Mild hypotonia and recurrent seizures in an 8-month-old boy: Answers

Intestinal hypomagnesemia in an Iranian patient with a novel TRPM6 variant: a case report and review of the literature

TRPM6 is predominantly expressed in the kidney and colon and encodes a protein containing an ion channel domain and a protein kinase domain. It is crucial for magnesium homeostasis and plays important roles in epithelial magnesium transport and the active magnesium absorption. In this study, we present a 70-day-old Iranian female patient from consanguineous parents with hypomagnesemia and secondary hypocalcemia. She presented with seizures 19 days after birth and refractory watery non-bloody diarrhea. She consequently had failure to thrive. Other features included hypotonia, wide anterior fontanel, ventriculomegaly, and pseudotumor cerebri following administration of nalidixic acid. She had severe hypomagnesemia and hypocalcemia which were treated with magnesium and calcium supplementation. Despite initial unstable response to supplemental magnesium, she eventually improved and the diarrhea discontinued. The patient was discharged by magnesium and calcium therapy. At the last follow-up at age 2.5 years, the patient remained well without any recurrence or complication. Genetic testing by whole-exome sequencing revealed a novel homozygous frameshift insertion-deletion (indel) variant in exon 26 of the TRPM6 gene, c.3693-3699del GCAAGAG ins CTGCTGTTGACATCTGCT, p.L1231Ffs*36. Segregation analysis revealed the TRPM6 heterozygous variant in both parents. Patients with biallelic TRPM6 pathogenic variants typically exhibit hypomagnesemia with secondary hypocalcemia and present with neurologic manifestations including seizures. In some patients, this is also complicated by chronic diarrhea and failure to thrive. Long-term complications are rare and most of the patients show a good prognosis with supplemental magnesium therapy.

Case Report: Novel TRPM6 Mutations Cause Hereditary Hypomagnesemia With Secondary Hypocalcemia in a Chinese Family and a Literature Review

BACKGROUND

Hereditary hypomagnesemia with secondary hypocalcemia (HSH) is a rare autosomal recessive disease due to biallelic TRPM6 mutations. Although the reports of HSH caused by TRPM6 mutations are not very rare, the age of onset in previously reported HSH cases were <1 year.

METHODS

We collected and analyzed the clinical data of twin brothers with onset age over 1 year old and performed whole exome sequencing in the patients and their parents. Confirmed by Sanger sequencing, missense mutation was analyzed in silico. We also searched Pubmed, and extracted clinical data from case reports and case series with full text in English, reporting original data of patients with TRPM6 mutations.

RESULTS

The twin patients had canonical HSH phenotype with compound novel TRPM6 mutations, p.T87K and c.705dupT, inherited from their father and mother, respectively. T87 is a highly conserved site and T87K is predicted to cause hydrogen bond disruption. We identified 26 articles published between May 28, 2002 to December 31, 2021 which reported a total of 88 patients with TRPM6 mutation. We found that the most common clinical phenotypes were hypomagnesemia, hypocalcemia, and convulsions. However, the age of onset in HSH patients almost always occurred under 12 months old, the twin patients of our study were 18 and 26 months old at onset.

CONCLUSION

We identified two novel TRPM6 mutations in a Chinses family with HSH, and showed that the age of onset with c.704c-c.705(exon7)insT and c.260(exon4)C>A mutation in TRPM6 was much later than other mutations and would be much less serious.

A novel synonymous homozygous variant [c.2538G>A (p.Thr846Thr)] in TRPM6 in a patient with hypomagnesemia with secondary hypocalcemia

OBJECTIVES

Hypomagnesemia 1, intestinal (HOMG1) is characterized by neurological symptoms that occur due to hypocalcemia and hypomagnesemia and caused by mutations in the TRPM6. Most of the identified variants in TRPM6 lead to premature termination: nonsense, frameshift, deletion, and splice site mutations.

CASE PRESENTATION

Herein, we report a 1.5 month-old case who presented with convulsion due to hypocalcemia and hypomagnesemia in the early infancy. Sequencing of TRPM6 revealed a novel homozygous synonymous variant [c.2538G > A (p.Thr846Thr)] in the last codon of exon 19, which is most likely to affect the splicing. We report a novel homozygous synonymous variant in the TRPM6 leading to HOMG1, expanding the mutational spectrum.

CONCLUSIONS

Synonymous mutations that were previously considered as harmless should be evaluated at the nucleotide level, keeping in mind that they may affect splicing and cause to the disease.

Long-term Clinical Follow-up of Patients with Familial Hypomagnesemia with Secondary Hypocalcemia

OBJECTIVE

Familial hypomagnesemia with secondary hypocalcemia (HSH) is an autosomal recessive disease caused by a mutation in the transient receptor potential melastatin 6 (TRPM6) gene and is characterized by selective magnesium malabsorption. Affected cases are usually diagnosed during infancy and usually present with seizures due to hypocalcemia and hypomagnesemia. Irreversible neurological deficits and arrhythmias can be observed without appropriate treatment. The aim was to evaluate the long-term follow-up of patients with genetically confirmed HSH.

METHODS

A total of six patients with HSH, two of whom were siblings, were included. Age at diagnosis, clinical, laboratory and follow-up data on admission were recorded. All 39 exons of the TRPM6 gene and flanking exon-intron junctions from genomic DNA were amplified and sequenced in all cases.

RESULTS

The median (range) follow-up duration was 12.1 (7.6-21.7) years. All cases were diagnosed in infancy. Four different mutations, three of which had not been previously reported, were detected in the TRPM6 gene. Treatment compliance was good and there were no severe complications in the long-term follow-up of cases. However, mental retardation, specific learning difficulty and attention deficit/hyperactive disorder were observed as comorbidities.

CONCLUSION

Of the four different TRPM6 mutations in this small cohort, three had not been previously reported. The long-term prognosis of HSH appears to be good, given early diagnosis and good treatment compliance. This long-term follow-up and prognostic data and the three novel mutations will contribute to the published evidence concerning this rare condition, HSH, and it is hoped will prevent negative outcomes.

Treatment Difficulties in Hypomagnesemia Secondary to the Transient Receptor Potential Melastatin 6 Gene: A Case Report with Novel Mutation

Hypomagnesemia is a rare cause of seizures in childhood but should be kept in mind in recurrent and intractable seizures and hypocalcemia in communities where consanguineous marriages are common. Familial hypomagnesemia with secondary hypocalcemia is a rare genetic cause of hypomagnesemia, due to variants in the transient receptor potential melastatin 6 (TRPM6) genes. Here, a three year-old boy with a novel variant in this gene and had difficulties with enteral hypomagnesemia treatment is presented. He had recurrent seizures since two years of age and was diagnosed with epilepsy and treated with multiple antiepileptic drugs. Subsequently, he was diagnosed with rickets due to severe hypocalcemia at another center. The patient was hypotonic and neurodevelopmentally poor. The most prominent laboratory finding was of hypomagnesemia with secondary hypocalcemia. The genetic analysis revealed a novel variant in the TRPM6 gene. After parental treatment of intravenous magnesium (Mg2⁺) sulfate and calcium, the treatment was switched to enteral Mg2⁺ medications, due to persistent hypomagnesemia and the gastrointestinal side-effects, different oral preparations were used. The patient was stable on an oral maintenance dose of Mg2⁺ oxide with borderline blood Mg2⁺ levels and resolution of hypocalcemia. Hypomagnesemia is one of the causes of hypocalcemia. Enteral replacement is the key treatment but the treatment should be individualized for each patient. Normalization of hypomagnesemia is not always easy and should not be the aim of the treatment.