Young‐onset parkinsonism due to homozygous duplication of α‐synuclein in a consanguineous family

Abstract

With a growing list of monogenetic causes of Parkinson's disease (PD),1–3 prioritization of genetic testing is made on the presumed mode of inheritance. In patients with early-onset PD from consanguineous families, the most common mode of inheritance is autosomal recessive. Rarely, an individual may be homozygous for a mutation inherited in an autosomal dominant manner, complicating the diagnostic process. The proband is a 36-year-old female from a consanguineous Pakistani family (Fig. 1A). She presented with a 5-year history of left-sided stiffness, micrographia, festinant gait, tremor, and falls. Past medical history included postpartum psychosis and severe depression. There were no symptoms of autonomic dysfunction. Neurological examination at the age of 35 years revealed severe bilateral bradykinesia, rigidity, worse on the left, and mild bilateral upper limb rest tremor. Gait was shuffling, with freezing and complete loss of postural reflexes. Bilateral ankle clonus was noted. Initially, there was a poor response to dopamine agonists. She responded well to 100/25 mg co-careldopa 3 times per day, but developed wearing offs and peak-dose dyskinesias after 4 months of treatment. There was no family history of parkinsonism. The proband's mother's neurological examination at the age of 72 years was normal. The proband's father, who died at age 64 years from a stroke, had no symptoms or signs of parkinsonism as described by his family. Her siblings and children were asymptomatic, but were unavailable for examination. Figure 1 A: Pedigree and clinical information on the family. A half filled symbol represents the proband (arrow), who is the only individual affected by parkinsonism. Clinically unaffected but confirmed mutation carrier (the proband's mother) is represented by ... Investigations in the proband included routine blood work, copper studies, and white cell enzymes, and were all normal. Dopamine transporter scan (DAT-SCAN) showed bilateral reduction in tracer uptake in the striatum, more significant on the right. Formal neuropsychological examination revealed a mild degree of intellectual under-functioning from premorbid estimates, with anterior/subcortical dysfunction. Mutations in PARKIN, PINK1, and common LRRK2 mutations were excluded. Multiplex ligation-dependent probe amplification (MLPA) showed 4 copies of SNCA (Fig. 1B). Comparative genomic hybridization (Nimblegen 135 K array Roche-Niblegen) showed copy number gain of 0.928 Mb, containing 5 genes including SNCA (Fig. 1C). Single-nucleotide polymorphism (SNP) array showed this to be homozygous. Fluorescent in situ-hybridization demonstrated duplication of SNCA on both alleles (Supplemental Figure 1). The proband's mother's MLPA showed SNCA duplication. Considering the proband's homozygosity for SNCA duplication, her father is likely to have had an SNCA duplication. Less likely, de novo duplication (in addition to duplication inherited from mother) occurred in the proband. SNCA multiplication is a rare cause of autosomal dominant PD.4,5 SNCA triplications (4 copies of SNCA) are fully penetrant; with early-onset and rapidly progressive parkinsonism associated with dementia, autonomic dysfunction, and psychiatric features.4 With SNCA duplications (3 SNCA copies), the disease clinically resembles idiopathic PD5 and reduced penetrance is recognized.6 Our patient has 4 SNCA copies and her clinical phenotype closely matches that of patients with SNCA triplications4 and is in keeping with the single previously described case of SNCA double duplication.7 Homozygous SNCA duplications are a rare cause of young-onset parkinsonism and should be considered in the patients with a family history compatible with recessive inheritance. Early cognitive dysfunction and depression are consistently reported in patients with 4 SNCA copies and may be regarded as “red flags” for SNCA multiplications.