A distinct clinical, neuropsychological and radiological phenotype is associated with progranulin gene mutations in a large UK series

Abstract

Mutations in the progranulin gene (GRN) are a major cause of frontotemporal lobar degeneration with ubiquitin-positive, tau-negative inclusions (FTLD-U) but the distinguishing clinical and anatomical features of this subgroup remain unclear. In a large UK cohort we found five different frameshift and premature termination mutations likely to be causative of FTLD in 25 affected family members. A previously described 4-bp insertion mutation in GRN exon 2 comprised the majority of cases in our cohort (20/25), with four novel mutations being identified in the other five affected members. Additional novel missense changes were discovered, of uncertain pathogenicity, but deletion of the entire gene was not detected. The patient collection was investigated by a single tertiary referral centre and is enriched for familial early onset FTLD with a high proportion of patients undergoing neuropsychological testing, MRI and eventual neuropathological diagnosis. Age at onset was variable, but four mutation carriers presented in their 40s and when analysed as a group, the mean age at onset of disease in GRN mutation carriers was later than tau gene (MAPT) mutation carriers and duration of disease was shorter when compared with both MAPT and FTLD-U without mutation. The most common clinical presentation seen in GRN mutation carriers was behavioural variant FTLD with apathy as the dominant feature. However, many patients had language output impairment that was either a progressive non-fluent aphasia or decreased speech output consistent with a dynamic aphasia. Neurological and neuropsychological examination also suggests that parietal lobe dysfunction is a characteristic feature of GRN mutation and differentiates this group from other patients with FTLD. MR imaging showed evidence of strikingly asymmetrical atrophy with the frontal, temporal and parietal lobes all affected. Both right- and left-sided predominant atrophy was seen even within the same family. As a group, the GRN carriers showed more asymmetry than in other FTLD groups. All pathologically investigated cases showed extensive type 3 TDP-43-positive pathology, including frequent neuronal cytoplasmic inclusions, dystrophic neurites in both grey and white matter and also neuronal intranuclear inclusions. Finally, we confirmed a modifying effect of APOE-E4 genotype on clinical phenotype with a later onset in the GRN carriers suggesting that this gene has distinct phenotypic effects in different neurodegenerative diseases.

Fig. 1

Schematic of GRN showing (A) exons (dark grey bars), UTR (light grey bars) and introns (not to scale, blue lines). Probable null mutations and mutations with unclear pathogenicity are shown above or below the introns/exons respectively (red). Known polymorphisms (green). (B) Screening strategy with six PCR primer pairs (F = forward primer; R = reverse primer); pathogenic and possibly pathogenic mutations (red) are shown above or below the introns/exons respecetively. Also shown are known polymorphisms (green) detected in our screen.

Fig. 2

Genealogies of pathogenic and possibly pathogenic mutation carriers. Individuals are identified by the family number (above genealogy)_individual number (adjacent to symbol). Filled symbols refer to definitely affected individuals. Half-filled symbols refer to possibly affected individuals who we were unable to clinically confirm.‘?’ refers to individuals who died aged <70 and may have transmitted the mutation without presentation with FTD.

Fig. 3

Clinical duration and clinical category of FTD is shown as a coloured bar in GRN mutation carriers (GRN-positive), MAPT mutation carriers (MAPT-positive) and wild-type GRN FTLD-U. See text for statistical analysis. Dotted vertical line indicates the mean age at clinical onset for each group.

Fig. 4

Liability curve derived from 25 GRN mutation carriers. These data are not a complete assessment of penetrance as we did not screen unaffected family members.

Fig. 5

Representative brain imaging from patients with GRN mutations. (A) and (B) Coronal T₁-weighted MR images showing asymmetrical right-sided fronto-temporo-parietal atrophy (Patient 7651_3)—4 years from symptom onset; (C) and (D) Coronal T₁-weighted MR images showing asymmetrical left-sided fronto-temporo-parietal atrophy (Patient DRC431_3)—2 years from symptom onset.