Posterior cortical atrophy as a primary clinical phenotype of corticobasal syndrome with a progranulin gene rs5848 TT genotype

Posterior cortical atrophy (PCA) represents a special clinicoradiologic syndrome characterized by progressive visuospatial and visuoperceptual deficits. PCA and corticobasal syndrome (CBS) may share similar pathogenetic mechanisms. We report the clinical, neuropsychological, imaging, and genetic features of a patient with initial visual problems, who further developed other cognitive impairments and asymmetric extrapyramidal signs fitting into the diagnosis of CBS. Genetic testing revealed homozygous for the T allele of the rs5848 GRN variant. This study provided an evidence for CBS belonging to the clinical spectrum of GRN genetic variant and demonstrated CBS may initially present with symptoms of PCA in rare cases.

The patient's initial symptoms were discovered in early 2009 at the age of 52, as it was noted that he took longer time to return home alone than before. He also began to complain of blurred vision, but it did not affect his daily life. Two years later, he began to suffer from progressive spatial disorientation. He was unable to fetch objects with either arm by visual guidance. His memory also began to decline and sometimes he spoke not fluently. In 2012, he exhibited problems in writing simple words, calculating and discerning fingers. He walked more slowly and sometimes exhibited tremor and myoclonus of his upper limbs, especially his right side. In May 2013, he complained of deterioration and difficulty in tracking specific objects in his visual field. He also had difficulty in riding, dressing himself, and distinguishing coats from pants. He urinated anywhere as he could not find the bathroom, and he even occasionally defecated on himself. Structural brain magnetic resonance imaging (MRI) in a local hospital revealed cerebral atrophy, particularly in the posterior cortex (Fig. 1a, b). He was diagnosed with AD. After 6 months, he stopped donepezil treatment since his symptoms got more severe.
Positive neurological system signs included reduced facial expression, increased muscle tone and bradykinesia especially on the right side and a little resting tremor on his right side. His performance in neuropsychological evaluations (Table 1) suggested that he had problems in visual spatial, memory, language, and motor functions. He had no significant past medical history and no family history of dementia.

Discussion
Posterior cortical atrophy is a rare clinicoradiologic neurodegenerative syndrome, and two sets of clinical diagnostic criteria for PCA have been proposed [7,8]. The core features of disease onset, progress forms, and main clinical manifestations are consistent, but do not illustrate its underlying pathological, genetic, or biomarker standards. Fig. 1 The clinical materials of the case. a-b Axial sections, T1-weighted sequences of MRI 4 years after disease onset (at 56 years of age) revealed cerebral atrophy, particularly in the posterior cortex. c-d One and a half years later, the secondary MRI revealed much more extensive atrophy within the frontal and occipital-parietal lobes, and bilateral partial parietal lobe. e-f 18F-FDG-PET showed diffuse bilateral hypometabolism in the frontal, parietal, temporal, and occipital lobes, particularly in the parieto-occipital regions. g Bilateral fundus examinations were normal. h Genetic testing showed a single nucleotide polymorphism rs5848 (c.*78C > T), located in the 3′-UTR of GRN In the initial phase of our case, the symptoms were visual problems, specifically, of disorientation and blurred vision. Over time he manifested additional symptoms including optic ataxia, oculomotor apraxia, right-left disorientation, acalculia, dressing apraxia, and agraphia, fitting into the dorsal subtype of PCA. His first cerebral MRI revealed posterior cortical atrophy, also supporting his initial clinical diagnosis. As the disease progressed, the patient developed further asymmetric extrapyramidal signs and progressive global cognitive impairment, but without vertical ophthalmoplegia or apparent hallucinations. The second MRI revealed much more extensive cerebral atrophy and FDG-PET confirmed hypometabolism of those cortical regions. Given the history, clinical findings, and supporting examinations, a diagnosis of CBS was then made [9].
Besides corticobasal degeneration, CBS can also be caused by such as progressive supranuclear palsy, AD, Pick's disease, FTLD with TDP-43 inclusions, mutations in GRN or MAPT, DLB, and CJD [10]. Considering current clinical criteria and various pathological etiologies for PCA and CBS, there is considerable overlap, meaning that some patients may fulfill criteria for both syndromes, or develop from one syndrome to the other [11].
Special biomarkers such as the CSF tau:Aβ ratio may predict pathology in CBS patients [12]. In our case, CSF analysis revealed normal tau and Aβ1-42 levels, which suggests low probability of AD pathology. In addition, genetic tests identified no reported pathogenic genetic mutations for early-onset AD, and an APOE E3/E3 genotype. FTLD-associated gene testing revealed a genetic variant, rs5848 (c.*78C > T) in the 3′-UTR of the GRN gene, which has been reported as a risk factor for TDP-43-positive frontotemporal dementia and other neurodegenerative diseases such as AD. The GRN rs5848 TT genotype is reported to improve microRNA binding efficiency to the 3′-UTR of GRN, leading to enhanced suppression of GRN translation and reduced GRN expression [13]. Tartaglia et al. [14] reported a typical patient with sporadic CBS, who was homozygous for this GRN variant (rs5848), and without mutations in MAPT or GRN genes. Recently, Caroppo et al., [6] reported a visual/ventral variant in a PCA patient carrying a heterozygous GRN mutation. Whether this PCA patient would develop other symptoms that meet the diagnosis of CBS, like our case, needs longitudinal follow-up and final autopsy evidence.
This study provides evidence for CBS belonging to the GRN genetic variant (rs5848) clinical spectrum, and demonstrates that in rare cases, CBS may initially present with symptoms of PCA. Longitudinal follow-up is required to ascertain the most likely etiology and determine the clinical-genetic-pathological mechanism of the T allele of the rs5848 polymorphism in CBS diagnosis.