Molecular Pathogenesis
To date biallelic pathogenic variants in one of six genes – CLPP, ERAL1, HARS2, HSD17B4, LARS2, and TWNK – are known to cause Perrault syndrome. However, many cases of Perrault syndrome are not molecularly defined.
CLPP and ERAL1 have roles in the formation of the mitochondrial ribosome; HARS2 and LARS2 are important for the translation of mitochondrial proteins; and TWNK maintains mitochondrial DNA. It is likely that defects of mitochondrial translation and protein homeostasis in the inner ear and ovary underlie the pathogenesis of Perrault syndrome.
CLPP
Gene structure.
CLPP is predicted to generate four protein-coding transcripts. The longest transcript of 1194 bp (NM_006012.2) has six exons. For a detailed summary of gene and protein information, see Table A, Gene.
Pathogenic variants. Initially three unrelated families – all of Pakistani ethnicity – were reported [Jenkinson et al 2013]. Affected individuals in each family were homozygous for the pathogenic variant c.433A>C, c.440G>C, or c.270+4A>G. Fewer than ten additional families from across the world have subsequently been reported, with biallelic missense variants. Six of ten CLPP pathogenic missense variants localized to a region of CLPP from amino acid residue 142 to 162, with a cluster around residues 144-147 [Demain et al 2017].
Theunissen et al [2016] reported three affected individuals who were compound heterozygous for at least one loss-of-function CLPP pathogenic variant and had more severe phenotypes (see Table 6).
Table 6.
CLPP Pathogenic Variants Discussed in This GeneReview
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DNA Nucleotide Change | Predicted Protein Change | Reference Sequences |
---|
c.270+4A>G | |
NM_006012.2
NP_006003.1
|
c.433A>C | p.Thr145Pro |
c.440G>C | p.Cys147Ser |
Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.
GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen.hgvs.org). See Quick Reference for an explanation of nomenclature.
Normal gene product.
CLPP encodes ATP-dependent Clp protease proteolytic subunit, mitochondrial. The longest transcript encodes the longest isoform of CLPP with 277 amino acids (NP_006003.1).
Abnormal gene product. Crystal-structure modeling suggests that many of the pathogenic missense variants would alter the structure of the CLPP barrel chamber that captures unfolded or misfolded proteins and exposes them to proteolysis [Jenkinson et al 2013].
ERAL1
Gene structure.
ERAL1 is predicted to generate three protein-coding transcripts. The longest transcript of 1923 bp (NM_005702.3) is generated from ten exons. See Table A, Gene for a detailed summary of gene and protein information.
Pathogenic variants. Three families from a genetically isolated population of Dutch ancestry have been reported [Chatzispyrou et al 2017]. All affected individuals were homozygous for the missense variant c.707A>T.
Table 7.
ERAL1 Pathogenic Variants Discussed in This GeneReview
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Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.
GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen.hgvs.org). See Quick Reference for an explanation of nomenclature.
Normal gene product.
ERAL1 encodes GTPase Era, mitochondrial, an ERA-like protein 1 chaperone of mitochondrial 12s rRNA, which is essential for assembly of the mitochondrial 28s ribosomal subunit. The longest coding isoform of ERAL1 comprises 437 amino acids (NP_001304914).
Abnormal gene product. The c.707A>T variant is predicted to interfere with GTP binding in ERAL1 and, therefore, its interaction with mitochondrial 12S RNA. Affected individuals had reduced levels of mitochondrial 12S rRNA and of the 28S mitochondrial subunit.
HARS2
Gene structure.
HARS2 is predicted to be expressed as a number of transcript variants. The longest transcript of 2515 bp (NM_012208.3) is generated from 13 exons.
For a detailed summary of gene and protein information, see Table A, Gene.
Pathogenic variants. Three families with Perrault syndrome with pathogenic missense variants in HARS2 have been reported. Two unrelated Moroccan families were reported with both affected individuals homozygous for c.1010A>G, which is suspected to be a founder variant due to shared haplotypes between affected individuals [Lerat et al 2016]. In a single family of European descent affected individuals were compound heterozygotes for c.598C>G and c.1102G>T [Pierce et al 2011].
Table 8.
HARS2 Pathogenic Variants Discussed in This GeneReview
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DNA Nucleotide Change | Predicted Protein Change | Reference Sequences |
---|
c.598C>G 1 | p.Leu200Val 1 |
NM_012208.3
NP_036340.1
|
c.1102G>T | p.Val368Leu |
c.1010A>G | p.Tyr337Cys |
Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.
GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen.hgvs.org). See Quick Reference for an explanation of nomenclature.
- 1.
Created an alternate splice site leading to deletion of 12 codons [Pierce et al 2011]
Normal gene product.
HARS2 encodes probable histidine--tRNA ligase, mitochondrial. Variable protein-coding isoforms of HARS2 have been reported. The canonic transcript variant NM_012208.3 encodes the 506 amino acids of the isoform NP_036340.1.
Abnormal gene product. The pathogenic variants in HARS2 reduced the aminoacylation activity of HARS2 [Pierce et al 2011].
HSD17B4
Gene structure.
HSD17B4 is predicted to encode multiple protein-coding transcripts. The canonic transcript comprises 2710 bp encoded by 24 exons (NM_000414.3). For a detailed summary of gene and protein information, see Table A, Gene.
Pathogenic variants. Three families with Perrault syndrome with pathogenic missense variants in HSD17B4 have been reported. Two affected sisters were compound heterozygotes for a nonsense and a missense variant, c.1704T>A and c.650A>G, respectively [Pierce et al 2010]. A single individual with PS was reported as compound heterozygous for the variants HSD17B4 c.46G>A and c.244G>T [Demain et al 2017]. Affected individuals of a family were homozygous for the variant c.298G>T [Chen et al 2017]. The variant c.46G>A is associated with D-bifunctional protein deficiency [Demain et al 2017] (see Table 9, footnote 1).
Table 9.
HSD17B4 Pathogenic Variants Discussed in This GeneReview
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DNA Nucleotide Change | Predicted Protein Change | Reference Sequences |
---|
c.46G>A 1 | p.Gly16Ser |
NM_000414.3
NP_000405.1
|
c.244G>T | p.Val82Phe |
c.298G>T | p.Ala100Ser |
c.650A>G | p.Tyr217Cys |
c.1704T>A | p.Tyr568Ter |
Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.
GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen.hgvs.org). See Quick Reference for an explanation of nomenclature.
- 1.
The most common pathogenic variant causing D-bifunctional protein deficiency (see Table 2)
Normal gene product.
HSD17B4 encodes the peroxisomal multifunctional enzyme type 2 protein (HSD17B4), also known as bifunctional enzyme, hydroxysteroid (17-beta) dehydrogenase 4 and D-bifunctional protein (DBP). It is involved in fatty acid beta-oxidation and steroid metabolism. The canonic isoform (NP_000405.1) has 736 amino acid residues.
Abnormal gene product. The HSD17B4 pathogenic missense variants in individuals with PS are predicted to destabilize the dehydrogenase domain of DBP or interrupt co-factor binding [Pierce et al 2010, Demain et al 2017]. Biallelic loss-of-function or pathogenic missense variants in HSD17B4 may cause the allelic disorder D-bifunctional protein deficiency (see Table 2), which is more severe than Perrault syndrome and usually fatal in childhood. The variants associated with Perrault syndrome are thought to be less deleterious than those causing D-bifunctional protein deficiency, thus resulting in the milder phenotype.
LARS2
Gene structure. The longest LARS2 transcript of 4203 bp (NM_015340.3) is generated from 21 exons. For a detailed summary of gene and protein information, see Table A, Gene.
Pathogenic variants. A number of individuals with biallelic pathogenic variants in LARS2 have been reported. Initially two individuals with Perrault syndrome were reported [Pierce et al 2013]: one with a homozygous pathogenic missense variant c.1565C>A, and another with a compound heterozygous frameshift variant c.1077delT and missense variant c.1886C>T (Table 10). The transversion c.1565C>A has been identified as a homozygous variant in two additional families and as a compound heterozygous variant in trans to c.351G>C in another family [Demain et al 2017, Zerkaoui et al 2017]. All four families with the variant c.1565C>A had low-frequency SNHL. The variant c.1886C>T has been reported in a second family in trans to the variant c.1358G>A. In a single family biallelic compound heterozygous variants in LARS2 c.880G>A and c.1556C>T were associated with a neurologic phenotype.
Table 10.
LARS2 Pathogenic Variants Discussed in This GeneReview
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DNA Nucleotide Change | Predicted Protein Change (Alias 1) | Reference Sequences |
---|
c.351G>C | p.Met117Ile |
NM_015340.3
NP_056155.1
|
c.880G>A | p.Glu294Lys |
c.1077delT | p.Ile360PhefsTer15 (Ile360fsTer) |
c.1358G>A | p.Arg453Gln |
c.1556C>T | p.Thr519Met |
c.1565C>A | p.Thr522Asn 2 |
c.1886C>T | p.Thr629Met |
Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.
GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen.hgvs.org). See Quick Reference for an explanation of nomenclature.
- 1.
Variant designation that does not conform to current naming conventions
- 2.
Normal gene product.
LARS2 encodes mitochondrial leucyl-tRNA synthetase 2. The longest coding isoform of LARS2 is 903 amino acids (NP_056155.1).
Abnormal gene product. The p.Thr522Asn variant occurs in a catalytic domain and has been shown to reduce the aminoacylation activity of LARS2 ninefold [Riley et al 2016]. Many of the other reported variants also lie in the catalytic domain and are predicted to reduce aminoacylation activity. The p.Thr629Met variant is in a site adjacent to a conserved catalytic loop [Pierce et al 2013].
TWNK
Gene structure. The longest TWNK transcript and major splice variant (NM_021830.4) comprises five exons and encodes the protein isoform twinkle (also known as isoform A).
Transcript variant NM_001163812.1 is a minor splice variant that encodes the distinct protein isoform known as twinky (also known as isoform B). This transcript results from the use of a downstream exon 4 splice-donor site and leads to a 43-bp insertion between the regular exon 4 - exon 5 sequence, which causes a premature stop codon [Spelbrink et al 2001]. See Table A, Gene for a detailed summary of gene and protein information.
Pathogenic variants. Biallelic variants in TWNK associated with Perrault syndrome were initially reported in affected individuals of two unrelated families who were compound heterozygotes for c.[1172G>A];[1754A>G] and c.[1321T>G];[1519G>A] [Morino et al 2014]. Additional affected individuals and variants have been reported, including c.1196A>G, which was reported in two unrelated families in trans to the variants c.968G>A [Demain et al 2017] and c.1802G>A [Ołdak et al 2017]. A family was also reported to be homozygous for the variant c. 793C>T. To date all affected individuals have reported neurologic features [Demain et al 2017, Ołdak et al 2017].
Table 11.
TWNK Pathogenic Variants Discussed in This GeneReview
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DNA Nucleotide Change | Predicted Protein Change | Reference Sequences |
---|
c.1172G>A | p.Arg391His |
NM_021830.4
NP_068602.2
|
c.1754A>G | p.Asn585Ser |
c.1321T>G | p.Trp441Gly |
c.1519G>A | p.Val507Ile |
c.968G>A | p.Arg323Gln |
c.1196A>G | p.Asn399Ser |
c.1802G>A | p.Arg601Gln |
c. 793C>T | p.Arg265Cys |
Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.
GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen.hgvs.org). See Quick Reference for an explanation of nomenclature.
Normal gene product.
TWNK encodes two splicing isoforms, twinkle and twinkie. Twinkle (NP_001157284.1) comprises 684 amino acids and forms hexamer complexes. Twinky (NP_068602.2) comprises 582 amino acids, shares the first 578 amino acids with twinkle, and terminates with four alternative amino acids. Twinkle is the mitochondrial DNA replicative helicase and localizes to mitochondrial nucleosomes. Twinkle contains three functional domains: a 3-prime helicase region, required for mtDNA replication; a linker region involved in hexamer formation; and a 5-primase domain. Twinkie has a mitochondrial localization and its function is currently unknown [Spelbrink et al 2001].
Abnormal gene product. Most of the pathogenic variants associated with Perrault syndrome are located in the linker and helicase domains of twinkle. Variants located in the linker region have been predicted to interfere with hexamer formation. Variants in the helicase region are predicted to affect the helicase function or interaction of subunits within the hexamer complex [Morino et al 2014, Demain et al 2017, Ołdak et al 2017].