CNNM2 Mutations Cause Impaired Brain Development and Seizures in Patients with Hypomagnesemia

Abstract

Intellectual disability and seizures are frequently associated with hypomagnesemia and have an important genetic component. However, to find the genetic origin of intellectual disability and seizures often remains challenging because of considerable genetic heterogeneity and clinical variability. In this study, we have identified new mutations in CNNM2 in five families suffering from mental retardation, seizures, and hypomagnesemia. For the first time, a recessive mode of inheritance of CNNM2 mutations was observed. Importantly, patients with recessive CNNM2 mutations suffer from brain malformations and severe intellectual disability. Additionally, three patients with moderate mental disability were shown to carry de novo heterozygous missense mutations in the CNNM2 gene. To elucidate the physiological role of CNNM2 and explain the pathomechanisms of disease, we studied CNNM2 function combining in vitro activity assays and the zebrafish knockdown model system. Using stable Mg2+ isotopes, we demonstrated that CNNM2 increases cellular Mg2+ uptake in HEK293 cells and that this process occurs through regulation of the Mg2+-permeable cation channel TRPM7. In contrast, cells expressing mutated CNNM2 proteins did not show increased Mg2+ uptake. Knockdown of cnnm2 isoforms in zebrafish resulted in disturbed brain development including neurodevelopmental impairments such as increased embryonic spontaneous contractions and weak touch-evoked escape behaviour, and reduced body Mg content, indicative of impaired renal Mg2+ absorption. These phenotypes were rescued by injection of mammalian wild-type Cnnm2 cRNA, whereas mammalian mutant Cnnm2 cRNA did not improve the zebrafish knockdown phenotypes. We therefore concluded that CNNM2 is fundamental for brain development, neurological functioning and Mg2+ homeostasis. By establishing the loss-of-function zebrafish model for CNNM2 genetic disease, we provide a unique system for testing therapeutic drugs targeting CNNM2 and for monitoring their effects on the brain and kidney phenotype.

Localizing PRL-2 expression and determining the effects of dietary Mg2+ on expression levels

“…Studies have demonstrated that Mg 2+ positively regulates synaptic plasticity in hippocampal neurons (Slutsky et al 2004) and that Mg 2+ supplementation enhances learning ability and memory in animals (Slutsky et al 2010). A recent study also identified mutations in the Mg 2+ transporter CNNM2, one of the potential PRL-2 binding partners, to be the cause of impaired brain development and seizures in patients with hypomagnesemia (Arjona et al 2014). Given that hypomagnesemia is associated with a wide variety of neurological diseases, such as migraine, depression, and epilepsy (de Baaij et al 2015), it is highly possible that PRL-2 plays a role in brain function, as well as brain development, by interacting with CNNM2 to regulate cellular Mg 2+ homeostasis.…”

“…CNNM1-4 (Cyclin M), also known as ancient conserved domain protein (ACDP), is a family of divalent cation transporters that was identified on the basis of strong amino acid homology to the bacterial metal ion transporter CorC (Wang et al 2004). As initially expected from homology, the CNNM family protein has been shown to function as a transporter for Mg 2+ and other divalent cations (Goytain and Quamme 2005;Yamazaki et al 2013;Stuiver et al 2011;Arjona et al 2014). Similar to PRLs, CNNMs are also localized to the plasma membrane (Quamme 2010), and later studies strongly suggested the PRL/ CNNM complex to be involved in the transport of Mg 2+ (Hardy et al 2014;Funato et al 2014).…”

Human CNNM2 is not a Mg2+ transporter per se

“…The identification of a schizophrenia risk variant rs7914558 is noteworthy, as is its possible impact on neural systems relevant to social cognition through the gray matter volumetric vulnerability of the orbital regions in the inferior frontal gyri [29,34]. Moreover, mutations directly associated with impaired brain development and seizures in patients with hypomagnesemia have been identified [1].…”

Structural Basis of the Oncogenic Interaction of Phosphatase PRL-1 with the Magnesium Transporter CNNM2

“…This includes the human "CBS domain divalent metal cation transport mediator" (CNNM) gene family, also referred to as ancient conserved domain proteins (20 -22), that include four isoforms (CNNM1-4) that are expressed in all human tissues (20) except for CNNM1, which is mainly expressed in the brain (23,24). Mutations in CNNM2 cause familial dominant hypomagnesemia (MIM613882) (23,25) and have been recently linked to major neuropsychiatric disorders (26), intellectual disability, and disturbed brain development (27). In contrast, mutations in CNNM4, the closest homolog of CNNM2, are considered the cause of the autosome-recessive cone-rod dystrophy with amelogenesis imperfecta (MIM217080) (28,29).…”