Tolerability and Efficacy of L-Serine in Patients With GRIN-related Encephalopathy
Tolerability and Efficacy of L-Serine in Patients With GRIN-related Encephalopathy
GRIN-related disorders encompass a new group of Inborn Errors of Metabolism according to the
 recent nosology published by Ferreira et al (Genet Med, 2019).

 These rare conditions represent a subtype of paediatric encephalopathies leading to
 intellectual disability, hypotonia, communication deficits and motor impairment (Orphanet
 entries: 178469, 289266, 101685, for GRIN1, GRIN2A and GRIN2B, respectively).

 Mutations leading to glutamatergic hypotransmission can be potentially treated with L-Serine
 leading to significant clinical benefits in patients according to a pilot study published by
 our group (Soto et al, 2019).

 In our study, the investigators will include about 20 spanish patients older than 2 years of
 age, harbouring GRIN variants functionally anotated as loss-of-function pathogenic variants.
 The investigators will evaluate dose tolerability, efficacy of the treatment according to
 neurocognitive and motor scales, as well as the effects of L-serine in microbiome
 composition.
GRIN-related disorders, a novel group of IEM (inborn errors of metabolism): Mutations on GRIN
 genes, encoding for the N-methyl-D-Aspartate receptor (NMDAR) subunits, have been recently
 associated with autosomic dominant GRIN-related disorders (Lemke et al., 2016; Platzer et
 al., 2017; XiangWei et al., 2018; Strehlow et al., 2019). These rare conditions represent a
 subtype of paediatric encephalopathies leading to intellectual disability, hypotonia,
 communication deficits and motor impairment (Orphanet entries: 178469, 289266, 101685, for
 GRIN1, GRIN2A and GRIN2B, respectively).

 There are about 200 patients affected with these diseases in Europe according to a data
 collection study conducted by the patients' families and presented at the first European
 meeting of GRIN disorders in October 2018 (Barcelona, Hospital Sant Joan de Déu).

 Along the last years, our research efforts were focused to delineate the pathophysiology and
 personalized therapies for these diseases that are now considered as a new category of IEM.
 In particular, the recent nosology of inherited metabolic disorders (Ferreira et al, 2018)
 classifies GRIN-related disorders in the category: "DISORDERS OF NITROGEN CONTAINING
 COMPOUNDS", and the subgroup: "20. Disorders of glutamate metabolism":

 GRIN1 (OMIM138249): Ionotropic glutamate receptor NMDA type subunit 1 dysregulation (AD, AR):
 Autosomal dominant mental retardation type 8; neurodevelopmental disorder with or without
 hyperkinetic movements and seizures. GRIN2A (OMIM138253): Ionotropic glutamate receptor NMDA
 type subunit 2A dysregulation (AD): Non related to human disease so far. GRIN2B (OMIM138252):
 Inotropic glutamate receptor NMDA type subunit 2B dysregulation (AD): Early infantile
 epileptic encephalopathy type 27; autosomal dominant mental retardation type 6. GRIN2D
 (OMIM602717): Ionotropic glutamate receptor NMDA type subunit 2D superactivity (AD): Early
 infantile epileptic encephalopathy type 46.

 Indeed, in a proof-of-concept, the investigators have determined that a GRIN2B
 loss-of-function variant can be rescued in vitro, by means of elevated doses of D-serine
 (NMDAR coagonist) administration. More importantly, the investigators showed that L-serine
 (the D-serine natural precursor) dietary supplement was associated with a significant
 improvement of motor and communication skills of the patient harbouring GRIN2B hypofunctional
 variant (Soto et al., 2019).

 Serine enantiomers: cognitive enhancement and potential use in GRIN-related disorders
 L-serine (C3H7NO3; 105.09 g/mol; synonym (S)-2-amino-3-hydroxypropanoic acid) is a
 naturally-occurring dietary amino acid. It is abundant in soy products, some edible seaweeds,
 sweet potatoes, eggs, and meat. Since some L-serine is produced by astrocytes in the brain,
 it is considered a non-essential amino acid. L-serine is directly involved in the
 biosynthesis of purines, pyrimidines, and other amino acids. Serine residues are found in
 most proteins and within proteins function as a site for phosphorylation.

 L-serine is considered as GRAS (generally recognized as safe) by the FDA and has been
 approved as a normal food additive under CFR172.320. It is widely sold as a dietary
 supplement. A randomized trial of L-serine in 18 patients with hereditary sensory and
 autonomic neuropathy type 1 has been published (ClinicalTrials.gov identifier NCT01733407).
 High-dose oral L-serine supplementation (400 mg/kg/day) appears safe in patients with HSAN1
 and is potentially effective at slowing disease progression.The authors did not report
 adverse effects at doses of 400mg/kg/day (Eichler et al) Further, L-serine dietary supplement
 has been used in Paediatrics at doses of 400 to 600 mg/kg/day since several decades ago for
 the treatment of a group of inherited metabolic disorders, namely: 3-phosphoglycerate
 dehydrogenase deficiency, phosphoserine aminotransferase deficiency and phosphoserine
 phosphatase deficiency, with no reported adverse effects. In patients with 3-phosphoglycerate
 dehydrogenase deficiency, neither 100 nor 200 mg/kg per day had any effects on the patients'
 symptoms or on CSF serine and glycine concentrations (de Koning et al 1998). Only with
 L-serine 500 mg/kg per day was a reduction in seizure activity noted and an increase in
 serine CSF concentrations observed (Koning 2006).

 D-serine, the natural coagonist of the NMDA receptor, results from the racemisation of
 L-serine in brain, and its biosynthetic alteration can lead to neuronal dysfunction (van de
 Crabben et al., 2013). In addition to its homeostatic role in neuronal function, the
 beneficial effect of D-serine supplement has been recently shown in healthy individuals
 (Levin et al., 2015). Heresco-Levy's group showed the procognitive effects of D-serine
 throughout NMDA receptor function. D-serine deficits have been associated with aging in rats,
 with a functional rescue observed following D-serine administration (Turpin et al., 2011;
 Billard, 2015).

 Additionally, L-serine biosynthesis defects (de Koning, 2006) cause neurological phenotypes
 (psychomotor retardation, microcephaly, seizures) that can be safely treated by L-serine.

 Moreover, and as already mentioned, our group has recently described the beneficial effect of
 a chronic L-serine dietary (500 mg/kg/per day) supplement in a 5 years GRIN2B patient (Soto
 et al., 2019). The patient has shown notable improvements in motor and cognitive performance
 and communication after 11 and 17 months of L-serine dietary supplementation. These data
 suggest that L-serine supplementation might ameliorate GRIN2B-related severe encephalopathy
 and other neurological conditions caused by glutamatergic signaling deficiency.

 Besides this direct effect, D-serine can prevent behavioral abnormalities in adult mice
 challenged by maternal immune activation (Fujita et al., 2016), and induces hippocampal
 neurogenesis (Sultan et al., 2015).

 These evidences indicate that 500 mg/kg/per day is a safe and effective dose, that could be
 used in patients with GRIN related disorders.

 HYPOTHESIS Overall, these evidences indicate that, independently on the molecular aetiology,
 NMDAR activity potentiation throughout L-serine dietary supplement can ameliorate
 glutamatergic function and improve the life quality of children suffering from GRIN-related
 disorders, a novel IEM. Dietary supplement of L-serine results on increased D-serine plasma
 levels and potentiates NMDA receptors leading to NMDA receptors functionality increase and
 hypofunctionality rescue, as shown in a pilot study (Soto et al., 2019).

 AIM The purpose of this study is to determine L-Serine dietary supplement efficacy for the
 treatment of patients with GRIN-related disorders caused by the presence of GRIN genetic
 variants leading to hypofunctional (loss-of-function) NMDA receptors.
Inclusion Criteria:

 -  Diagnosis of encephalopathy associated with genetic GRIN variant pathogenic or likely
 pathogenic, causing a loss-of-function

 -  Parent(s)/legal representative give written informed consent for participation in the
 trial; patient assent (if possessing adequate understanding, in the investigator's)

 -  Patient & caregiver are willing and able (in the investigator's opinion) to comply
 with all trial requirements (including the completion of all caregiver assessments by
 the same caregiver throughout the trial).

 Exclusion Criteria:

 -  Diagnosis of encephalopathy with absence of genetic GRIN variant pathogenic or likely
 pathogenic.

 -  Currently using or has used within the 30 days prior to screening L-Serine supplement

 -  Patient has been previously randomized into this trial