Lowe syndrome is an X-linked disorder that has a complex phenotype that includes progressive renal failure and blindness. The disease is caused by mutations in an inositol polyphosphate 5-phosphatase designated OCRL. It has been shown that the OCRL protein is found on the surface of lysosomes and that a renal tubular cell line deficient in OCRL accumulated substrate phosphatidylinositol 4,5-bisphosphate. Because this lipid is required for vesicle trafficking from lysosomes, we postulate that there is a defect in lysosomal enzyme trafficking in patients with Lowe syndrome that leads to increased extracellular lysosomal enzymes and might lead to tissue damage and contribute to the pathogenesis of the disease. We have measured seven lysosomal enzymes in the plasma of 15 patients with Lowe syndrome and 15 age-matched male controls. We find a 1.6- to 2.0-fold increase in all of the enzymes measured. When the data was analyzed by quintiles of activity for all of the enzymes, we found that 95% of values in the lowest quintile come from normal subjects whereas in the highest quintile 85% of the values are from patients with Lowe syndrome. The increased enzyme levels are not attributable to renal insufficiency because there was no difference in enzyme activity in the four patients with the highest creatinine levels compared with the six patients with the lowest creatinine values.

Lowe syndrome is a disorder characterized by cognitive impairment with a discrete behavioral phenotype including temper tantrums, irritability, complex repetitive behaviors, and unusual mannerisms. Cataracts and severe visual impairment are present at birth, and most patients are blind eventually (1, 2). There is severe renal fanconi syndrome with progressive renal impairment. Growth failure is a prominent feature of the disorder, with most boys attaining a height of less than 5 feet. The disease is caused by mutations in an X-linked gene that encodes the OCRL (oculocerebrorenal:Lowe) protein (3–8). OCRL is an inositol polyphosphate 5-phosphatase that hydrolyzes all of the substrates of the enzymes in this family, including the inositol phosphates inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate and the inositol lipids phosphatidylinositol 4,5-bisphosphate [PtdIns(4, 5)P₂] and phosphatidylinositol 3,4,5-trisphosphate (9). A study using extracts from a renal proximal tubule cell line suggests that OCRL is mainly a lipid phosphatase (10). Thus, cultured cells from a patient accumulate PtdIns(4,5)P₂ despite the fact that these cells contain several other isozymes of inositol polyphosphate 5-phosphatase. This result implies that the different isozymes have discrete functions that are nonredundant. In renal proximal tubule cell lines from normal subjects, OCRL was shown to be localized to lysosomes by immunofluorescence (10). In the absence of OCRL, lysosomes may therefore accumulate PtdIns(4, 5)P₂. PtdIns(4, 5)P₂ is important for the formation of clathrin coated vesicles on lysosomes (11). Thus, in the presence of excess PtdIns(4, 5)P₂ in lysosomal membranes, there may be aberrant or increased membrane trafficking. We postulate that, in the case of Lowe syndrome, there may be mistrafficking of lysosomal enzymes, resulting in increased extracellular release of lysosomal enzymes. In the face of cells being continuously bathed in an excess of such hydrolytic enzymes, there may be tissue damage that contributes to the phenotype seen in Lowe syndrome. To evaluate this hypothesis, we have measured a panel of seven lysosomal enzymes in the plasma of boys with Lowe syndrome and age-matched male controls.

In preliminary studies, we measured lysosomal enzyme activity in fresh control plasma, in plasma frozen at −20°C for up to 4 months, and in plasma stored overnight at room temperature. There were no significant differences in the results obtained from these sources, and, thus, we instituted a study. We have measured seven lysosomal enzymes in 15 boys with Lowe syndrome and 15 age-matched male controls. All assays were done by the fluorometric measurement of released 4-methyl umbelliferone from the appropriate substrate. We measured β-d-glucuronidase, α-l-fucosidase, α-d-mannosidase, N-acetyl-β-d-glucosaminidase, N-acetyl-α-d-glucosaminidase, β-d-galactosidase, and α-d-galactosidase. The levels of all seven enzymes were increased in Lowe syndrome plasma from 1.6- to 2-fold, as shown in Fig. Fig.1.1. All but one of the increases were statistically significant, as determined by Student’s t test. The P values obtained ranged from P < 0.1 for N-acetyl-α-d-glucosaminidase to P < 0.001 for N-acetyl-β-d-glucosaminidase. The difference between Lowe syndrome patients and controls is more strikingly demonstrated when all of the assay values were pooled and ranked in quintiles from lowest to highest values, as shown in Fig. Fig.2.2. In this case, the lowest quintile is almost all from controls with decreasing representation in each succeeding quintile. The converse is seen in Lowe syndrome, with few values in the lowest quintile and increasing numbers of values in the succeeding quintiles. We also measured plasma creatinine levels and plotted them vs. age to compare renal function in Lowe syndrome with that in controls. The normal increase in creatinine levels with age is defined by the equation creatinine = 0.35 + 0.03 × (age in years) as shown in Fig. Fig.3.3. It is apparent that the slope of the increase is steeper in Lowe syndrome patients, indicating the slow progression of renal function impairment with age that has been reported previously (1). We considered the possibility that the high level of lysosomal enzymes in plasma from patients with Lowe syndrome might result from mild renal insufficiency. Thus, we compared the levels of enzyme activity in the six boys with the lowest creatinine values [x = 0.47 +/−0.05 (SD)] with the four with the highest values [x = 1.15 +/− 0.21 (SD)]. As shown in Fig. Fig.4,4, there was no difference between the enzyme levels that could be related to creatinine level. Of interest, it has been reported previously that plasma levels of the lysosomal enzyme acid phosphatase are markedly elevated in Lowe syndrome (1). In an effort to further elucidate the mechanism for increased lysosomal enzyme release in Lowe syndrome, we studied the uptake and release of β-glucuronidase in cultured renal tubular cell lines from a patient with Lowe syndrome compared with lines from control subjects. We could not detect any significant differences in either uptake or release of enzymes by these cell lines. This may reflect the fact that these cell lines do not mimic the physiological situation in the patients.