Phosphoserine Phosphatase Deficiency (PSPHD) Cell Models for Research

Phosphoserine Phosphatase Deficiency (PSPHD) is an extremely rare autosomal recessive disorder caused by mutations in the PSPH gene. The exact prevalence is unknown, but fewer than 50 cases have been reported worldwide. The condition is characterized by intellectual disability, microcephaly, and psychomotor delay, with symptoms typically presenting in infancy. The disease is not associated with increased mortality, but it significantly impacts quality of life. There is no cure, and treatment is symptomatic, often involving serine supplementation. The rarity of the disease makes it a challenging area for research, but also a critical one for understanding serine metabolism and neurodevelopment.

PSPHD is an ideal model for studying serine biosynthesis and its role in neuronal development. The PSPH gene encodes phosphoserine phosphatase, a key enzyme in the L-serine synthesis pathway. Research on PSPHD can provide insights into metabolic disorders, neurodevelopmental processes, and potential therapeutic targets. Public datasets such as ClinVar and UniProt contain information on PSPH mutations, but functional studies are limited. Gene-edited cell models, such as CRISPR knockout and knock-in lines, are essential for investigating the molecular consequences of PSPH mutations and for screening potential therapeutic compounds.

PSPHD results from impaired L-serine biosynthesis. The pathway involves three enzymes: phosphoglycerate dehydrogenase (PHGDH), phosphoserine aminotransferase (PSAT1), and phosphoserine phosphatase (PSPH). Defects in any of these enzymes lead to serine deficiency, which affects multiple cellular processes.

• • Serine is a precursor for the synthesis of proteins, nucleotides, and phospholipids.
• • Serine is also a precursor for glycine and cysteine, which are important for glutathione synthesis and redox balance.
• • In the central nervous system, serine acts as a neuromodulator and is essential for neuronal survival and function.

Mutations in PSPH lead to reduced enzyme activity, causing a bottleneck in the pathway and subsequent serine depletion. This results in impaired cell proliferation, particularly in rapidly dividing neural progenitor cells, leading to microcephaly and intellectual disability.

Data from ClinVar and UniProt. Most mutations are private and inherited in an autosomal recessive manner. Common mutations include p.Arg183Trp and p.Leu141Pro, which have been reported in multiple families.

Serine deficiency impacts several signaling pathways:

• • mTOR signaling: Serine is required for mTOR activation, which regulates cell growth and proliferation. Deficiency leads to reduced mTOR activity and impaired neurogenesis.
• • One-carbon metabolism: Serine is a major source of one-carbon units for the folate cycle, which is essential for nucleotide synthesis and methylation reactions. Deficiency disrupts DNA synthesis and epigenetic regulation.
• • Redox homeostasis: Serine is a precursor for glutathione, a key antioxidant. Deficiency increases oxidative stress, leading to cellular damage.
• • Neurotransmitter synthesis: Serine is a precursor for glycine and D-serine, which are co-agonists at NMDA receptors. Deficiency affects glutamatergic signaling and synaptic plasticity.

Organoids, particularly cerebral organoids, are valuable for studying neurodevelopmental defects in a 3D context. They can be generated from patient-derived iPSCs or edited with CRISPR to introduce PSPH mutations.

• • Knockout mouse: PspH knockout mice have been generated and show growth retardation, microcephaly, and impaired neurogenesis, recapitulating the human phenotype.
• • Zebrafish models: Zebrafish with pspH knockdown show developmental defects, including reduced head size and impaired motor function.
• • Drosophila models: Drosophila with PSPH knockdown exhibit reduced lifespan and locomotor deficits.

These models are useful for studying the pathophysiology and testing potential therapies, but they are not suitable for high-throughput drug screening. Gene-edited cell models are more practical for such applications.

CRISPR-Cas9 technology enables the generation of isogenic cell lines with specific PSPH mutations. These models are essential for studying the functional consequences of mutations in a controlled genetic background.

• • Knockout lines: Complete loss of PSPH function can be achieved by introducing frameshift mutations. These lines are useful for studying the effects of complete serine deficiency.
• • Knock-in lines: Patient-specific point mutations can be introduced to model the exact genetic defect. These lines are valuable for studying mutation-specific effects and for drug screening.

Commercially available, sequence-verified gene-edited cell lines are available from several suppliers. These models are validated for the absence of off-target effects and are provided with detailed characterization data, accelerating research.

Gene-edited cell lines are used to validate the role of PSPH in serine metabolism and neurodevelopment. For example, PSPH knockout lines can be used to confirm the enzyme's role in the pathway by measuring serine levels and downstream metabolites. Knock-in lines with specific mutations can be used to assess the impact of those mutations on enzyme activity and cellular phenotype. These models are also useful for identifying genetic modifiers and synthetic lethal interactions.

Isogenic cell lines with and without PSPH mutations can be used in high-throughput screens to identify compounds that rescue the serine deficiency phenotype. For example, screening libraries of small molecules for those that increase cell viability or restore serine levels. Additionally, these models can be used to test the efficacy of serine supplementation and other potential therapies. Resistance mechanisms can be studied by exposing cells to increasing concentrations of drugs and selecting for resistant clones.

CRISPR-based screens can identify genes that, when knocked out, exacerbate or rescue the PSPH deficiency phenotype. This can reveal novel biomarkers and therapeutic targets. For example, a synthetic lethal screen in PSPH knockout cells could identify genes that are essential only in the absence of PSPH, providing potential targets for drug development. Additionally, proteomic and metabolomic analyses of gene-edited cells can identify biomarkers for disease progression and treatment response.