Lactase Deficiency, Congenital (COLACD) Cell Models for Research

Congenital lactase deficiency (COLACD) is an ultra-rare autosomal recessive disorder characterized by absent or severely reduced lactase-phlorizin hydrolase (LPH) activity from birth. The exact global incidence is unknown, but it is estimated to affect fewer than 1 in 1,000,000 live births. Cases have been reported worldwide, with a higher prevalence in Finland, where the founder mutation c.4177C>T (p.Arg1393Ter) accounts for most cases. Without early diagnosis and dietary intervention (lactose-free formula), affected infants develop severe osmotic diarrhea, dehydration, and failure to thrive, which can be life-threatening. With prompt treatment, prognosis is excellent, and patients can lead normal lives on a lactose-restricted diet. However, long-term complications may include nutritional deficiencies and metabolic bone disease if dietary management is suboptimal. The disease serves as a model for understanding intestinal enzyme deficiencies and the molecular basis of disaccharidase disorders.

COLACD is an ideal model for studying the molecular mechanisms of lactase expression, processing, and function. The disease is caused by mutations in the LCT gene, which encodes the lactase enzyme. Research on COLACD can elucidate genotype-phenotype correlations, the role of specific LCT mutations in enzyme activity, and the regulation of intestinal gene expression. Additionally, because lactase is a brush-border enzyme, COLACD provides a system to investigate protein trafficking, glycosylation, and membrane anchoring. Public datasets, such as those in ClinVar and gnomAD, provide variant frequencies and pathogenicity annotations. Open questions include the identification of modifier genes, the impact of epigenetic factors, and the development of enzyme replacement or gene therapy approaches. Gene-edited cell models, such as LCT knockout or knock-in lines, are essential tools for functional validation of variants and for drug screening aimed at enhancing residual enzyme activity.

The primary mechanism is loss-of-function mutations in the LCT gene, leading to absent or non-functional lactase enzyme. The pathogenic mechanisms include:

• • Nonsense mutations: Introduce premature stop codons, leading to truncated proteins that are degraded by nonsense-mediated decay (e.g., p.Arg1393Ter).
• • Missense mutations: Alter amino acid residues critical for enzyme activity or protein folding (e.g., p.Gly232Arg, p.Thr666Ile).
• • Splice-site mutations: Disrupt normal mRNA splicing, resulting in aberrant transcripts (e.g., c.1993-2A>G).
• • Frameshift mutations: Cause a shift in the reading frame, producing non-functional proteins (e.g., c.1345delC).

These mutations result in reduced or absent lactase activity in the intestinal brush border, leading to lactose malabsorption and the clinical symptoms.

Data from ClinVar and literature. The LCT gene is the only gene known to cause COLACD. The frequency of specific mutations varies by population; for example, the Finnish founder mutation accounts for ~90% of cases in Finland.

Since COLACD is a monogenic disorder, the primary deregulated pathway is the lactase enzyme activity itself. However, secondary effects include:

• • Intestinal osmotic stress: Undigested lactose draws water into the gut, activating signaling pathways related to fluid secretion and inflammation.
• • Microbiome alterations: Unabsorbed lactose is fermented by gut bacteria, producing gas and short-chain fatty acids, which can influence immune signaling.
• • Nutritional deficiency: Chronic diarrhea leads to malabsorption of other nutrients, affecting metabolic pathways.

At the molecular level, the LCT gene is regulated by the transcription factors CDX2, HNF1α, and GATA factors, which are involved in intestinal development. Disruption of lactase activity does not directly affect these pathways, but the disease phenotype is a consequence of enzyme loss.

Caco-2 cells are the most commonly used model for intestinal absorption studies due to their ability to differentiate into enterocyte-like cells. However, they are cancerous and may not fully represent normal intestinal physiology. Organoids derived from intestinal stem cells offer a more physiologically relevant model, as they can be generated from patients with COLACD or edited using CRISPR to introduce specific LCT mutations. Organoids can be cultured in 3D and differentiated to express brush-border enzymes, making them valuable for studying lactase function and testing therapeutic strategies.

Animal models for COLACD are limited, but include:

• • Lct knockout mice: Generated by homologous recombination to disrupt the mouse Lct gene. These mice exhibit lactose intolerance and serve as a model for the disease.
• • Patient-derived xenografts (PDX): Not applicable for COLACD, as it is not a cancer.
• • Genetically engineered mouse models (GEMM): Knock-in mice carrying specific human LCT mutations (e.g., p.Arg1393Ter) can be created to study the effects of mutations in vivo.
• • Induced models: Chemical induction of lactose intolerance is not feasible; however, dietary manipulation can mimic symptoms in wild-type mice.

These models are useful for studying the pathophysiology and testing gene therapy approaches, but they are time-consuming and costly. Cell-based models are often preferred for initial screening.

CRISPR-based gene editing enables the creation of isogenic cell lines with specific LCT mutations, providing powerful tools for functional studies. Examples include:

• • LCT knockout cell lines: Using CRISPR-Cas9 to introduce frameshift mutations in the LCT gene, resulting in complete loss of lactase expression. These lines can be used to study the consequences of enzyme absence and to screen for compounds that upregulate alternative enzymes.
• • LCT knock-in cell lines: Introducing specific patient mutations (e.g., p.Arg1393Ter) into wild-type cells to create disease-relevant models. These lines allow the study of mutation-specific effects on protein expression, trafficking, and activity.
• • Reporter lines: Engineering cells with a fluorescent reporter under the control of the LCT promoter to monitor gene expression in real-time.

Commercially available, sequence-verified gene-edited cell lines accelerate research by providing consistent and validated models. These models are essential for drug discovery, target validation, and functional genomics studies.

Gene-edited cell lines are used to validate the functional impact of LCT variants. For example:

• • Knockout lines can be used to confirm that loss of LCT is sufficient to cause lactase deficiency.
• • Knock-in lines with specific mutations can be compared to wild-type to assess the effect on enzyme activity, protein stability, and cellular localization.
• • CRISPR screens can be performed in LCT knockout backgrounds to identify genes that compensate for lactase loss or that are involved in lactose metabolism.

These approaches help elucidate the molecular basis of COLACD and identify potential therapeutic targets.

Isogenic cell lines are invaluable for drug screening:

• • High-throughput screening: LCT knockout or knock-in lines can be used to screen for compounds that increase lactase activity or upregulate alternative disaccharidases.
• • Resistance modeling: In the context of enzyme replacement therapy, cells can be used to test the efficacy of recombinant lactase formulations.
• • Gene therapy testing: CRISPR-corrected cell lines can be used to evaluate the efficiency of gene editing approaches, such as homology-directed repair to correct disease-causing mutations.

These applications are critical for developing new treatments for COLACD.

Gene-edited cells can be used to discover biomarkers for disease severity or response to therapy. For example:

• • Transcriptomic analysis of LCT knockout vs. wild-type cells can identify differentially expressed genes that may serve as biomarkers.
• • Proteomic profiling can reveal changes in protein expression related to lactase deficiency.
• • CRISPR synthetic lethality screens can identify genes that are essential in LCT-deficient cells, which could be targeted therapeutically.

These biomarkers may help monitor disease progression and treatment efficacy in patients.