Molecular Basis of DFNB73: Mutations of BSND Can Cause Nonsyndromic Deafness or Bartter Syndrome

Abstract

BSND encodes barttin, an accessory subunit of renal and inner ear chloride channels. To date, all mutations of BSND have been shown to cause Bartter syndrome type IV, characterized by significant renal abnormalities and deafness. We identified a BSND mutation (p.I12T) in four kindreds segregating nonsyndromic deafness linked to a 4.04-cM interval on chromosome 1p32.3. The functional consequences of p.I12T differ from BSND mutations that cause renal failure and deafness in Bartter syndrome type IV. p.I12T leaves chloride channel function unaffected and only interferes with chaperone function of barttin in intracellular trafficking. This study provides functional data implicating a hypomorphic allele of BSND as a cause of apparent nonsyndromic deafness. We demonstrate that BSND mutations with different functional consequences are the basis for either syndromic or nonsyndromic deafness.

Barttin Activates ClC-K Channel Function by Modulating Gating

“…Absolute open probabilities could not be accurately determined for WT rClC-K1/barttin. However, for V166E rClC-K1/barttin and hClC-Ka/ barttin, we could demonstrate that the absolute open probability of the slow gate is 1 in whole-cell 11 and cell-attached recordings 14 (Figure 5), respectively. Taken together, these results demonstrate that protopore gating is fast both in the presence and in the absence of barttin and that barttin constitutively opens the common slow gate of ClC-K/barttin channels.…”

“…In cell-attached patches, the behavior of hClC-Ka/ barttin thus resembled a channel with two independently gated conduction pathways without a cooperative gating process. The high absolute open probabilities of hClC-Ka/barttin 14 made possible the accurate determination of the number of ion pores per patch and thus allowed another approach to distinguish individual or common gating. If the observed fast gating affects individual protopores, an even number of conductance states must be observed in each patch.…”

“…13 In contrast, a BSND mutation that affects only the chaperone function of barttin and leaves the function of ClC-K/barttin channels unaffected caused nonsyndromic deafness. 14 To further understand how barttin modifies the function of ClC-K channels, we studied voltage-dependent gating of two ClC-K isoforms, rat ClC-K1 and human ClC-Ka, expressed either alone or together with barttin. ClC channels exhibit a unique double-barreled architecture with two independent ion conduction pathways, the so-called protopores.…”

Tryptophan Scanning Mutagenesis Identifies the Molecular Determinants of Distinct Barttin Functions

“…The functional analysis of disease-associated mutations found in patients with Bartter syndrome IV revealed that many mutations did not modify the different functions of barttin in a similar way. Some abolished hClC-Ka/barttin and hClC-Kb/barttin functions but left intracellular CLC-K/barttin trafficking unaltered (15), whereas another one exclusively impaired surface membrane insertion (18). To identify amino acids crucial for the different functions of barttin, we individually substituted each residue within the predicted transmembrane core of barttin by tryptophan (19 -24) and studied the effects of these mutations by heterologous expression and functional as well as biochemical analyses.…”

Modulation of ClC-K Channel Function by the Accessory Subunit Barttin

“…13,14 Partial loss-of-function mutations of barttin may result in nonsyndromal hearing loss. 15 ClC-K/barttin channels are under control of multiple signal cascades. ClC-K/barttin is inhibited by acid pH and stimulated by extracellular calcium.…”