Hyperglycinemia in captive-bred vervet monkeys with cataracts: genetic dynamics and associations

Abstract

A small percentage (8%) of the captive-bred vervet monkeys (Chlorocebus aethiops) maintained at Primate Unit and Delft Animal Centre (PUDAC) of the South African Medical Research Council (SAMRC) were found to have high levels of glycine in their plasma (457795 µmol/L) and cerebrospinal fluid (CSF) (7.5-12.7 µmol/L). Additionally, these hyperglycinemic monkeys developed cataracts, a condition which has been previously characterized and reported in this specific colony of captive-bred vervet monkeys. This type of association has never been reported in literature before, therefore, this study will be the first of its kind to be investigated in non-human primates (NHPs). Nonketotic hyperglycinemia (NKH), also known as glycine encephalopathy, is well characterised in humans. The symptoms are exclusively neurological in nature, and clinically patients are diagnosed with abnormally high glycine levels in plasma (normal <350 µmol/L) and CSF (normal range 0-8). This neurological disorder is transmitted in an autosomal recessive form and is mainly instigated by a defective glycine cleavage system (GCS). In contrast to GCS, glycine transporter (GlyT1) which regulates glycine concentration at synapses and valproate administration have been associated with NKH. In this study, it was hypothesized that a correlation exists between hyperglycinemia and cataract in vervet monkeys. Since there is a close genetic relatedness between humans and NHPs, underlying genetic factors associated with cataract and hyperglycinemia in vervet monkeys are similar to those found in humans. Hence, genes that are implicated in cataract and NKH in humans were considered candidate genes in vervet monkeys. Two approaches were followed: (1) The animal intervention approach with valproate, sodium benzoate and dextromethorphan was conducted to compare the effectiveness of the current NKH treatment; (2) Molecular aspects of NKH were investigated using genotyping and gene expression techniques for valproate glucuronidation (UGT1A6 and UGT1A9), GlyT1 (SLC6A9), GCS (AMT and GLDC) and cataract (CRYAA and GCNT2) genes. Based on the findings from the animal intervention study, valproate induction in phase one elevated alkaline phosphatase, phosphate and platelet count. In phase two, the effect of valproate on the aforementioned biochemical parameters were reversed by sodium benzoate and dextromethorphan. The treatment was more effective in reducing glycine levels in plasma and CSF of the spontaneous group. Furthermore, the genotyping results for UGT1A6 revealed four missense variants, three silent variants in UGT1A9 and one silent variants in SLC6A9, and these sequence changes were not identified in the control group. Therefore, it is possible that these sequence variants played a role in valproate metabolism during the intervention study. A similar observation was made between mutated spontaneous individuals compared to the controls, and the results showed that both AMT and SLC6A9 genes were down-regulated in the spontaneous group. Therefore, AMT and SLC6A9 gene expression confirmed that there is a link between cataract formation and hyperglycinemia. The findings of the study conclusively suggest that a combination of drug therapy of sodium benzoate and dextromethorphan can be considered as treatment for normalizing glycine levels in plasma and CSF. Additionally, GCS and GlyT1 sequence variants may be responsible for the spontaneous hyperglycinemia in captive-bred vervet monkeys.