Mutations in HTRA2 are not a common cause of familial classic ET

Recently Gulsuner1 described a rare single-nucleotide variant (SNV, rs72470545, G399S) in the HTRA2 gene in a very large Turkish family as causing essential tremor (ET) as well as ET with Parkinson disease (PD). We tested the hypothesis that SNVs in HTRA2 also might be the cause of classical ET without PD. We analyzed all eight protein coding exons of HTRA2 (NM_013247) and the exon/intron boundaries by Sanger sequencing in 29 unrelated, earlyonset, German patients with classic ET with at least three affected siblings (mean age at onset, 19.2 6 13.4 y [standard deviation (SD)], mean age at examination 64.2 6 18.2 y). All patients fulfilled the diagnostic criteria of classic ET. None of the patients was reported to have relatives suffering from PD. Ethics review board approval and written consents were obtained. We found one heterozygous exonic missense SNV in HTRA2 (rs72470544, A141S, NM_013247) in a single sample. This SNP had been mentioned as potentially associated with, but not causing, PD in the initial study associating genetic variants in HTRA2 with PD. However, later large-scale studies did not reveal any significant association between PD and rs72470544. Exome-Variant-Server data show that rs72470544 is quite common. Approximately 5% of all European Americans carry at least one minor allele, whereas rs72470545 is much rarer, with approximately 1% heterozygote minor allele carriers and 0/4300 homozygotes (http://evs.gs.washington.edu/EVS/). The Gulsuner study implicates HTRA2 in the pathogenesis of ET and ET with PD. However, replication and convincing functional evidence for rs72470545 will be needed to regard disease causation as proven, especially in light of the conflicting, largely negative data for this variant in PD. Gulsuner attributes the coexistence of ET with PD in the same patient to homozygosity for rs72470545 in HTRA2. We do not agree completely with this notion, because three patients with ET with PD are homozgous and two are heterozygous. We agree with Gulsuner that zygosity is a possible factor contributing to disease severity. However, age rather than zygosity might be the pivotal factor determining progression from ET to ET with PD. Patients with ET only (mean age, 53.3 y) were significantly younger than ET with PD patients (mean age, 77.2 y) (t test, P value: 0.013; data extracted from Fig. 1 of Gulsuner). This and the fact that very slow progression and FIG. 1. Substantia nigra MRI scan and striatal I-FP-CIT uptake in LRRK2 carriers. Target MRI of the SN (upper row) is obtained at the level of the inferior third of the red nucleus (left image) and at the level of superior cerebellar peduncles decussation (right image) Arrows point out the normal substantia nigra aspect. On the bottom row, the I-FP-CIT scan feature is appreciable. (A) Case 1, LRRK2 carrier. Target MRI of the SN shows the loss of nigrosome formation and trilaminar structure of the right SN with the preservation of the normal anatomy of the left SN at the red nucleus level. On the bottom row, bilateral abnormal I-FP-CIT scan, predominant on the right side, is appreciable (38% reduction in the right putamen, 25% in the left putamen). (B) Case 2, LRRK2 carrier. Target MRI of the SN shows normal aspect of nigrosome formation and trilaminar structure on both sides; on the bottom row bilateral abnormal I-FP-CIT scan, predominant on the right side, is appreciable (48% reduction in the right putamen, 40% in the left putamen). (C) Case 3, affected patient. Target MRI of the SN shows a complete bilateral loss of the normal anatomy of the SN. On the bottom row, bilateral abnormal I-FP-CIT scan is appreciable (86% reduction in the right putamen, 33% in the left putamen). MRI, magnetic resonance imaging; [123 I]N-x-fluoropropyl-2b-carbomethoxy-3b-{4-iodophenyl}nortropane, 123I-FP-CIT; Leucine-rich repeat kinase 2, LRRK2; SN, substantia nigra. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com. ]