Atypical Hypotonia-Cystinuria Syndrome

• SLC3A1 encodes rBAT, a kidney transporter subunit; when missing, cystine and some dibasic amino acids are not reabsorbed and spill into urine, forming stones. MedlinePlus

• PREPL encodes a cytoplasmic enzyme involved in vesicle trafficking at cholinergic synapses; its loss produces neonatal hypotonia and a treatable congenital? myasthenic-like picture in infancy. PMC+2Frontiers+2

• C2orf34/CAMKMT encodes calmodulin-lysine N-methyltransferase; when deleted with PREPL and SLC3A1, the phenotype broadens and has been called “atypical HCS.” ScienceDirect+1

How common and when it starts: Atypical HCS is ultra-rare (estimated <1 per million). It usually begins in the neonatal period or early infancy. Orpha

Other names

Doctors and articles may use several names for the same condition: Atypical hypotonia-cystinuria syndrome; 2p21 microdeletion syndrome, atypical HCS subtype; C2orf34/CAMKMT-PREPL-SLC3A1 contiguous gene deletion; and older terms like HCS with seizures and lactic acidosis when the deletion is larger. These names refer to overlapping clinical pictures tied to the 2p21 region. PMC+1

Types

Clinicians think of two main groupings along a spectrum of deletions on chromosome 2p21:

1. Classic HCS – deletion or biallelic defects affecting PREPL and SLC3A1; key signs are neonatal hypotonia and cystinuria type I. Orpha+1

2. Atypical HCS – larger contiguous deletions that also remove C2orf34/CAMKMT (and sometimes additional nearby genes), adding features like seizures, lactic acidosis, and more global developmental involvement. PMC+1

(You may also see isolated PREPL deficiency—without cystinuria—which looks like a congenital? myasthenic syndrome in early life; this helps explain why pyridostigmine can help some babies with HCS-related hypotonia.) PMC

Causes

All “causes” are genetic mechanisms that knock out transporter and neuronal functions in the 2p21 region; below are the common ones doctors document in case reports and reviews:

1. Homozygous deletion of SLC3A1 + PREPL + C2orf34/CAMKMT (contiguous deletion) → atypical HCS with broader neurologic signs. PMC+1

2. Homozygous deletion of SLC3A1 + PREPL → classic HCS; cystinuria type I plus neonatal hypotonia. Nature

3. Compound heterozygous microdeletions across 2p21 removing the same gene set. PubMed

4. Large 2p21 microdeletion syndrome extending into adjacent genes → seizures and lactic acidosis. Orpha

5. Pathogenic SLC3A1 variants combined with 2p21 deletion on the other allele (trans) → cystinuria within the HCS spectrum. BioMed Central

6. Founder microdeletions in small populations reported historically (France/Belgium), with later worldwide detection. Nature

7. Isolated PREPL loss (when present with SLC3A1 deletion it becomes HCS) explaining hypotonia/myasthenia component. PMC

8. PREPL functional loss disrupting AP-1–mediated trafficking of the acetylcholine vesicular transporter. Frontiers

9. Loss of CAMKMT (C2orf34) altering calmodulin methylation pathways and cellular signaling. ScienceDirect

10. SLC3A1 loss abolishing renal? reabsorption of cystine and dibasic amino acids (rBAT defect). MedlinePlus

11. Contiguous deletions of varying sizes (≈24–76 kb or larger) creating clinical variability. PMC

12. De-novo deletions (new in the child) when parents are carriers of small variants or are unaffected. PubMed

13. Biallelic SLC3A1 + SLC7A9 variants cause cystinuria in general (not specific to aHCS) and can complicate aHCS diagnosis?. BioMed Central

14. Modifier genes near 2p21 may influence severity in reported families (inferred from variable expressivity). PMC

15. PREPL long/short isoform effects with mitochondrial vs cytosolic localization may affect phenotype (shown in model systems). ScienceDirect

16. Growth hormone axis involvement due to PREPL loss, explaining short stature and GH deficiency seen in some patients. MalaCards

17. Transporter complex disruption of rBAT/b^0,+AT affecting cystine solubility and stone risk. MedlinePlus

18. Autosomal recessive inheritance requiring two non-working copies for disease. Orpha

19. Population-specific alleles identified across Europe and North America after initial clustering. Nature

20. Clinical under-recognition delaying genetic testing; once testing is done, deletions are confirmed and explain the cause. PMC

Common symptoms and signs

1. Floppy infant (generalized hypotonia) at birth. Orpha

2. Poor feeding and failure to thrive in early months. Orpha

3. Delayed motor milestones (late head control, sitting, walking). Orpha

4. Cystine kidney stones (nephrolithiasis), sometimes starting in childhood. Orpha

5. Recurrent renal? colic and urinary infections linked to stones. PMC

6. Growth retardation, sometimes with growth hormone deficiency. MalaCards

7. Seizures (more often in atypical, larger deletions). Orpha

8. Elevated blood lactate/lactic acidosis in some atypical cases. Orpha

9. Facial features such as depressed nasal bridge or tented upper lip (mild, variable). NCBI

10. Neonatal hypoglycemia? and metabolic instability in some infants. NCBI

11. Swallowing/aspiration risk from poor oropharyngeal coordination. Frontiers

12. Congenital? myasthenic-like symptoms (fatigable weakness?) in early life. PMC

13. Developmental and learning challenges (range is wide). Orpha

14. Low muscle mass and endurance on exam. Orpha

15. Laboratory cystinuria (very high urinary cystine and dibasic amino acids). MedlinePlus

Diagnostic tests

Physical examination 

1. General tone assessment showing diffuse hypotonia without focal weakness? at birth. Orpha

2. Feeding and swallow evaluation detecting poor suck and fatigue?. Frontiers

3. Growth and pubertal tracking to screen for GH deficiency and delayed growth. MalaCards

4. Neurologic exam for fatigability suggesting a myasthenic component. PMC

Manual/bedside tests 

1. Urine cystine screening? with nitroprusside test as a quick screen. PMC
2. Stone straining during colic to capture and analyze passed stones. PMC
3. Serial urine pH checks (home dipsticks) to guide alkalinization. PMC
4. Bedside swallow study (thickened feeds trials) if aspiration risk. Frontiers

Laboratory and pathology 

1. Quantitative aminoaciduria (urinary cystine and dibasic amino acids). MedlinePlus
2. Cystine stone analysis confirming cystine composition. PMC
3. Serum lactate and acid-base status (atypical cases may show elevation). Orpha
4. Endocrine work-up (IGF-1, GH stimulation) if short stature or poor growth. MalaCards
5. Genetic testing (chromosomal microarray or exome) to detect 2p21 contiguous deletions. PMC
6. Targeted gene analysis for SLC3A1, PREPL, CAMKMT to define exact breakpoints. PMC

Electrodiagnostic 

1. Repetitive nerve stimulation in infants with fatigability to assess neuromuscular transmission. PMC
2. EMG (as feasible) to characterize myasthenic-like features. PMC

Imaging 

1. Renal? ultrasound? for stones and hydronephrosis with no radiation. PMC
2. Low-dose non-contrast CT if ultrasound? is unclear or complications suspected. PMC
3. Brain MRI if seizures or developmental regression are present. Orpha
4. Videofluoroscopic swallow study if aspiration is suspected clinically. Frontiers

Non-pharmacological treatments (therapies & others)

(Each includes a simple description, purpose, and mechanism. These are supportive pillars alongside medicines.)

1. Structured high-fluid plan: Drink enough to make ≥3 liters of urine per day (adjust by age/size in children). Purpose: dilute cystine to below its solubility. Mechanism: more urine volume lowers cystine concentration and reduces stone formation. PMC

2. Dietary sodium restriction: Keep salt intake low. Purpose: reduce cystine excretion. Mechanism: sodium and cystine reabsorption are linked in the kidney; less dietary sodium reduces urinary cystine. PMC

3. Moderation of animal protein (especially methionine-rich foods): Purpose: reduce cystine production and acid load. Mechanism: methionine is a cystine precursor; lower intake lowers urinary cystine and acidity. PMC

4. Citrate-rich beverages (e.g., lemon/lime juice as part of fluid intake): Purpose: support urinary citrate and alkalinity. Mechanism: dietary citrate can raise urinary citrate and pH, complementing medical alkalinization. (Diet helps; medicines are stronger.) PMC

5. Home urine pH monitoring: Purpose: keep urine around pH 7.0–7.5 under a clinician’s guidance. Mechanism: cystine is more soluble at higher pH. PMC

6. Swallow/feeding therapy: Purpose: improve safe feeding in hypotonic infants. Mechanism: oromotor training and texture modification reduce aspiration and improve caloric intake. Frontiers

7. Physiotherapy: Purpose: strengthen core and postural control. Mechanism: repeated, targeted exercises improve motor unit recruitment and endurance despite baseline hypotonia. Orpha

8. Occupational therapy: Purpose: functional independence in daily tasks. Mechanism: activity-based practice and adaptive tools counter low tone and fatigue?. Orpha

9. Speech-language therapy: Purpose: communication and safe swallowing. Mechanism: motor-speech drills and compensatory strategies support oromotor function. Frontiers

10. Nutritional support & growth monitoring: Purpose: achieve adequate calories/protein without high methionine load. Mechanism: dietitian-guided plans optimize growth and reduce stone risk. MalaCards

11. Timed voiding and stone straining: Purpose: reduce urinary stasis and collect stones for analysis. Mechanism: frequent voiding lowers crystal dwell time; straining confirms stone type. PMC

12. Hydration during illness/heat: Purpose: prevent dehydration?-triggered stones. Mechanism: maintaining urine volume avoids cystine supersaturation. PMC

13. Genetic counseling (family planning): Purpose: explain inheritance and carrier testing. Mechanism: autosomal recessive risk assessment and options for future pregnancies. Orpha

14. Educational supports/IEP planning: Purpose: address learning or motor delays. Mechanism: structured educational interventions target developmental needs. Orpha

15. Physical activity pacing & energy conservation: Purpose: manage fatigability and low endurance. Mechanism: planned rest, gradual conditioning, and assistive devices. PMC

16. Renal? ultrasound? surveillance schedule: Purpose: detect stones early. Mechanism: periodic imaging guides timely intervention and prevents obstruction. PMC

17. UTI prevention hygiene and early testing: Purpose: lower infection? risk around stones. Mechanism: prompt culture and hydration minimize complications. PMC

18. Caregiver training for colic red flags: Purpose: rapid recognition of obstruction. Mechanism: education improves time-to-care during renal? colic. PMC

19. Seizure? safety plans (if present): Purpose: reduce injury and ensure rescue care. Mechanism: individualized emergency instructions for atypical cases with seizures. Orpha

20. Multidisciplinary clinic follow-up: Purpose: coordinate nephrology, neurology, endocrinology, nutrition, and therapies. Mechanism: integrated care matches the multi-system nature of aHCS. Orpha

Drug treatments

(Each includes what it does, class, typical dosing guidance to discuss with the clinician, when to take, purpose, mechanism, and key side effects. Doses are general pediatric/adult ranges from clinical reviews—individualize with your doctor.)

1. Potassium citrate (first-line): Class: urinary alkalinizer. Dose: individualized to maintain urine pH ~7.0–7.5; given 2–3 times daily. Time/Purpose: taken with meals/bedtime to raise and stabilize urine pH. Mechanism: citrate salts buffer urine, increase cystine solubility, and raise urinary citrate. Side effects: GI upset, hyperkalemia risk in renal? impairment. PMC

2. Potassium bicarbonate / potassium-magnesium citrate: Class: alkalinizing salts. Dose/Timing: titrated to target pH, divided doses. Purpose/Mechanism: same as above; magnesium may also reduce crystal formation. Side effects: GI bloating; potassium issues if kidney function is low. PMC

3. Sodium bicarbonate (when potassium can’t be used): Class: alkalinizer. Dose: individualized; divided. Mechanism: raises urine pH but added sodium can raise cystine excretion, so clinicians prefer potassium salts if possible. Side effects: edema?, hypertension? risk. PMC

4. Tiopronin (α-mercaptopropionylglycine): Class: thiol cystine-binding drug. Dose: weight-based (children often 15–40 mg/kg/day divided; adults commonly 800–1,200 mg/day divided). Purpose: for patients who still form stones despite fluids/diet/alkali. Mechanism: splits cystine into two cysteine-drug mixed disulfides that are more soluble. Side effects: rash?, proteinuria?, GI upset; requires monitoring. PMC

5. D-Penicillamine: Class: thiol cystine-binding drug. Dose: similar weight-based schedules; divided dosing. Mechanism: same principle as tiopronin. Side effects: more frequent adverse effects (rash?, leukopenia?, nephrotic syndrome); used when tiopronin is not available/tolerated. PMC

6. Captopril (select cases): Class: ACE inhibitor with a thiol group. Dose: standard ACE-inhibitor ranges. Purpose: off-label; sometimes tried when thiols are not tolerated. Mechanism: forms soluble complexes with cystine, though less effective than tiopronin/penicillamine. Side effects: cough, hyperkalemia, renal? effects. PMC

7. Pyridostigmine (for myasthenic-like hypotonia in infancy): Class: acetylcholinesterase inhibitor. Dose: weight-based q6–8h. Purpose: improve early fatigable weakness? from PREPL-related synaptic dysfunction. Mechanism: increases acetylcholine at neuromuscular junction. Side effects: abdominal cramps, bradycardia?. PMC

8. Growth hormone therapy (when GH deficiency is proven): Class: recombinant human GH. Dose: standard pediatric dosing nightly. Purpose: improve linear growth. Mechanism: replaces deficient hormone in some PREPL-related cases. Side effects: benign? intracranial hypertension?, edema? (monitor). MalaCards

9. Tamsulosin (medical expulsive therapy during stone passage): Class: α-1 blocker. Dose: standard daily dosing per age/weight. Purpose: facilitate ureteral stone passage. Mechanism: relaxes ureteral smooth muscle. Side effects: dizziness, orthostasis. PMC

10. NSAIDs for renal? colic (e.g., ibuprofen, ketorolac under supervision): Class: analgesic/anti-inflammatory. Dose: per standard pain protocols. Purpose: control colic pain and reduce ureteral spasm. Mechanism: prostaglandin inhibition. Side effects: GI and renal risks; avoid dehydration. PMC

11. Opioids (short course if severe colic): Class: analgesic. Purpose: breakthrough pain only, brief use. Mechanism: central μ-receptor agonism. Side effects: sedation, constipation; use cautiously. PMC

12. Antiemetics (ondansetron) during colic: Class: 5-HT3 antagonist. Purpose: control vomiting, support oral hydration. Mechanism: blocks serotonin receptors in gut/chemoreceptor trigger zone. Side effects: headache, constipation. PMC

13. Antibiotics for UTI associated with stones: Class: per culture. Purpose: treat infection that can complicate obstruction. Mechanism: eradicates bacteria; choice guided by sensitivities. Side effects: drug-specific. PMC

14. Citrate solution preparations (liquid forms): Class: alkalinizer. Purpose: pediatric-friendly dosing to sustain target pH overnight. Mechanism/Side effects: as in potassium citrate. PMC

15. Magnesium supplements (adjunct): Class: mineral supplement. Purpose: sometimes used with citrate; limited evidence. Mechanism: may reduce crystal aggregation. Side effects: diarrhea at higher doses. PMC

16. Bicarbonate-containing alkaline waters (adjunct): Class: dietary alkali source. Purpose: mild pH support with fluids. Mechanism: bicarbonate load slightly raises urine pH; not a replacement for prescription alkalinizers. Side effects: sodium load if high-sodium waters. PMC

17. Topical anesthetics/lidocaine patches (post-procedure discomfort): Class: local anesthetic. Purpose: reduce pain after stone procedures. Mechanism: sodium channel blockade. Side effects: local irritation. PMC

18. Vitamin D and calcium per deficiency (not stone-promoting when balanced): Class: supplements. Purpose: treat proven deficiencies, support bone health in limited mobility. Mechanism: corrects deficiency; dosing guided by labs. Side effects: hypercalciuria if over-supplemented—monitor. PMC

19. Anticonvulsants (if seizures present): Class: antiseizure meds. Purpose: control seizures seen in broader 2p21 microdeletions. Mechanism: drug-specific neuronal effects. Side effects: drug-specific; choose with neurology. Orpha

20. Proton pump inhibitors (peri-operative or reflux-related): Class: acid suppression. Purpose: protect GI tract with NSAIDs or treat reflux. Mechanism: blocks gastric acid secretion. Side effects: hypomagnesemia with long-term use (monitor). PMC

Dietary molecular supplement options

These can support the medical plan; primary prevention still relies on high fluids, sodium restriction, and prescription alkalinizers. Discuss each with your clinician.

1. Lemon or lime juice as part of daily fluids: supplies citrate to support urinary citrate and alkalinity. Typical use: add to water through the day; exact volume individualized. Mechanism: dietary citrate → urinary citrate ↑, urine pH ↑ slightly. PMC

2. Potassium citrate powder solutions (food-grade under medical guidance): flexible dosing to sustain target pH overnight. Mechanism: systemic alkali load → urine pH ↑. PMC

3. Potassium bicarbonate powder (medical supervision): alternative alkali when tablets are hard to swallow. Mechanism: bicarbonate → urine pH ↑. PMC

4. Magnesium citrate (low-dose): may reduce crystal aggregation; watch for diarrhea. Mechanism: magnesium complexes anions and can interfere with crystallization. PMC

5. Alkaline mineral waters with bicarbonate: modest adjunct to raise urine pH as part of fluid plan. Mechanism: bicarbonate load. PMC

6. Citrate-containing oral rehydration drinks (low-sugar): practical during illness/heat to maintain volume and citrate. Mechanism: fluid + citrate delivery. PMC

7. Low-sodium diet products (reading labels): reduce cystine excretion by limiting sodium. Mechanism: less renal sodium → lower urinary cystine. PMC

8. Plant-forward protein choices to moderate methionine load: swap portions of animal protein with legumes/tofu as dietitian advises. Mechanism: lower methionine intake reduces cystine production and acid load. PMC

9. Caffeine moderation (keeps hydration quality high, avoids diuresis swings). Mechanism: steadier hydration reduces supersaturation swings. PMC

10. Vitamin D repletion if deficient to support muscle and bone health given hypotonia and limited mobility risks. Mechanism: endocrine normalization; dose per labs. MalaCards

Important note on “immunity booster / regenerative / stem-cell drugs

At this time, there are no approved immune-boosting, regenerative, or stem-cell drugs that treat or reverse atypical HCS or cystinuria. The condition arises from missing transporter and signaling genes in the kidney and nervous system; management focuses on hydration, diet, urine alkalinization, thiol drugs for cystine, treatment of the PREPL-related myasthenic component, growth hormone if truly deficient, and timely stone procedures. Offering “stem-cell” or “regenerative” drugs here would be misleading and unsafe. If you’d like, I can check for active clinical trials from reputable registries and summarize eligibility. PMC+1

Surgeries and procedures

1. Ureteroscopy with laser lithotripsy: A tiny scope goes up the ureter to visualize and laser-fragment stones; fragments are removed. Why: for symptomatic ureteral stones or when medical passage fails. PMC

2. Percutaneous nephrolithotomy (PCNL): A small tract from the back into the kidney allows instruments to remove large or complex stones. Why: for large cystine stones that resist shockwave therapy. PMC

3. Shockwave lithotripsy (ESWL): Focused external sound waves fragment stones. Why: selected smaller stones; cystine is relatively hard, so success varies. PMC

4. Ureteral stent or nephrostomy tube: Temporary drainage to bypass obstruction. Why: relieve blockage, infection risk, or post-procedure swelling. PMC

5. Feeding tube (NG or gastrostomy) in severe neonatal feeding failure: Why: secure nutrition and growth while hypotonia improves. Frontiers

Preventions

1. Keep urine volume high every day (age-appropriate targets from your clinician). PMC

2. Monitor urine pH at home and adjust alkalinizer per plan. PMC

3. Limit sodium intake by reading food labels. PMC

4. Moderate animal protein; avoid high-methionine binges. PMC

5. Spread fluids through day and include a bedtime dose. PMC

6. Increase fluids during fever, heat, or exercise. PMC

7. Do scheduled ultrasounds per nephrology to catch stones early. PMC

8. Treat UTIs promptly to avoid complications around stones. PMC

9. Keep therapy appointments (PT/OT/SLP) to improve tone and feeding. Orpha

10. Genetic counseling for future pregnancies to understand risks. Orpha

When to see doctors urgently

Seek immediate care for severe flank pain, vomiting with dehydration, fever with urinary symptoms, reduced urine output, visible blood in urine, or signs of obstruction (pain + nausea + little urine). Babies with HCS need urgent help for poor feeding, choking, blue spells, poor weight gain, lethargy, or seizures in atypical cases. Early visits prevent kidney damage and support nutrition and development. PMC+1

What to eat and what to avoid

1. Drink water steadily all day; include a bedtime drink as advised. PMC

2. Add natural citrus (lemon/lime) to some water for citrate support. PMC

3. Choose low-sodium foods (fresh rather than processed). PMC

4. Moderate portions of animal protein (especially red meat and fish high in methionine). PMC

5. Use herbs/spices instead of salt for flavor. PMC

6. Plan balanced meals with vegetables, fruits, and whole grains. PMC

7. Limit very salty snacks (chips, instant noodles, pickles). PMC

8. Avoid dehydration during school, travel, or hot weather—carry a bottle. PMC

9. Coordinate formula/feeds with the dietitian for infants who need thickening or tube feeds. Frontiers

10. Track urine pH and volume to see how diet changes work for you. PMC

Frequently Asked Questions (FAQ)

1) What exactly makes this “atypical”?
The deletion usually includes C2orf34/CAMKMT in addition to PREPL and SLC3A1, so symptoms can include seizures and lactic acidosis beyond classic HCS. PMC

2) Is it definitely genetic and inherited?
Yes. It is autosomal recessive; each parent usually carries one non-working copy without symptoms. Orpha

3) How is cystinuria connected to the weak muscles?
Both problems come from the same contiguous deletion: SLC3A1 loss causes cystine stones, and PREPL loss causes hypotonia via synaptic vesicle dysfunction. MedlinePlus+1

4) Can medicines stop stones completely?
They greatly reduce risk. Fluids + sodium restriction + alkalinizers are first-line; thiol drugs are added if stones keep forming. PMC

5) Why is urine pH so important?
Cystine dissolves better at higher pH, so keeping urine around pH 7.0–7.5 lowers stone risk. PMC

6) Are thiol drugs safe for children?
They are used when needed, but require monitoring for rash, proteinuria, and blood count changes. PMC

7) Why might pyridostigmine help a floppy baby with HCS?
PREPL loss causes a myasthenic-like problem; boosting acetylcholine at the junction can improve strength in early life. PMC

8) Is growth hormone always needed?
No. It is considered only if testing proves GH deficiency. MalaCards

9) Can diet alone fix cystinuria?
Diet helps, but fluids + alkalinizers are the backbone; thiols are added if needed. PMC

10) Do citrus drinks replace potassium citrate?
They support citrate intake but are generally not strong enough to maintain target pH alone. PMC

11) Why do some children have seizures or high lactate?
Those features are tied to larger 2p21 deletions affecting nearby genes (the “atypical” form). Orpha

12) What imaging is best for stones?
Start with renal ultrasound; use low-dose CT if needed. PMC

13) Is surgery always needed for stones?
No. Some pass with medical expulsive therapy; others need ureteroscopy or PCNL depending on size/location. PMC

14) Can adults be diagnosed later?
Yes. Some are detected after recurrent stones; genetics then reveal the 2p21 deletion spectrum. PMC

15) Where can I read more reliable summaries?
Trusted overviews exist at Orphanet and peer-reviewed reviews on cystinuria and PREPL deficiency. Orpha+2PMC+2

Disclaimer: Each person’s journey is unique, treatment plan, life style, food habit, hormonal condition, immune system, chronic disease condition, geological location, weather and previous medical  history is also unique. So always seek the best advice from a qualified medical professional or health care provider before trying any treatments to ensure to find out the best plan for you. This guide is for general information and educational purposes only. Regular check-ups and awareness can help to manage and prevent complications associated with these diseases conditions. If you or someone are suffering from this disease condition bookmark this website or share with someone who might find it useful! Boost your knowledge and stay ahead in your health journey. We always try to ensure that the content is regularly updated to reflect the latest medical research and treatment options. Thank you for giving your valuable time to read the article.

The article is written by Team RxHarun and reviewed by the Rx Editorial Board Members

Last Updated: September 28, 2025.