We presented a long-term follow-up of a patient with a homozygous PRODH gene mutation and mild clinical manifestations who showed an important decline in plasma proline levels and a slight improvement in clinical symptoms with an increase in intelligence scores after antioxidant therapy.
The type of the mutation or the length of deletion can determine the clinical variability and severity of the disease and the levels of serum proline. High serum proline levels are related to serious clinical findings, such as epilepsy, severe neuromotor retardation, and behavioral problems [10]. The presence of mild and atypical clinical symptoms, such as speech disturbance, mild neuromotor development delay, attention deficit, and learning disability, can be related to mildly elevated proline levels. The proline levels of our patient are moderately high and may be associated with mild clinical findings and even good therapy response.
NM_016335.4:c.1357C>T (p. Arg453Cys) (rs3970559) mutation is a frequent variant in the population (gnomAD Exomes: 0.01065). The UniProt database classifies this variant as a disease-causing mutation for both HPI and schizophrenia. The Dann Score (0.9985), Mutation taster, FATHMM-MKL, LRT, Mutation Assessor, SIFT and Provean tools predict this variant as a pathogenic variant, while others predict it as a benign variant (FATHMM, MetaSVM, MetalR). There are conflicting reports in the ClinVar database from benign variant to pathogenic [3]. As our case does not have a classical clinical phenotype, unlike previous reports, this variant is most likely patological variant but does not cause severe clinical symptoms. The mother of our patient also has mild developmental delay, learning difficulties, and cranial MRI with white matter involvement and the same genotype as her daughter. We see that the older sister and the father, who have no clinical findings and whose proline levels are within normal limits, are heterozygous.
Although the exact pathophysiologic mechanism is not yet known, several mechanisms have been suggested to explain how high proline concentrations can affect brain functions. In the literature, the most commonly emphasized and proposed mechanism is the alteration in glutamatergic homeostasis. Glutamate is an excitatory neurotransmitter that is important in normal brain function. Glutamate excitotoxicity has been shown to be linked with mitochondrial dysfunction, with energy impairment and subsequent partial membrane depolarization with resultant relief of magnesium blockage of the N-methyl-D-aspartate channel, which results in excessive influx of Ca+2. It has been suggested that hyperprolinemia by causing a reduction in glutamate reuptake and Na+, K+ ATPase activity leads to glutamatergic excitotoxity, which leads to pathological signaling and in turn contributes to cell injury and death via the production of free radicals [11]. This cell death may lead to cellular damage in the brain.
It has been shown that increased proline levels induce oxidative stress in rat brains [12] as well as alter glutamatergic transmission at hippocampal synapses [13]. Crabtree et al. found that L-proline is a GABA (gamma aminobutyric acid)-mimetic and can act at multiple GABAergic targets, but disease-relevant concentrations lead to disturbances in GABAergic production by blocking GAD blockage. Researchers suggested that this disturbance in the GABAergic system leads to the accumulation of neuroactive metabolites that cause molecular and synaptic dysfunction and finally psychotic disorders. Abnormalities in the GABAergic system have been shown in many neurodevelopmental disorders, such as attention-deficient hyperactivity disorder and schizophrenia [14].
Recent studies mentioned that low PRODH activity not only causes hyperprolinemia but also has additional effects on the electron transport chain (ETC). PRODH directly and indirectly regulates the ETC [10]. Hancock et al found that PRODH binds directly to coenzyme Q1 and that CoQ1-dependent PRODH activity requires functional Complex III and Complex IV, so PRODH supports respiration independent of Complex I and II activity. It has been shown that in the presence of increasing CoQ1, PRODH activity increases in living cell cultures [15]. We have no additional laboratory findings of mitochondrial dysfunction in our case. A negative correlation with proline levels and antioxidant therapy was demonstrated. In addition to laboratory improvements, it can be accepted that there is objective progress in speech disturbance and intellectual disability.
We may speculate that antioxidant therapy (including coenzyme Q, carnitine, B complex) has an important influence on the activity of PRODH, and this treatment approach decreases proline levels and provides optimal acting ETC. Further studies with larger sample sizes are needed to evaluate the clinical and laboratory efficiency of antioxidant therapy in patients with HPI.