Hypomagnesemia with secondary hypocalcemia

Hypomagnesemia with secondary hypocalcemia (HSH) is an autosomal recessive genetic disorder affecting intestinal magnesium absorption. Decreased intestinal magnesium reabsorption and the resulting decrease in serum magnesium levels is believed to cause lowered parathyroid hormone (PTH) output by the parathyroid gland. This results in decreased PTH and decreased serum calcium levels (hypocalcemia). This manifests in convulsions and spasms in early infancy which, if left untreated, can lead to mental retardation or death. HSH is caused by mutations in the TRPM6 gene.

Hypomagnesemia with secondary hypocalcemia
SpecialtyEndocrinology

Pathophysiology

HSH is caused by decreased intestinal magnesium reabsorption through TRPM6 channels. When expressed in cells, TRPM6 produces outwardly rectifying currents with the outward portion composed of Na+ ions and the inward portion of divalent cations (particularly magnesium and calcium). Inward flow of sodium ions is blocked by extracellular divalent cations. Increased intracellular magnesium concentrations also decrease current through TRPM6 channels. There are currently more than 30 known mutations in TRPM6 that are associated with HSH and these mutations are spreading throughout the gene (table 1). Of the eight HSH mutations that have been tested, none have shown to produce whole-cell current. The S141L mutation, one of the few missense mutations, has been of particular interest to researchers. They have found that it prevents coassembly with TRPM7 (and presumably other TRPM6 subunits) and lacks the ability to traffic to the membrane. Whether other mutants are able to traffic properly to the surface or coassemble has not yet been further studied.

While the hypomagnesemia in patients with HSH is a direct result of TRPM6 mutations, hypocalcemia is an indirect, secondary result. Parathyroid gland secretion of PTH can be altered by changes in serum magnesium levels. The decreased serum magnesium levels seen in HSH result in decreased PTH secretion. PTH, in turn, controls the availability of serum calcium. Decreasing PTH levels cause a decrease in calcium availability in serum and, thus, the neurological symptoms of HSH.

Table 1. TRPM6 mutations associated with hypomagnesemia with secondary hypocalcemia
Mutation Location Functional? Reference
Nucleotide Amino acid
c.C166T R56X N-terminus [1]
c.C422T S141L N-terminus No [2],[3],[4],[5]
c.G469T E157X N-terminus [4]
c.T521G I174R N-terminus [6]
c.668delA D223fsX263 N-terminus [4]
c.1010+5G→C Splicing N-terminus [1]
c.A1060C T354P N-terminus [6]
c.1134+5G→A Splicing N-terminus [6]
c.1208-1G→A Splicing N-terminus [4]
c.1280delA H427fsX429 N-terminus No [2],[4]
c.1308+1G→A Splicing N-terminus [4]
c.C1437A Y479X N-terminus [6]
c.C1450T R484X N-terminus [1]
c.C1769G S590X N-terminus No [2],[4],[7]
c.del1796-1797 P599fsX609 N-terminus [4]
c.2009+1G→A Splicing N-terminus [1],[6]
c.G2120A C707Y N-terminus [6]
c.2207delG R736fsX737 N-terminus No [2],[4],[7]
c.2537-2A→T Splicing N-terminus [4]
c.2667+1G→A Splicing [2],[4]
c.C2782T R928X M3 No [4]
c.del Ex 21 M4 [4]
c.Del2831-2832insG I944fsX959 M4-M5 [4]
c.3209-68A→G Splicing [1]
c.del Ex 22 + 23 M5-6 [4]
C.3537-1G→A Splicing [2],[4]
c.3779-91del Q1260fsX1283 C-terminus [2],[4]
c.del Ex 25 - 27 C-terminus [4]
c.del Ex 26 Y1533X C-terminus No [4]
c.5017-18delT L1673fsX1675 C-terminus No [4]
c.del Ex 31 + 32 C-terminus No [4]
c.5057+2T→C Splicing C-terminus [4]
c.A5775G Splicing C-terminus [4]

Diagnosis

Diagnosis typically occurs during the first 6 months of life due to characteristic neurological symptoms. These symptoms include muscle spasms, tetany, and seizures. Laboratory testing indicates hypomagnesemia (decreased serum magnesium levels), hypocalcemia (decreased serum calcium levels), and little to no measurable parathyroid hormone levels. Diagnosis is confirmed with these symptoms and can be further solidified with genetic sequencing of the TRPM6 gene.

Treatment

Treatment of HSH involves administration of high doses of magnesium salts. These salts may be taken orally or otherwise (e.g. subcutaneously). This treatment works by increasing magnesium absorption through the non-TRPM6 mediated paracellular uptake pathways. This treatment must be continued throughout life.

History

HSH was originally believed to be an X-linked disorder due to the preponderance of affected males. With the finding that mutations in TRPM6 (on chromosome 9) are causative for the disorder this is no longer the case. Of recent interest, however, is the characterization of a patient with symptoms similar to HSH who has a translocation of the chromosomes 9 and X.[6]

See also

References
  • Konrad M, Schlingmann K, Gudermann T (2004). "Insights into the molecular nature of magnesium homeostasis". Am J Physiol Renal Physiol. 286 (4): F599–605. doi:10.1152/ajprenal.00312.2003. PMID 15001450.

Footnotes
  1. Walder R, Landau D, Meyer P, Shalev H, Tsolia M, Borochowitz Z, Boettger M, Beck G, Englehardt R, Carmi R, Sheffield V (2002). "Mutation of TRPM6 causes familial hypomagnesemia with secondary hypocalcemia". Nat Genet. 31 (2): 171–4. doi:10.1038/ng901. PMID 12032570. S2CID 33192419.
  2. Schlingmann K, Weber S, Peters M, Niemann Nejsum L, Vitzthum H, Klingel K, Kratz M, Haddad E, Ristoff E, Dinour D, Syrrou M, Nielsen S, Sassen M, Waldegger S, Seyberth H, Konrad M (2002). "Hypomagnesemia with secondary hypocalcemia is caused by mutations in TRPM6, a new member of the TRPM gene family". Nat Genet. 31 (2): 166–70. doi:10.1038/ng889. PMID 12032568. S2CID 40990544.
  3. Chubanov V, Waldegger S, Mederos y Schnitzler M, Vitzthum H, Sassen M, Seyberth H, Konrad M, Gudermann T (2004). "Disruption of TRPM6/TRPM7 complex formation by a mutation in the TRPM6 gene causes hypomagnesemia with secondary hypocalcemia". Proc Natl Acad Sci USA. 101 (9): 2894–9. Bibcode:2004PNAS..101.2894C. doi:10.1073/pnas.0305252101. PMC 365716. PMID 14976260.
  4. Schlingmann K, Sassen M, Weber S, Pechmann U, Kusch K, Pelken L, Lotan D, Syrrou M, Prebble J, Cole D, Metzger D, Rahman S, Tajima T, Shu S, Waldegger S, Seyberth H, Konrad M (2005). "Novel TRPM6 mutations in 21 families with primary hypomagnesemia and secondary hypocalcemia". J Am Soc Nephrol. 16 (10): 3061–9. doi:10.1681/ASN.2004110989. PMID 16107578.
  5. Li M, Jiang J, Yue L (2006). "Functional Characterization of Homo- and Heteromeric Channel Kinases TRPM6 and TRPM7". J Gen Physiol. 127 (5): 525–37. doi:10.1085/jgp.200609502. PMC 2151519. PMID 16636202.
  6. Jalkanen R, Pronicka E, Tyynismaa H, Hanauer A, Walder R, Alitalo T (2006). "Genetic background of HSH in three Polish families and a patient with an X;9 translocation". Eur J Hum Genet. 14 (1): 55–62. doi:10.1038/sj.ejhg.5201515. PMID 16267500.
  7. Voets T, Nilius B, Hoefs S, van der Kemp A, Droogmans G, Bindels R, Hoenderop J (2004). "TRPM6 forms the Mg2+ influx channel involved in intestinal and renal Mg2+ absorption". J Biol Chem. 279 (1): 19–25. doi:10.1074/jbc.M311201200. PMID 14576148.
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