Formation of claudin-10 based tight junctions (TJs) is paramount to paracellular Na

+

 transport in multiple epithelia. Sequence variants in

CLDN10

have been linked to HELIX syndrome

a salt-losing tubulopathy with altered handling of divalent cations accompanied by dysfunctional salivary

sweat

and lacrimal glands. Here

we investigate molecular basis and phenotypic consequences of a newly identified homozygous

CLDN10

variant that translates into a single amino acid substitution within the fourth transmembrane helix of claudin-10. In addition to hypohidrosis (H)

electrolyte (E) imbalance with impaired urine concentrating ability

and hypolacrimia (L)

phenotypic findings include altered salivary electrolyte composition and amelogenesis imperfecta but neither ichthyosis (I) nor xerostomia (X). Employing cellular TJ reconstitution assays

we demonstrate perturbation of

cis

- and

trans

-interactions between mutant claudin-10 proteins. Ultrastructures of reconstituted TJ strands show disturbed continuity and reduced abundance in the mutant case. Throughout

both major isoforms

claudin-10a and claudin-10b

are differentially affected with claudin-10b showing more severe molecular alterations. However

expression of the mutant in renal epithelial cells with endogenous TJs results in wild-type-like ion selectivity and conductivity

indicating that aberrant claudin-10 is generally capable of forming functional paracellular channels. Thus

mutant proteins prove pathogenic by compromising claudin-10 TJ strand assembly. Additional

ex vivo

investigations indicate their insertion into TJs to occur in a tissue-specific manner.