Physiological and behavioural characterisation of a novel steroid sulfatase-deficient mouse

This study characterizes a novel *Sts*-deficient mouse model that lacks steroid sulfatase activity and exhibits increased activity, higher normalized heart weights, and sex-specific behavioral differences, providing a valuable tool for investigating the physiological and behavioral consequences of STS deficiency.

The Gist

Imagine your body is a massive, bustling factory. Inside this factory, there are thousands of different machines (enzymes) that take raw materials and process them into useful products. One specific machine, called Steroid Sulfatase (or STS), has a very important job: it acts like a "key remover."

Many hormones in your body arrive at the factory doors with a heavy, waterproof "sulfate" lock on them. They can't do their work until the STS machine snaps that lock off, turning them into active, water-soluble keys that can unlock doors and start processes.

The Problem:
In some people, the blueprints for this "key remover" machine are missing or broken. This condition is called STS Deficiency. In humans, it causes a skin condition called X-linked Ichthyosis (scaly skin), but it also seems to be linked to brain issues (like ADHD or anxiety) and heart problems.

The Challenge:
Scientists wanted to study this in mice to understand why these problems happen. But there was a catch: the gene for this machine is located in a very tricky, repetitive part of the mouse DNA. It's like trying to cut out a single specific page from a book where every page looks exactly the same. Previous attempts to create a "broken machine" mouse failed because the genetic scissors kept cutting the wrong pages.

The Solution (The New Mouse):
The team in this paper, led by Trevor Humby and William Davies, used a high-tech tool called CRISPR (think of it as molecular "Find and Replace" software) to precisely snip out the broken part of the gene in a new batch of mice. They created a mouse that completely lacks this "key remover" machine.

What They Found (The Results):

1. The Factory is Quiet: As expected, these new mice had almost zero "key remover" activity in their livers and brains. The machine was effectively gone.
2. They Are Healthy (Mostly): Surprisingly, these mice didn't look sick. They grew up, had babies, and lived normal lifespans. They didn't have the severe health crises you might expect from losing such an important machine.
3. The "Hyper" Mice: When the scientists watched the mice play, they noticed something interesting. The mice without the machine were more active. They ran around more in open fields and explored more.
   • Analogy: Imagine a car with the parking brake slightly released. It doesn't crash, but it rolls a bit faster than the others. This suggests these mice might be a good model for studying ADHD or hyperactivity in humans.
4. The "Brave" vs. "Shy" Twist: Here is where it gets funny. The scientists put the mice in a maze with open, scary arms and safe, enclosed arms.
   • Male mice without the machine acted like brave explorers, running into the scary open arms faster.
   • Female mice without the machine acted like cautious worriers, sticking to the safe corners.
   • Analogy: It's like a group of kids at a playground. The boys without the machine are the ones jumping off the high slide, while the girls are the ones holding onto the railing. This suggests the missing gene affects boys and girls differently, perhaps explaining why anxiety disorders look different in men and women.
5. The Heavy Hearts: The scientists weighed the hearts of the older mice. The hearts of the mice without the machine were heavier relative to their body size.
   • Analogy: It's like an engine that has been running at high RPMs for too long; the muscles get bigger and heavier. This is a warning sign for heart disease, which matches what doctors see in some humans with this condition.
6. The Hormone Mystery: The scientists checked the blood to see if the "keys" were piling up or running out. Surprisingly, the levels of most hormones looked normal. The factory was managing to keep things balanced, even without the key remover. This tells us the problems aren't just about "too much" or "too little" hormone, but perhaps about how the cells react to the missing machine.

The Big Picture:
This paper is like finding a new, perfect test subject for a medical mystery. Before, scientists were trying to study a broken machine using a model that was also missing other important tools (the old mouse models). Now, they have a "clean" model where only the STS machine is missing.

Why does this matter?

• For ADHD: The "hyper" behavior in the mice helps us understand why some people with this genetic issue are impulsive or can't sit still.
• For the Heart: The heavier hearts suggest we should start checking the hearts of people with this skin condition, just in case.
• For Anxiety: The difference between the "brave" males and "shy" females helps explain why anxiety disorders often affect men and women differently.

In short, the scientists built a new, precise "broken machine" mouse. These mice are healthy but act a bit differently, giving scientists a powerful new tool to figure out how to fix the heart and brain problems associated with this condition in humans.

Technical Summary

1. Problem Statement

Steroid sulfatase (STS) is an enzyme responsible for cleaving sulfate groups from steroid hormones (e.g., converting DHEAS to DHEA), thereby regulating their bioavailability and activity. In humans, STS deficiency (caused by deletions or mutations at Xp22.31) leads to X-linked ichthyosis (XLI), a dermatological disorder, and is associated with significant comorbidities including:

• Neurodevelopmental disorders (ADHD, autism, epilepsy).
• Mood disorders and anxiety.
• Cardiac arrhythmias and structural heart abnormalities.

The Gap: Prior to this study, no single-gene "knockout" mammalian model existed to investigate these systemic associations. Previous models were limited to:

1. Skin-only phenotypes: Existing models focused solely on dermatological aspects.
2. The 39,XY*O mouse: A model with an X-Y fusion that lacks STS but also lacks adjacent genes (e.g., Nlgn4), making it impossible to isolate STS-specific effects.
3. Pharmacological inhibition: Acute inhibitor administration does not model developmental loss of function.

The authors aimed to generate and characterize a novel, CRISPR-generated mouse model with a specific deletion in the Sts gene to isolate the physiological and behavioral consequences of STS deficiency.

2. Methodology

Model Generation:

• Strain: C57BL/6J background.
• Modification: A CRISPR-mediated small frameshift deletion in exon 2 of the Sts gene (encoding the sulfatase domain), predicted to cause a premature stop codon.
• Breeding: Heterozygous parents were crossed to generate wildtype (+/+), heterozygous (+/-), and homozygous (-/-) littermates.

Experimental Design:

• Subjects: 92 adult mice (3–5 months) for behavioral testing; 80 mice for tissue collection (including older mice >20 weeks for cardiac analysis).
• Genotyping: Performed via real-time PCR on ear biopsies; confirmed post-mortem.
• Behavioral Paradigms: Conducted in a randomized order with blind scoring where possible:
  • Anxiety/Risk: Elevated Plus Maze (EPM), Open Field Test (OFT).
  • Consummatory/Novelty: Drinking behavior (water vs. 10% milk).
  • Motor/Coordination: Rotarod.
  • Working Memory/Exploration: Spontaneous Alternation (T-maze).
  • Sensorimotor Gating: Acoustic Startle Response (ASR) and Prepulse Inhibition (PPI).
• Physiological & Molecular Assays:
  • Enzyme Activity: Measured in liver and brain tissues.
  • Gene Expression: Quantitative PCR (qPCR) for Sts in liver.
  • Steroidomics: Liquid chromatography-mass spectrometry (LC-MS) of 21 serum steroids.
  • Cardiac Morphometry: Heart weight normalized to bodyweight and tibia length (indices of hypertrophy).

Statistical Analysis:

• Two-way ANCOVA (factors: Genotype, Sex; covariate: Age).
• Non-parametric tests (Mann-Whitney U, Kruskal-Wallis) used where normality assumptions were violated.

3. Key Contributions

1. First Single-Gene Knockout Model: Successfully generated the first viable, single-gene Sts knockout mouse model that avoids the confounding deletion of adjacent genes found in the 39,XY*O model.
2. Validation of Enzyme Loss: Confirmed >99% reduction in STS activity in the liver and >85% reduction in the brain of homozygous mice, validating the model's efficacy.
3. Comprehensive Phenotyping: Provided the first broad characterization of STS deficiency beyond skin pathology, covering general health, breeding, behavior, endocrinology, and cardiac morphology.
4. Sex-Specific Findings: Identified significant genotype-by-sex interactions, suggesting that STS deficiency impacts males and females differently regarding bodyweight and anxiety-like behaviors.

4. Key Results

A. Molecular and Physiological Validation

• Enzyme Activity: Homozygous mice showed near-complete loss of STS activity in both liver and brain.
• Health & Breeding: Mice exhibited normal breeding performance, Mendelian genotype ratios at weaning, and grossly normal health up to 12 months.
• Bodyweight: A significant Genotype × Sex interaction was observed. Homozygous males were ~5% lighter than wildtype males, while homozygous females were ~5% heavier than wildtype females.

B. Behavioral Characterization

• Hyperactivity: Homozygous mice were significantly more active than wildtypes in the Open Field Test (~10% increase in distance moved) and the Spontaneous Alternation test (more arm entries).
• Anxiety & Risk-Taking (Genotype × Sex Interaction):
  • Males: Homozygous males showed signs of reduced anxiety (more time in open arms of EPM, higher latency to enter open arms, higher milk consumption).
  • Females: Homozygous females showed signs of increased anxiety (less time in open arms, lower milk consumption).
• Null Findings: No significant differences were found in motor coordination (Rotarod), sensorimotor gating (PPI), or acoustic startle response.

C. Endocrinology

• Steroid Levels: Serum analysis of 21 steroids revealed expected sex differences (higher testosterone in males) but no significant genotype-dependent alterations in steroid hormone levels. This suggests that the observed phenotypes are not driven by gross systemic hormonal imbalances.

D. Cardiac Morphology

• Cardiac Hypertrophy: Older homozygous mice (>20 weeks) exhibited significantly higher normalized heart weights (relative to bodyweight and tibia length) compared to wildtypes, suggesting a predisposition to cardiac hypertrophy.

5. Significance and Future Directions

• Translational Relevance: The model recapitulates key features of human STS deficiency, specifically hyperactivity (relevant to ADHD comorbidity in XLI) and cardiac structural changes (relevant to arrhythmia risk).
• Mechanistic Insight: The lack of gross steroid hormone changes suggests that STS deficiency may act through local tissue mechanisms or non-hormonal pathways (e.g., altered Hippo signaling/YAP1 activation, as suggested by recent skin studies).
• Sex Differences: The divergent effects on bodyweight and anxiety in males vs. females highlight the complexity of STS function, potentially linked to its escape from X-inactivation in humans.
• Next Steps: The authors propose investigating the molecular basis of the cardiac hypertrophy (e.g., ECG, histology) and testing whether the hyperactivity phenotype can be reversed by ADHD medications. They also emphasize the need to test human XLI carriers in analogous behavioral paradigms to validate the model's translational value.

Conclusion: This study establishes a robust, experimentally tractable mouse model for STS deficiency, confirming that the loss of this single enzyme is sufficient to drive specific behavioral (hyperactivity, sex-dependent anxiety) and cardiac phenotypes without gross endocrine disruption.