Mitochondrial serine protease HTRA2 p.G399S in a kindred with essential tremor and Parkinson disease

Abstract

Essential tremor is one of the most frequent movement disorders of humans and can be associated with substantial disability. Some but not all persons with essential tremor develop signs of Parkinson disease, and the relationship between the conditions has not been clear. In a six-generation consanguineous Turkish kindred with both essential tremor and Parkinson disease, we carried out whole exome sequencing and pedigree analysis, identifying HTRA2 p.G399S as the allele likely responsible for both conditions. Essential tremor was present in persons either heterozygous or homozygous for this allele. Homozygosity was associated with earlier age at onset of tremor (P < 0.0001), more severe postural tremor (P < 0.0001), and more severe kinetic tremor (P = 0.0019). Homozygotes, but not heterozygotes, developed Parkinson signs in the middle age. Among population controls from the same Anatolian region as the family, frequency of HTRA2 p.G399S was 0.0027, slightly lower than other populations. HTRA2 encodes a mitochondrial serine protease. Loss of function of HtrA2 was previously shown to lead to parkinsonian features in motor neuron degeneration (mnd2) mice. HTRA2 p.G399S was previously shown to lead to mitochondrial dysfunction, altered mitochondrial morphology, and decreased protease activity, but epidemiologic studies of an association between HTRA2 and Parkinson disease yielded conflicting results. Our results suggest that in some families, HTRA2 p.G399S is responsible for hereditary essential tremor and that homozygotes for this allele develop Parkinson disease. This hypothesis has implications for understanding the pathogenesis of essential tremor and its relationship to Parkinson disease.

Keywords: DNA sequencing; gene identification; mitochondrial dysfunction; mutation; neurodegenerative disease.

Fig. 1.

Pedigree of family ET-1 segregating essential tremor, with genotypes at HTRA2 p.G399S. Individuals with essential tremor (ET) are shown with black symbols, and those with Parkinson disease (PD) with red symbols. Age at onset of tremor for affected individuals, current ages, and genotypes at HTRA2 p.G399S are indicated in this order under the symbols. N indicates the wild-type allele, glycine; V indicates the variant allele, serine, at HTRA2 p.G399S. Individuals who underwent exome sequencing (IV:3, IV:4, VI:5) are indicated with arrows. Subject VI:1, who is unaffected and heterozygous for HTRA2 p.G399S, is presently 37 y old, younger than the mean age at onset of essential tremor among heterozygotes in the family. Phenotypes of four relatives are unknown: I:1, I:2, and III:1 are deceased, and V:11 refused clinical examination.

Fig. 2.

Archimedes spiral tests of individuals of various ages and genotypes at HTRA2 p.G399S. For all individuals, R was the dominant right hand and L was the nondominant left hand.

Fig. 3.

Relationship between HTRA2 genotype and age at onset of essential tremor and severity of tremors. V indicates the variant allele serine, and N indicates the wild-type allele glycine at HTRA2 p.G399S. Subjects heterozygous for the variant allele are indicated NV, and those homozygous for the variant allele are indicated VV. (A) Essential tremor age at onset varies significantly by genotype, P < 0.0001. (B) Severity of postural tremor of homozygous and heterozygous subjects differs significantly; analysis of covariance by genotype with age at examination as covariate yields F = 18.68, (2, 17 df), P < 0.0001. (C) Severity of kinetic tremor of homozygous and heterozygous subjects differs significantly; analysis of covariance by genotype with age at examination as covariate yields F = 9.24, (2, 17 df), P = 0.0019. Individuals with NN genotype and +1 postural or kinetic tremor scores did not fulfill the criteria for essential tremor diagnosis.

Fig. 4.

Schematic representation of the HTRA2 protein and its activation against mitochondrial stress. (A) Locations of all reported mutations in HTRA2 in persons with essential tremor or Parkinson disease. The full-length HTRA2 protein consists of a transmembrane domain (TM; residues 105–124), a conserved catalytic trypsin-like serine protease domain (Tryp_SPc; residues 178–342), and a C-terminal PDZ domain (residues 363–445). Phosphorylation sites are at Ser142 and Ser400. (B) Oxidative stress results in the activation of p38 stress kinase pathway. In HTRA2, p38 phosphorylates serine at residue 142 in a PINK1-dependent manner; CDK5 phosphorylates serine at residue 400, increasing the proteolytic activity of HTRA2. Active HTRA2 induces stress-responsive caspases. PINK1 also recruits Parkin, an E3 ubiquitin protein ligase, from cytosol to mitochondria to induce mitophagy. Red diamonds represent phosphorylation sites at residues 142 and 400. P, phosphorylation. Adapted by permission from Macmillan Publishers Ltd: Nature Cell Biology (20), copyright (2007).