Inherited Tubulopathies of the Kidney

Abstract

The kidney tubules provide homeostasis by maintaining the external milieu that is critical for proper cellular function. Without homeostasis, there would be no heartbeat, no muscle movement, no thought, sensation, or emotion. The task is achieved by an orchestra of proteins, directly or indirectly involved in the tubular transport of water and solutes. Inherited tubulopathies are characterized by impaired function of one or more of these specific transport molecules. The clinical consequences can range from isolated alterations in the concentration of specific solutes in blood or urine to serious and life-threatening disorders of homeostasis. In this review, we focus on genetic aspects of the tubulopathies and how genetic investigations and kidney physiology have crossfertilized each other and facilitated the identification of these disorders and their molecular basis. In turn, clinical investigations of genetically defined patients have shaped our understanding of kidney physiology.

Introduction

Homeostasis refers to the maintenance of the “milieu interieur,” which, as expressed by the physiologist Claude Bernard, is a “condition for a free and independent existence” (1). Simplified, homeostasis with regard to the kidney follows the principle of balance: “what goes in, must come out.” Our bodies must maintain balance of fluids, electrolytes, and acid-base despite great fluctuations from diet, metabolism, and environmental conditions. If laboring in high temperatures with limited fluid intake, water must be preserved; if the diet contains a high acid load, this needs to be excreted, and so on: the kidneys will adjust excretion of fluid and solutes so that the total body balance is preserved. Specifically, it is the kidney tubules that perform this critical task by adjusting reabsorption and secretion so that the final urine volume and composition matches the diet and environmental stressors. A whole array of transport proteins in the tubular epithelial cells is involved in this, and their importance becomes most apparent when one or more of them are defective. Collectively, these disorders are called kidney tubulopathies and a list of these is compiled in Table 1. The clinical consequences reflect the role of the affected transport protein: it can be isolated loss of specific solutes, which may even be of clinical benefit, considering, for instance, isolated renal glycosuria owing to mutations in the sodium-glucose cotransporter type 2. But they can also be devastating if global homeostasis is affected, impairing even the most basic physiologic functions, such as respiration (2).

Table 1.

List of inherited tubulopathies

Disorder	Inheritance	Gene	Protein	OMIM
Proximal tubule
Fanconi renotubular syndrome 1	AD	GATM	L-ARGININE:GLYCINE AMIDINOTRANSFERASE	#134600
Fanconi renotubular syndrome 2	AR	SLC34A1	NaPi2A	#182309
Fanconi renotubular syndrome 3	AD	EHHADH	PBFE	#607037
Fanconi renotubular syndrome 4	AD	HNF4A	HNF-4	#600281
Fanconi Bickel syndrome	AR	SLC2A2	GLUT-2	#138160
Dent disease type 1	XLR	CLCN5	CLC-5	#300008
Dent disease type 2/Lowe syndrome	XLR	OCRL	OCRL	#300535
Renal tubular acidosis type 3	AR	CA2	Carbonic anhydrase 2	#611492
Hereditary hypophosphatemic rickets with hypercalciuria	AR	SLC34A3	NaPi2c	#241530
X-linked hypophosphatemic rickets	XLD	PHEX	PHEX	#307800
Cystinuria A	AD	SLC3A1	rBAT	#104614
Cystinuria B	AR	SLC7A9	b(0,+)AT1	#604144
Lysinuric protein intolerance	AR	SLC7A7	y(+)LAT1	#222700
Hartnup disorder	AR	SLC6A19	B(0)AT1	#234500
Iminoglycinuria	AR/digenic	SLC36A2+SLC6A20/SLC6A19	SLC36A2+SLC6A20/SLC6A19	#242600
Dicarboxylic aminoaciduria	AR	SLC1A1	?	#222730
Thick ascending limb
Bartter type 1	AR	SLC12A1	NKCC2	#600839
Bartter type 2	AR	KCNJ1	ROMK	#600359
Bartter type 3	AR	CLCNKB	CLC-Kb	#602023
Bartter type 4a	AR	BSND	Barttin	#606412
Bartter type 4b	Digenic	CLCNKA+CLCNKB	CLC-Ka+CLC-Kb	#602024
Bartter type 5	XR	MAGED2	MAGED2	#601199
Hypomagnesemia type 3/familial hypomagnesemia with hypercalciuria and nephrocalcinosis	AR	CLDN16	Claudin16	#603959
Hypomagnesemia type 5/familial hypomagnesemia with hypercalciuria and nephrocalcinosis	AR	CLDN19	Claudin19	#610036
Autosomal dominant hypocalcemia	AD	CaSR	Calcium-sensing receptor	#601198
Kenny−Caffey syndrome type 2	AD	FAM111A	FAM111A	#127000
Distal convoluted tubule
Gitelman syndrome	AR	SLC12A3	NCCT	#600968
EAST/SeSAME syndrome	AR	KCNJ10	Kir4.1	#602028
Pseudohypoaldosteronism type 2b	AD	WNK4	WNK4	#601844
Pseudohypoaldosteronism type 2c	AD	WNK1	WNK1	#605232
Pseudohypoaldosteronism type 2d	AD/AR	KLHL3	KLHL3	#614495
Pseudohypoaldosteronism type 2e	AD	CUL3	CUL3	#614496
Hypomagnesemia type 1/hypomagnesemia with secondary hypocalcemia	AR	TRPM6	TRPM6	#607009
Hypomagnesemia type 2	AD	FXYD2	Na-K-ATPase	#154020
Autosomal dominant hypomagnesemia	AD	KCNA1	Kv1.1	#176260
HNF1B-related kidney disease	AD	HNF1B	HNF1B	#137920
Hyperphenylalaninemia BH4-deficient	AR	PCBD1	PCDB1	#264070
Hypomagnesemia type 4	AR	EGF	EGF	#611718
Neonatal inflammatory skin and bowel disease type 2	AR	EGFR	EGFR	#616069
Hypomagnesemia, seizures, and mental retardation type 1	AD/AR	CNNM2	CNNM2	#613882
Hypomagnesemia, seizures, and mental retardation type 2	De novo	ATP1A1	ATP1A1	#618314
Collecting duct
Pseudohypoaldosteronism type 1	AR	SCNN1A	ENaC α subunit	#600228
Pseudohypoaldosteronism type 1	AR	SCNN1B	ENaC β subunit	#600760
Pseudohypoaldosteronism type 1	AR	SCNN1G	ENaC γ subunit	#600761
Pseudohypoaldosteronism type 1A	AD	NR3C2	MR	#600983
Liddle syndrome	AD	SCNN1B	ENaC β subunit	#600760
Liddle syndrome	AD	SCNN1G	ENaC γ subunit	#600761
Apparent mineralocorticoid excess	AR	HSD11B2	11-β-HSD2	#614232
Glucocorticoid remediable aldosteronism	AD	CYP11B1/CYP11B2	11-β-hydroxylase/ALDOS	#610613
Congenital adrenal hyperplasia type 1	AR	CYP21A2	21-hydroxylase	#613815
Congenital adrenal hyperplasia type 2	AR	HSD3B2	3-β-HSD2	#613890
Congenital adrenal hyperplasia type 4	AR	CYP11B1	11-β-hydroxylase	#610613
Congenital adrenal hyperplasia type 5	AR	CYP17A1	17-α-hydroxylase	#609300
Nephrogenic diabetes insipidus	XLR	AVPR2	AVPR2	#300538
Nephrogenic diabetes insipidus	AR/AD	AQP2	AQP-2	#107777
Nephrogenic syndrome of inappropriate antidiuresis	XLR	AVPR2	V2R	#300538
Nephrogenic syndrome of inappropriate antidiuresis	AD	GNAS	G-α s
Distal RTA	AD/AR	SLC4A1	AE1	#109270
Distal RTA	AR	ATP6V1B1	V-ATPase subunit B1	#192132
Distal RTA	AR	ATP6V0A4	V-ATPase subunit a4	#605239
Distal RTA	AR	FOXI1	Forkhead box protein I1	#601093
Distal RTA	AR	WDR72	WD repeat-containing protein 72	#613214

Listed are primary renal tubulopathies grouped by affected nephron segment, the underlying gene(s), and encoded protein(s), as well as their OMIM entry number. OMIM, Online Mendelian Inheritance in Man; AD, autosomal dominant; AR, autosomal recessive; XLR, x-linked recessive; RTA, renal tubular acidosis.

Kidney physiologists often study genetically modified animals to better understand the role of the respective gene. In contrast, clinicians studying patients with suspected tubulopathies find clues as to the potentially underlying cause through clinical observations. It is clinicians, who, together with genetic and physiologic scientists, have often led the discovery of these transport molecules and their encoding genes and thereby provided fundamental insights into kidney physiology. In this review, rather than providing an exhaustive discussion of all tubulopathies, we focus on the contribution of genetics to our current understanding of tubulopathies and, in turn, kidney physiology.

The Central Role of Salt

On average, the human adult kidney filters approximately 150–180 L of water and 20–25 mol of sodium per day, of which typically >99% is then reabsorbed back into the circulation by multiple sodium transport systems along the nephron (Figure 1) (3). The engine that drives tubular transport is the Na⁺-K⁺-ATPase, located on the basolateral aspect of the tubular epithelial cells. The stoichiometry of 3 sodium versus 2 potassium ion counter transport in concert with a potassium conductance establishes the crucial electrochemical gradient that facilitates sodium entry from the tubular lumen. Consequently, this gradient is used for the transport of many other solutes, such as glucose, various amino acids and phosphate (by cotransport), protons and potassium (by exchange), or calcium and uric acid (by facilitated diffusion) (3).

Figure 1.

Inherited tubulopathies are caused by dysfunction of specific transport molecules in the renal tubule. Shown is an overview of a nephron, detailing selected transporters involved in disorders of tubular sodium handling. For associated disorders, see Table 1.

Genetic and clinical investigations into rare disorders of kidney salt handling have facilitated the cloning of many of the most important salt transport systems. Often, this was in parallel with kidney physiologic studies. The identification of the genetic basis of Bartter and Gitelman syndromes, for instance, fit perfectly with the understanding of kidney physiology and the clinical experience from loop and thiazide diuretics (4). But in some instances, genetic investigations also provided very surprising results that prompted a revision of our understanding of physiology.

A key insight from these genetic studies is the recognition of kidney salt handling at the center of BP regulation and thus volume homeostasis (5). In the 1970s, Guyton et al. (6) described in detail the numerous physiologic processes implicated in affecting BP and beautifully illustrated the integration of all these factors in a famous diagram. But subsequent genetic studies of rare disorders associated with abnormal BP clearly established kidney salt handling at the center of long-term BP regulation (5).

An interesting observation from these disorders is the evolutionary ranking of the various aspects of homeostasis. Because the transport of many solutes in the tubule is directly or indirectly linked to sodium, this can create conflicts; for instance, in the collecting duct, the reabsorption of sodium via the epithelial sodium channel creates the favorable electrical gradient for the secretion of potassium and protons (Figure 1). Consequently, the biochemical “fingerprint” of enhanced sodium reabsorption in this segment is hypokalemic alkalosis (7). In Bartter and Gitelman syndromes, the kidneys excrete potassium and protons to facilitate sodium reabsorption, demonstrating the precedence of volume homeostasis over potassium and acid-base balance (3). It follows naturally from this insight that salt supplementation should be beneficial also for normalizing plasma potassium levels in these salt-wasting syndromes (8).

Genetic investigations have also revealed how evolution has devised mechanisms to cope with competing homeostatic demands. The identification of the genetic basis of pseudohypoaldosteronism type 2 (Gordon syndrome) elegantly solved the “aldosterone paradox,” i.e., the question of how aldosterone could be involved in the potentially conflicting demands of volume and potassium homeostasis (9). If both sodium and potassium need to be preserved, sodium reabsorption is shifted to the distal convoluted tubule (DCT); if sodium must be preserved, but potassium excreted (for instance, after a salt-poor, potassium-rich meal typical for the paleolithic diet), then sodium reabsorption is shifted to the collecting duct, facilitating potassium secretion (9). These and other disorders also revealed the potassium sensing role of KCNJ10, the basolateral potassium channel in the DCT, in which mutations cause epilepsy, ataxia, sensorineural-deafness, tubulopathy syndrome (10,11). From a clinical perspective, this has important implications, as it explains why a high-potassium intake facilitates excretion of the high-salt content of a typical Western diet, thus mitigating the associated cardiovascular complications (12).

Genetic studies have also revealed the sometimes haphazard nature of evolution. Intelligent design would likely have made the various steroid hormones specific for their respective receptors. However, the mineralocorticoid receptor is highly sensitive to the glucocorticoid cortisol, which is present in plasma in almost 1000-fold higher concentration than aldosterone (13). Fortunately, specificity is provided by the enzyme HSD11B2, which metabolizes cortisol to cortisone and thereby protects the mineralocorticoid receptor. Mutations in this enzyme lead to the rare hypertensive disorder of apparent mineralocorticoid excess (13). The study of this disorder also illustrates the importance of a comprehensive clinical investigation for a correct diagnosis, as this disorder shares many clinical features with Bartter syndromes types 1 and 2: hypokalemic alkalosis, hypercalciuria, and, commonly, secondary nephrogenic diabetes insipidus (14). Only BP and hormone levels can distinguish one disorder from the other: in apparent mineralocorticoid excess, BP is elevated and aldosterone/renin levels are suppressed, whereas in the Bartter syndromes, the opposite is true.

Approximately 65% of sodium reabsorption occurs in the proximal tubule, and experiments in mice show that the bulk of this occurs via the sodium-proton exchanger Nhe3 (15). Consequently, loss of function of NHE3 in humans was expected to be either incompatible with life, or at least lead to a severe form of proximal tubular acidosis (16). Yet, when such loss-of-function mutations were identified, they were associated with the intestinal disorder congenital secretory sodium diarrhea with no apparent kidney phenotype (17). Further studies in mice demonstrated that a kidney-specific knockout of Nhe3 does exhibit some bicarbonate wasting, albeit mild (18). Potentially, downstream isoforms of NHE provide compensation for NHE3-specific dysfunction in the proximal tubule, thereby explaining the mild kidney phenotype.

Disorders of Water

Disorders of kidney water handling were among the first tubulopathies to be genetically solved, perhaps because of the typical clinical presentation: impaired kidney water conservation (nephrogenic diabetes insipidus) is associated with hypernatremic dehydration, whereas impaired water excretion (nephrogenic syndrome of inappropriate antidiuresis) is associated with hyponatremia (19,20).

Primary nephrogenic diabetes insipidus is associated with mutations in two genes, AVPR2 and AQP2, encoding for the type 2 arginine-vasopressin (AVP) receptor and a water channel, respectively (Figure 2) (21,22). Detailed clinical observations in affected patients have revealed also extrarenal roles of AVPR2, expressed in the vasculature, where activation of the receptor results in vasodilation and release of coagulation factors (23).

Figure 2.

Disorders of water are caused by impaired regulation of water reabsorption in the collecting duct. Shown is a schematic of a principal cell in the collecting duct (CD), with relevance to water transport. Note the expression of AVPR2 on the basolateral side. Activation of this receptor results in insertion of AQP2 into the apical membrane.

Nephrogenic syndrome of inappropriate antidiuresis is characterized by an inability to dilute the urine, with a consequent risk of hyponatremia from water overload (20,24). It is thus the mirror image of nephrogenic diabetes insipidus: nephrogenic diabetes insipidus reflects loss of function in the urinary concentration pathway, whereas nephrogenic syndrome of inappropriate antidiuresis is caused by gain-of-function mutations in AVPR2 (20). Yet, no corresponding gain-of-function mutations in AQP2 have so far been described. Instead, mutations in GNAS have been identified as another cause of nephrogenic syndrome of inappropriate antidiuresis, either isolated or as part of a more complex syndrome, which reflects the association of GNAS with several G protein–coupled receptors (25,26). GNAS encodes the stimulatory G-α protein, which links AVPR2 activation to adenyl cyclase in the AVP signaling pathway (Figure 2) (27). Why some GNAS mutations lead to syndromic features and others seem to cause isolated nephrogenic syndrome of inappropriate antidiuresis is currently unclear. It has been speculated that AVPR2 signaling may be the most sensitive GNAS-associated pathway, so that milder mutation manifest clinically only in impaired urinary dilution (25).

These disorders therefore provide important clinical insights, corresponding to those from salt-handling disorders: impaired kidney water handling is reflected in dysnatremia, whereas disorders of kidney sodium handling primarily result in altered volume homeostasis. Moreover, they demonstrate the precedence of volume homeostasis over maintenance of normal plasma tonicity: in hypernatremic dehydration in nephrogenic diabetes insipidus, the kidneys will preserve sodium, whereas in hyponatremia water overload in the nephrogenic syndrome of inappropriate antidiuresis, sodium is excreted. Such insight also informs the diagnosis and treatment of the more common but related disorder, the syndrome of inappropriate antidiuresis, which, owing to its elevated urinary sodium concentration, is commonly misdiagnosed as cerebral or pulmonary salt wasting (7). Treatment with salt supplementation thus only re-establishes the volume overload and consequently risks hypertension. Instead, asymptomatic syndrome of inappropriate antidiuresis should be treated by water removal (28).

Acid-Base Homeostasis

Early clinical and physiologic investigations distinguished renal tubular acidosis (RTA) into a proximal and distal form, to which a mixed form was later added (29,30).

Subsequent discovery of the underlying genes beautifully confirmed the initial astute clinical observations as these genes encoded proteins involved in either proximal bicarbonate reabsorption (proximal RTA) or distal proton secretion (distal RTA) or both (Figure 3). Proximal RTA typically occurs in people with generalized proximal tubular dysfunction (Fanconi renotubular syndrome). Isolated proximal RTA is exceedingly rare, is associated with eye abnormalities, and is caused by mutations in the basolateral sodium/bicarbonate exchanger SLC4A4 (31). A further family with apparently isolated proximal RTA and dominant inheritance has been described, but the genetic basis remains so far unsolved (32). SLC9A3 (see above) was considered a strong candidate, but this could not be confirmed, illustrating how our understanding of kidney physiology, derived mainly from animal models, can at times be critically informed by insights from human genetics (16).

Figure 3.

Disorders of acid-base homeostasis are caused by impaired bicarbonate reabsorption in the proximal tubule or impaired acid secretion in the collecting duct. Shown is a schematic of a tubular epithelial cell in the proximal tubule (PT) and an intercalated cell (type A) in the collecting duct (CD), highlighting proteins involved in bicarbonate reabsorption and acid secretion. For associated disorders, see Table 1.

Genetic investigations into distal RTA provided insights into the different functions of the chloride/bicarbonate anion exchanger AE1. This transport protein, encoded by SLC4A1 and initially identified in Western blots from red cells (“Band 3”), was already associated with East Asian ovalocytosis and hereditary spherocytosis, but was subsequently found to also cause distal RTA (33).

Physiologic investigations have highlighted the role of both the H⁺-ATPase and H⁺/K⁺-ATPase in the collecting duct for distal acid secretion (34,35). Yet, so far, mutations have been identified in only two of the 14 different subunits of the H⁺-ATPase (ATP6V0A4 and ATP6V1B1) that cause distal RTA, and in none of the H⁺/K⁺-ATPase (36,37). The remaining genes are obviously strong candidate genes for the 30%–40% of genetically unsolved patients with a clinical diagnosis of primary distal RTA, yet so far, no convincing mutations have been reported (38). Instead, two different genes have emerged as novel distal RTA disease genes. The first, FOXI1, fits perfectly with current understanding of physiology (39). It encodes a transcription factor regulating the expression of several proteins involved in acid secretion, and mice deleted for the murine paralogue had been previously described to have distal RTA (40). Mutations in transcription factors are usually inherited in a dominant fashion, probably reflecting the fact that complete loss is not compatible with life (41). The fact that FOXI1 mutations are recessively inherited highlights the specific and restricted role of this transcription factor in acid-secreting epithelia.

In contrast to FOXI1, the discovery of WDR72 as a distal RTA disease gene was completely unexpected, illustrating the power of genetics to provide novel insights (42). Mutations in WDR72 were identified in several patients with a syndromic combination of amelogenesis imperfecta and distal RTA. The exact mechanisms of how WDR72 mutations are linked to distal RTA remain unclear at present. Interestingly, WDR72 mutations had been previously identified as a cause of amelogenesis imperfecta (43). It remains unclear whether specific WDR72 mutations can cause isolated dental problems, others only distal RTA, and yet others both clinical features. Potentially, this just reflects incomplete phenotyping, so that the dentist may have missed the presence of distal RTA, just as the nephrologist may have overlooked amelogenesis imperfecta. Interestingly, the identification of WDR72 mutations in a cohort of patients with distal RTA led to the subsequent diagnosis of amelogenesis imperfecta in all affected patients by “reverse phenotyping” (44). This is reminiscent of the identification of FAM20A mutations first as a cause of amelogenesis imperfecta, and subsequently, of nephrocalcinosis (45). Arguably, these examples illustrate the narrow clinical focus of individual medical specialties and the importance of comprehensive phenotyping.

Clinical studies in patients with acid-base abnormalities reveal the critical importance of precise acid-base balance for proper physiologic function of our body, reflected in the exquisitely tight control of arterial pH between 7.37 and 7.43, a change of <±3 nmol/L. Protons can bind to proteins, especially to histidine residues, and thereby affect protein charge, which in turn affects protein folding and function. Consequently, disturbances of acid-base balance are potentially catastrophic and can include dysrhythmias, apnea, coma, and death (46). And even relatively mild forms of distal RTA reveal that acidosis is associated not only with rickets and nephrocalcinosis/urolithiasis, but also with more general symptoms, such as impaired growth (47).

Magnesium Homeostasis

Magnesium is the second-most abundant intracellular cation and a cofactor in numerous enzymatic reactions, yet it is often called the “forgotten ion” (48). Magnesium homeostasis is primarily maintained through reabsorption in the thick ascending limb of the loop of Henle (TAL) and DCT, and clinically, this can be typically distinguished by the concurrence of hypercalciuria (TAL) or normo-/hypocalciuria (DCT) (49). All known genetic causes of hypomagnesemia encode proteins expressed in these functional segments of the nephron (Figure 4). In the TAL, calcium and magnesium are reabsorbed through a common paracellular pathway, lined by CLDN16 and CLDN19, mutations in which cause hypercalciuric hypomagnesemia (49). Interestingly, mutations in CLDN10, another claudin that lines this paracellular pore, enhance magnesium reabsorption and thereby cause hypermagnesemia as part of a more complex phenotype called HELIX syndrome (50).

Figure 4.

Disorders of magnesium homeostasis are caused by impaired magnesium transport in TAL or DCT. Shown is a schematic of a tubular epithelial cell in thick ascending limb of the loop of Henle (TAL) and distal convoluted tubule (DCT), highlighting proteins involved in tubular magnesium handling. For associated disorders, see Table 1.

Although paracellular reabsorption of cations depends on a transepithelial voltage gradient, which is established by the combined function of NKCC2 and ROMK1, the corresponding recessive disorders Bartter syndromes type 1 and 2 are usually not associated with hypomagnesemia (3). In contrast, dominant gain-of-function mutations in the calcium-sensing receptor that suppress ROMK function do associate with hypomagnesemia, likely because of other functional roles of this receptor (49).

Although only about 10% of filtered magnesium is reabsorbed in the DCT, this represents the final regulated pathway, and genetic investigations have revealed a surprising complexity: at least 15 genes have so far been identified in which mutations impair magnesium transport in this segment (Figure 4, Table 1). These are associated with hypocalciuria if not only magnesium but also salt reabsorption is affected (Gitelman-like hypomagnesemias [49]), as the resulting hypovolemia leads to increased proximal sodium and thus calcium reabsorption (51). In contrast, disorders affecting magnesium transport only are associated with normocalciuria. The most prominent example for the latter is familial hypomagnesemia with secondary hypocalcemia, and the identification of the underlying genetic basis as mutations in TRPM6 established this channel as the key magnesium transport pathway in the apical membrane of the DCT (52).

There are some interesting observations in genetically defined patients with hypomagnesemia. For instance, mutations impairing the function of the basolateral Na⁺-K⁺-ATPase seem to predominantly affect magnesium transport. Mutations in the α (ATP1A1) (53) or γ (FXYD2) (54) subunit, or in their regulators, such as HNF1B (55), manifest primarily with hypomagnesemia, although a trend to a Gitelman-like tubulopathy may be observed (56). Presumably, this reflects the critical role of the DCT for magnesium homeostasis, whereas other segments may be able to compensate more with regard to sodium (57).

Another interesting observation is the apparent absence of hypokalemia in patients with TRPM6 mutations (58). Because ROMK is inhibited by intracellular magnesium, hypomagnesemia has been proposed as a cause for refractory hypokalemia (59). Yet, these patients, despite their often profound hypomagnesemia, are not reported to suffer from hypokalemia, seriously questioning the clinical relevance of hypomagnesemia for potassium control.

A relevant physiologic question arising from the genetic findings in patients with hypomagnesemia is regarding the link between impaired sodium transport in DCT and magnesium reabsorption. Presumably, apical sodium uptake via SCL12A3 is necessary to maintain the activity of the Na⁺-K⁺-ATPase, which, in turn, establishes the electrochemical gradient that facilitates magnesium uptake. To do so, an apical potassium channel is needed that can translate this electrochemical gradient into a lumen-negative membrane potential to enable magnesium uptake via TRPM6. The identification of a familial form of hypomagnesemia associated with a mutation in the apically expressed potassium channel KCNA1 fits nicely with this hypothesis, except that this is a voltage-gated potassium channel with essentially zero open probability at the membrane voltages observed in DCT (60). Moreover, other mutations in KCNA1 are the cause of episodic ataxia type 1, which is not associated with hypomagnesemia (49). Further studies are needed to resolve these apparent conundrums.

Genetic investigations have also provided some insights with regard to the identity of the basolateral transport pathway for magnesium in DCT, although questions remain. Initially, CNNM2 was thought to constitute this pathway in the form of a basolateral sodium-magnesium exchanger, but later it was also proposed to be an intracellular magnesium sensor (61,62). Thus, whether CNNM2 constitutes the main basolateral exit pathway or whether other transport proteins are involved remains currently unclear.

Genetic Testing in Tubulopathies

Currently, more than 50 disease genes for kidney tubulopathies are recognized (Table 1), and the list keeps on expanding. Because of the specific phenotype associated with mutations in most genes, an accurate clinical diagnosis can usually be established. However, even in expert centers, genetic testing can sometimes further specify or even correct the clinical diagnosis, so that genetic confirmation is usually recommended because of potentially important implications not only for genetic counseling, but also for treatment (63,64). For instance, Bartter syndrome is occasionally misdiagnosed as nephrogenic diabetes insipidus, and the common therapeutic use of thiazides for the latter could have life-threatening consequences in the former disorder (27). Whether genetic testing is done by sequencing of single genes, a targeted panel, or whole exome/genome depends on the certainty of the clinical diagnosis and local availability/affordability. If no causative variant(s) are identified by selected gene sequencing, a subsequent more comprehensive approach is necessary to reach a genetic diagnosis. For this reason, whole-exome/-genome sequencing is increasingly used as a first-line approach. In the United Kingdom, for instance, the aim is to use whole-genome sequencing for the vast majority of genetic tests (https://www.england.nhs.uk/genomics/nhs-genomic-med-service/). To minimize the analytical workload and the risk of incidental findings, only a panel of relevant disease genes is assessed. Obviously, these gene panels need to be constantly updated, and various resources for this exist (65).

Yet, although the diagnostic yield of genetic testing in tubulopathies is much higher than in most other kidney disorders, in about a third of patients with pediatric onset tubulopathy, a genetic diagnosis cannot be established and this increases to more than two thirds in those with adult onset (63,64). This likely reflects the diagnostic uncertainty regarding some identified variants, or genes, as well as acquired causes, especially in adult-onset patients.

Translation of Genetic Insights

Genetic investigations have provided unparalleled insights into the molecular basis of tubular physiology and thus the maintenance of homeostasis. Although discoveries of new Mendelian disease genes are still ongoing, their number is necessarily limited. Other genetic approaches, such as genome-wide association studies, may provide further insights into the genetic architecture of tubulopathies and the maintenance of homeostasis (66).

Translating genetic discoveries into targeted treatments is an obvious key aim. Many of the identified molecular causes were already known as targets of drugs, such as diuretics. For disorders with a gain-of-function disease mechanism, treatment can be logical and targeted, such as the use of thiazides in pseudohypoaldosteronism type 2 or amiloride in Liddle syndrome. In some instances, understanding of the molecular mechanism has provided a basis for the development of a specific treatment. One that has now entered the clinical arena is the anti-FGF23 antibody burosumab in X-linked hypophosphatemic rickets (67). Another example is the recognition of the central role of cyclic AMP in kidney water handling, which has spurred investigations into novel treatments for nephrogenic diabetes that enhance AQP2 expression in the apical membrane independent of AVPR2 receptor activation (68(–70).

Yet, for most tubulopathies, treatment has remained essentially unchanged over the past decades and is mainly supportive; for instance, in the form of electrolyte supplements. Fortunately, the advent of gene-editing technologies, such as CRISPR/CAS9, may provide opportunities for the future treatment of tubulopathies (71). In contrast to most other genetic kidney disorders, such as inherited kidney malformations or nephronophthisis, in which global kidney architecture and function is typically irreversibly altered at the time of diagnosis, tubulopathies affect only very specific aspects of kidney function. Although there may be some irreversible changes, such as nephrocalcinosis or interstitial fibrosis, most patients with tubulopathies would have sufficient kidney function if the genetic defect could be repaired.

A key obstacle remains the targeted delivery of the gene-editing machinery to the kidney. So far, clinical trials of gene editing address mostly hematologic or immune disorders, where relevant blood cells can be obtained for gene editing ex vivo and then returned to the patient (72). However, if precise delivery to the tubule can be achieved, then tubulopathies will constitute an ideal group of disorders for the therapeutic application of this promising new technology. Another approach currently trialed is delivery of the correct gene via hematopoietic stem cells (73).

Conclusions

Tubulopathies of the kidney illustrate the synergistic potential of clinical, genetic, and physiologic investigations to facilitate accurate diagnosis and targeted management of affected patients, in turn enhancing our understanding of kidney physiology. They reveal the superior importance of sodium in overall tubular transport, the primate of volume over other aspects of homeostasis, and its control through sodium reabsorption. Tubulopathies also sometimes dramatically illustrate the critical importance of homeostasis for overall body function.

The high rate of identifiable underlying genetic causes, and the fact that they affect very specific aspects of tubular function while generally preserving overall kidney function, make them ideal targets for future treatments using gene-editing techniques.

Disclosures

All authors have nothing to disclose.

Funding

M.L. Downie, R. Kleta, and D. Bockenhauer were supported by St. Peter’s Trust for Kidney, Bladder & Prostate Research. S.C. Lopez Garcia, R. Kleta, and D. Bockenhauer are grateful to Dr. Magdi Yaqoob for support through the William Harvey Paediatric Fellowship. R. Kleta and D. Bockenhauer were supported by Kids Kidney Research and Kidney Research UK. R. Kleta was supported by the David and Elaine Potter Foundation. D. Bockenhauer was supported by the National Institute for Health Research Biomedical Research Centre at Great Ormond Street Hospital/University College London Institute of Child Health.