General University Hospital Prague Ke Karlovu 2 128 08 Prague 2 Czech Republic Institute of Histology and Embryology 1st Faculty of Medicine Charles University Prague Czech Republic

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