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Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC): compound heterozygous mutation in the claudin 16 (CLDN16) gene.

Hampson G, Konrad MA, Scoble J - BMC Nephrol (2008)

Bottom Line: Therapy with thiazide diuretics and magnesium supplements failed to halt the progression of the disorder.Mutation analysis revealed 2 heterozygous mutations in the claudin 16 gene (CLDN16) in both affected siblings; one missense mutation in exon 4: C646T which results in an amino acid change Arg216Cys in the second extracellular loop of CLDN16 and loss of function of the protein and a donor splice site mutation which changes intron 4 consensus splice site from 'GT' to 'TT' resulting in decreased splice efficiency and the formation of a truncated protein with loss of 64 amino acids in the second extracellular loop.The clinical course and molecular findings suggest complete loss of function of the protein in the 2 affected cases and highlight the case for molecular diagnosis in individuals with FHHNC.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Chemical Pathology, St Thomas Hospital, London, UK. geeta.hampson@kcl.ac.uk

ABSTRACT

Background: Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC) is an autosomal recessive disorder of renal calcium and magnesium wasting frequently complicated by progressive chronic renal failure in childhood or adolescence.

Methods: A 7 year old boy was investigated following the findings of marked renal insufficiency and nephrocalcinosis in his 18-month old sister. He too was found to have extensive nephrocalcinosis with increased fractional excretion of magnesium: 12.4% (<4%) and hypercalciuria: 5.7 mmol (< 2.5/24 hours). He had renal impairment, partial distal renal tubular acidosis and defective urinary concentrating ability. Therapy with thiazide diuretics and magnesium supplements failed to halt the progression of the disorder. Both children subsequently underwent renal transplantation. Both children's parents are unaffected and there is one unaffected sibling.

Results: Mutation analysis revealed 2 heterozygous mutations in the claudin 16 gene (CLDN16) in both affected siblings; one missense mutation in exon 4: C646T which results in an amino acid change Arg216Cys in the second extracellular loop of CLDN16 and loss of function of the protein and a donor splice site mutation which changes intron 4 consensus splice site from 'GT' to 'TT' resulting in decreased splice efficiency and the formation of a truncated protein with loss of 64 amino acids in the second extracellular loop.

Conclusion: The mutations in CLDN16 in this kindred affect the second extra-cellular loop of claudin 16. The clinical course and molecular findings suggest complete loss of function of the protein in the 2 affected cases and highlight the case for molecular diagnosis in individuals with FHHNC.

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Single strand polymorphism analysis of the amplified products of exon 4 of CLDN16 gene showing the conformational variants. Heterozygous carriers: Lanes 1: Unaffected sibling, Lane 3: Mother, Lane 4: Father. Affected cases: Lanes 2 and 5. Control: Lane 6. Blank: Lane 7.
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Figure 1: Single strand polymorphism analysis of the amplified products of exon 4 of CLDN16 gene showing the conformational variants. Heterozygous carriers: Lanes 1: Unaffected sibling, Lane 3: Mother, Lane 4: Father. Affected cases: Lanes 2 and 5. Control: Lane 6. Blank: Lane 7.

Mentions: Two types of conformational variants were identified in exon 4 in all family members compared to controls. This is illustrated in Figure 1. Lanes 2 and 5 represent the affected children. Lanes 1, 3, 4 correspond to the unaffected family members who are heterozygote carriers. Direct sequencing of exon 4 revealed 2 heterozygous mutations; one mis-sense mutation 646 C> T (inherited from the maternal allele) and a second novel mutation which is a splice site mutation 784 +1 G>T (inherited from the paternal allele) as shown in Figure 2 (Genbank Acc number NM_006580). The unaffected sibling was a heterozygous carrier of the paternal mutation (784 +1 G>T). The missense mutation is located in the second extracellular loop of claudin 16 and leads to an amino acid change from Arginine to Cysteine. This missense mutation has been shown following expression analysis to result in complete loss of function of the protein [8]. The 784 +1 G>T splice mutation results in a change in the intronic consensus splice site such that 'GT' is changed to 'TT'. This leads to a 224-fold decrease in splice efficiency as calculated from the automated splice site analysis. The mutation leads to skipping of exon 4 with a loss of 64 amino acids in the second transmembrane domain and is likely to lead also to loss of function.


Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC): compound heterozygous mutation in the claudin 16 (CLDN16) gene.

Hampson G, Konrad MA, Scoble J - BMC Nephrol (2008)

Single strand polymorphism analysis of the amplified products of exon 4 of CLDN16 gene showing the conformational variants. Heterozygous carriers: Lanes 1: Unaffected sibling, Lane 3: Mother, Lane 4: Father. Affected cases: Lanes 2 and 5. Control: Lane 6. Blank: Lane 7.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2562370&req=5

Figure 1: Single strand polymorphism analysis of the amplified products of exon 4 of CLDN16 gene showing the conformational variants. Heterozygous carriers: Lanes 1: Unaffected sibling, Lane 3: Mother, Lane 4: Father. Affected cases: Lanes 2 and 5. Control: Lane 6. Blank: Lane 7.
Mentions: Two types of conformational variants were identified in exon 4 in all family members compared to controls. This is illustrated in Figure 1. Lanes 2 and 5 represent the affected children. Lanes 1, 3, 4 correspond to the unaffected family members who are heterozygote carriers. Direct sequencing of exon 4 revealed 2 heterozygous mutations; one mis-sense mutation 646 C> T (inherited from the maternal allele) and a second novel mutation which is a splice site mutation 784 +1 G>T (inherited from the paternal allele) as shown in Figure 2 (Genbank Acc number NM_006580). The unaffected sibling was a heterozygous carrier of the paternal mutation (784 +1 G>T). The missense mutation is located in the second extracellular loop of claudin 16 and leads to an amino acid change from Arginine to Cysteine. This missense mutation has been shown following expression analysis to result in complete loss of function of the protein [8]. The 784 +1 G>T splice mutation results in a change in the intronic consensus splice site such that 'GT' is changed to 'TT'. This leads to a 224-fold decrease in splice efficiency as calculated from the automated splice site analysis. The mutation leads to skipping of exon 4 with a loss of 64 amino acids in the second transmembrane domain and is likely to lead also to loss of function.

Bottom Line: Therapy with thiazide diuretics and magnesium supplements failed to halt the progression of the disorder.Mutation analysis revealed 2 heterozygous mutations in the claudin 16 gene (CLDN16) in both affected siblings; one missense mutation in exon 4: C646T which results in an amino acid change Arg216Cys in the second extracellular loop of CLDN16 and loss of function of the protein and a donor splice site mutation which changes intron 4 consensus splice site from 'GT' to 'TT' resulting in decreased splice efficiency and the formation of a truncated protein with loss of 64 amino acids in the second extracellular loop.The clinical course and molecular findings suggest complete loss of function of the protein in the 2 affected cases and highlight the case for molecular diagnosis in individuals with FHHNC.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Chemical Pathology, St Thomas Hospital, London, UK. geeta.hampson@kcl.ac.uk

ABSTRACT

Background: Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC) is an autosomal recessive disorder of renal calcium and magnesium wasting frequently complicated by progressive chronic renal failure in childhood or adolescence.

Methods: A 7 year old boy was investigated following the findings of marked renal insufficiency and nephrocalcinosis in his 18-month old sister. He too was found to have extensive nephrocalcinosis with increased fractional excretion of magnesium: 12.4% (<4%) and hypercalciuria: 5.7 mmol (< 2.5/24 hours). He had renal impairment, partial distal renal tubular acidosis and defective urinary concentrating ability. Therapy with thiazide diuretics and magnesium supplements failed to halt the progression of the disorder. Both children subsequently underwent renal transplantation. Both children's parents are unaffected and there is one unaffected sibling.

Results: Mutation analysis revealed 2 heterozygous mutations in the claudin 16 gene (CLDN16) in both affected siblings; one missense mutation in exon 4: C646T which results in an amino acid change Arg216Cys in the second extracellular loop of CLDN16 and loss of function of the protein and a donor splice site mutation which changes intron 4 consensus splice site from 'GT' to 'TT' resulting in decreased splice efficiency and the formation of a truncated protein with loss of 64 amino acids in the second extracellular loop.

Conclusion: The mutations in CLDN16 in this kindred affect the second extra-cellular loop of claudin 16. The clinical course and molecular findings suggest complete loss of function of the protein in the 2 affected cases and highlight the case for molecular diagnosis in individuals with FHHNC.

Show MeSH
Related in: MedlinePlus