Holoprosencephaly 2 (HPE2) Cell Models for Research

Holoprosencephaly (HPE) is a rare congenital brain malformation with an estimated prevalence of 1 in 10,000 to 1 in 20,000 live births, but the incidence may be higher due to early fetal loss. HPE2 is a genetic subtype caused by mutations in the SIX3 gene. The clinical spectrum ranges from severe alobar HPE, which is often fatal, to milder microforms with facial dysmorphism and normal intelligence. According to the National Organization for Rare Disorders (NORD) and Orphanet, HPE2 accounts for a small percentage of HPE cases. The condition poses significant challenges in prenatal diagnosis and genetic counseling. Research into HPE2 is crucial for understanding early brain development and for developing potential therapeutic strategies, though currently there are no disease-modifying treatments.

HPE2 serves as an excellent model for studying forebrain development and the molecular mechanisms underlying midline patterning. The SIX3 gene encodes a homeodomain transcription factor that plays a critical role in the development of the forebrain and eyes. Mutations in SIX3 disrupt the Sonic Hedgehog (SHH) signaling pathway, which is essential for the specification of the ventral forebrain. Research using HPE2 models can provide insights into gene-environment interactions, the role of transcription factors in neurodevelopment, and potential targets for therapeutic intervention. Public datasets, such as those from the Developmental Genotype-Tissue Expression (dGTEx) project and the Allen Brain Atlas, offer valuable resources for studying SIX3 expression and regulation. Open questions include the precise downstream targets of SIX3 and how specific mutations lead to variable phenotypic severity.

Although HPE2 is a developmental disorder, the underlying pathways are relevant to cancer biology due to the role of SHH signaling in both development and tumorigenesis. The major pathways involved in HPE2 pathogenesis include:

1. Sonic Hedgehog (SHH) Signaling Pathway:

• • SHH ligand binds to Patched (PTCH1), relieving inhibition of Smoothened (SMO).
• • SMO activates GLI transcription factors, which regulate target genes involved in cell proliferation and differentiation.
• • SIX3 acts as a repressor of SHH signaling in the forebrain, and its loss leads to overactivation of SHH, causing abnormal patterning.

2. Wnt Signaling Pathway:

• • SIX3 also interacts with Wnt signaling, which is critical for forebrain development.
• • Mutations in SIX3 can disrupt Wnt/β-catenin signaling, leading to impaired cell fate specification.

3. Notch Signaling Pathway:

• • Notch signaling is involved in neurogenesis and is modulated by SIX3.
• • Aberrant Notch signaling may contribute to the neural progenitor defects observed in HPE2.

Data from ClinVar and the Human Gene Mutation Database (HGMD) indicate that SIX3 mutations are the most common cause of HPE2, with over 100 pathogenic variants reported. The frequency of SIX3 mutations in HPE is estimated at 1-2% based on large cohort studies.

The deregulation of signaling networks in HPE2 primarily involves the SHH and Wnt pathways, with crosstalk to other pathways. Key nodes include:

• • SHH pathway: SIX3 represses SHH expression in the rostral diencephalon. Loss of SIX3 leads to ectopic SHH expression, causing abnormal ventralization of the forebrain.
• • Wnt pathway: SIX3 interacts with TCF/LEF transcription factors to modulate Wnt target genes. Mutant SIX3 may fail to repress Wnt signaling, leading to excessive proliferation and impaired differentiation.
• • TGF-β pathway: TGIF1, another HPE gene, interacts with SMAD proteins to regulate TGF-β signaling. Mutations in TGIF1 can disrupt this pathway, contributing to HPE phenotypes.
• • FGF signaling: FGF8 is a target of SIX3 and is essential for forebrain patterning. Altered FGF signaling may contribute to the craniofacial defects seen in HPE2.

Organoids derived from patient iPSCs with SIX3 mutations provide a three-dimensional model to study the effects of mutations on brain development. These organoids can be used to screen for potential therapeutic compounds that rescue the phenotype.

• • Genetically Engineered Mouse Models (GEMMs):
• • SIX3 knockout mice exhibit severe holoprosencephaly and embryonic lethality, making them valuable for studying early developmental defects.
• • Conditional knockouts allow for tissue-specific deletion to study later stages.
• • Zebrafish Models:
• • Zebrafish with six3b mutations show forebrain defects, providing a high-throughput model for drug screening.
• • Patient-Derived Xenografts (PDX):
• • Not commonly used for HPE2 due to the developmental nature, but xenografts of SIX3-mutant cells could be used to study tumorigenic potential if relevant.

CRISPR-based gene editing has enabled the creation of isogenic cell lines with specific SIX3 mutations, providing powerful tools for functional studies. These models include:

• • SIX3 Knockout Cell Lines:
• • Using CRISPR-Cas9 to introduce frameshift mutations in SIX3 in human cell lines (e.g., SH-SY5Y, HEK293) allows for the study of loss-of-function effects on SHH and Wnt signaling.
• • These lines are commercially available and sequence-verified, ensuring reproducibility.
• • SIX3 Point-Mutation Knock-In Lines:
• • Introducing specific pathogenic missense mutations (e.g., p.R227Q) into wild-type cell lines enables the study of dominant-negative effects.
• • These models are valuable for understanding genotype-phenotype correlations and for drug screening.
• • Reporter Cell Lines:
• • Cell lines with fluorescent reporters under the control of SHH or Wnt responsive elements can be used to monitor pathway activity in real-time.

These gene-edited models accelerate research by providing consistent, genetically defined systems that can be used in high-throughput screening and mechanistic studies.

Gene-edited cell lines are essential for functional genomics studies to validate the role of SIX3 and its downstream targets. For example:

• • Loss-of-function screens: CRISPR knockout libraries can be used to identify genes that interact with SIX3, revealing novel pathways involved in forebrain development.
• • Transcriptomic analysis: RNA-seq on SIX3 knockout cells can identify differentially expressed genes, providing insights into the regulatory networks controlled by SIX3.
• • Protein interaction studies: Using tagged SIX3 in knock-in lines, co-immunoprecipitation followed by mass spectrometry can identify binding partners.

Isogenic cell line pairs (wild-type vs. SIX3 mutant) are powerful tools for drug screening. They can be used to:

• • Identify compounds that rescue the mutant phenotype: For example, screening for small molecules that inhibit SHH signaling in SIX3 knockout cells may identify potential therapeutic leads.
• • Study drug resistance: In cancer contexts, SIX3 mutations may confer resistance to SHH pathway inhibitors. Using isogenic pairs, researchers can test the efficacy of drugs and understand resistance mechanisms.
• • High-throughput screening: The reproducibility of gene-edited cell lines makes them suitable for large-scale compound libraries.

CRISPR-based synthetic lethality screens can identify vulnerabilities in SIX3-mutant cells that could serve as biomarkers or therapeutic targets. For example:

• • Synthetic lethal partners: By knocking out SIX3 and screening for genes whose loss is lethal only in the mutant background, researchers can identify potential drug targets.
• • Biomarker panels: Gene expression signatures from SIX3-mutant cells can be used to develop diagnostic biomarkers for HPE2.
• • Pathway activation markers: Reporter cell lines can be used to measure SHH pathway activity as a pharmacodynamic biomarker in drug development.