Delayed Sleep Phase Disorder (DSPD) Cell Models for Research

Delayed Sleep Phase Disorder (DSPD) is a circadian rhythm sleep-wake disorder characterized by a persistent delay in the timing of the major sleep episode relative to desired or socially acceptable times. The prevalence in the general population is estimated at 0.15% to 3%, with higher rates among adolescents and young adults. According to the World Health Organization (WHO), sleep disorders affect a significant portion of the global population, though specific DSPD statistics are limited. DSPD can lead to chronic sleep deprivation, impaired daytime functioning, and increased risk of comorbidities such as depression and metabolic disorders. The National Cancer Institute (NCI) does not track DSPD as it is not a cancer, but the disorder's impact on quality of life and productivity is substantial. Research into DSPD is crucial for developing effective treatments and understanding circadian biology.

DSPD serves as an excellent model for studying circadian rhythm regulation and its genetic underpinnings. The disorder has a strong genetic component, with mutations in core clock genes (e.g., PER3, CLOCK, CRY1) implicated. Public datasets such as the UK Biobank and the National Sleep Research Resource provide valuable phenotypic and genetic data. Open questions include the molecular mechanisms linking genetic variants to circadian misalignment and the development of targeted therapies. Gene-edited cell models allow researchers to dissect these pathways in vitro, offering a controlled system to study gene function and screen potential drugs.

The circadian clock is governed by interconnected transcriptional-translational feedback loops. Key pathways include:

• • Core Clock Loop: CLOCK and BMAL1 heterodimerize and activate transcription of Period (PER) and Cryptochrome (CRY) genes. PER and CRY proteins accumulate, dimerize, and translocate to the nucleus to inhibit CLOCK-BMAL1 activity, creating a negative feedback loop.
• • Stabilizing Loop: The nuclear receptors REV-ERBα/β and RORα/γ regulate BMAL1 expression. REV-ERBs repress BMAL1 transcription, while RORs activate it, providing additional stability.
• • Post-translational Regulation: Casein kinase 1 (CK1) isoforms phosphorylate PER proteins, marking them for degradation. Mutations in CK1 or PER phosphorylation sites can alter period length.

Genetic variants associated with DSPD are often rare but can be identified in familial cases. The following table summarizes key genes implicated in DSPD based on ClinVar and literature (frequencies are approximate and may vary by population):

DSPD-associated mutations disrupt the circadian network, leading to downstream effects on multiple signaling pathways:

• • Circadian Output Pathways: Altered clock gene expression affects downstream targets such as melatonin synthesis, cortisol secretion, and body temperature regulation.
• • Neurotransmitter Systems: Circadian disruption impacts dopaminergic and serotonergic signaling, which may contribute to mood disorders often comorbid with DSPD.
• • Metabolic Pathways: Clock genes regulate glucose and lipid metabolism; dysregulation can lead to metabolic syndrome.
• • Immune Function: Circadian genes modulate immune responses; disruption may increase susceptibility to infections and inflammatory diseases.

Common cell lines used in circadian research include:

Organoids, such as brain organoids, offer a more physiologically relevant model to study circadian rhythms in a 3D context, though they are more complex to maintain.

Animal models for DSPD are limited, but several approaches exist:

• • Genetically Engineered Mouse Models (GEMMs): Mice with mutations in clock genes (e.g., ClockΔ19, Per2Brdm1) exhibit circadian phenotypes and are used to study mechanisms.
• • Induced Models: Pharmacological or light-induced phase shifts can model DSPD in rodents.
• • Patient-Derived Xenografts (PDX): Not applicable for DSPD as it is not a cancer, but xenografts of human cells can be used to study circadian rhythms in vivo.

CRISPR-based gene editing enables the creation of isogenic cell lines with precise mutations in clock genes. For example:

• • PER3 Knockout Cell Line: Complete loss of PER3 function to study its role in circadian period regulation.
• • CLOCK Point Mutation Knock-In: Introduction of a specific DSPD-associated mutation (e.g., p.Arg362Trp) to assess its effect on circadian transcription.
• • CRY1 Splice Mutation Knock-In: Mimicking a splice-site mutation to study altered CRY1 isoforms.

These models are sequence-verified and can be generated in various cell backgrounds (e.g., U2OS, HEK293). Commercially available, they accelerate research by providing consistent, reproducible tools for functional studies and drug screening.

Knockout and knock-in lines are essential for validating the function of genes implicated in DSPD. For example, a PER3 knockout line can be used to assess changes in circadian period using bioluminescent reporters. Similarly, a CLOCK mutant knock-in line can reveal how the mutation alters transcriptional activity. These models enable high-throughput screening of genetic interactions and pathway mapping.

Isogenic pairs (wild-type vs. mutant) are powerful for drug screening. For DSPD, compounds that modulate circadian period or phase can be tested for their ability to correct the mutant phenotype. Resistance studies can identify compensatory mechanisms that cells employ to maintain circadian rhythms in the presence of mutations.

CRISPR-based synthetic lethality screens can identify genes that are essential in the context of specific clock mutations. For example, a screen in a CLOCK mutant background may reveal kinases that compensate for altered phosphorylation, providing potential therapeutic targets. Additionally, gene-edited cells can be used to identify biomarkers of circadian disruption for diagnostic purposes.