Plasma progranulin levels predict progranulin mutation status in frontotemporal dementia patients and asymptomatic family members

Abstract

Mutations in the progranulin gene (GRN) are an important cause of frontotemporal lobar degeneration (FTLD) with ubiquitin and TAR DNA-binding protein 43 (TDP43)-positive pathology. The clinical presentation associated with GRN mutations is heterogeneous and may include clinical probable Alzheimer's disease. All GRN mutations identified thus far cause disease through a uniform disease mechanism, i.e. the loss of functional GRN or haploinsufficiency. To determine if expression of GRN in plasma could predict GRN mutation status and could be used as a biological marker, we optimized a GRN ELISA and studied plasma samples of a consecutive clinical FTLD series of 219 patients, 70 control individuals, 72 early-onset probable Alzheimer's disease patients and nine symptomatic and 18 asymptomatic relatives of GRN mutation families. All FTLD patients with GRN loss-of-function mutations showed significantly reduced levels of GRN in plasma to about one third of the levels observed in non-GRN carriers and control individuals (P < 0.001). No overlap in distributions of GRN levels was observed between the eight GRN loss-of-function mutation carriers (range: 53-94 ng/ml) and 191 non-GRN mutation carriers (range: 115-386 ng/ml). Similar low levels of GRN were identified in asymptomatic GRN mutation carriers. Importantly, ELISA analyses also identified one probable Alzheimer's disease patient (1.4%) carrying a loss-of-function mutation in GRN. Biochemical analyses further showed that the GRN ELISA only detects full-length GRN, no intermediate granulin fragments. This study demonstrates that using a GRN ELISA in plasma, pathogenic GRN mutations can be accurately detected in symptomatic and asymptomatic carriers. The approximately 75% reduction in full-length GRN, suggests an unbalanced GRN metabolism in loss-of-function mutation carriers whereby more GRN is processed into granulins. We propose that plasma GRN levels could be used as a reliable and inexpensive tool to identify all GRN mutation carriers in early-onset dementia populations and asymptomatic at-risk individuals.

Figure 1

Plasma GRN levels in FTLD patients and control individuals. Plasma GRN protein levels (ng/ml) in patients with FTLD carrying loss-of-function GRN mutations (n = 8), FTLD patients without GRN mutations (n = 191) and healthy control individuals (n = 70). Each data point represents an individual. Plasma levels are significantly decreased in loss-of-function GRN mutation carriers compared to non-GRN mutation carriers (P < 0.001; Mann–Whitney test). For each group the median plasma level is indicated with a wide horizontal line. For larger groups (FTLD patients without GRN mutation carriers and healthy control individuals) 25% and 75% quantiles are shown with short horizontal lines.

Figure 2

Specific plasma GRN levels in all types of GRN mutations. GRN expression is plotted for each patient with FTLD from our cohort carrying a GRN mutation. Black bars represent patients with FTLD carrying pathogenic GRN loss-of-function mutations; grey bars represent patients with FTLD carrying GRN mutations with unknown significance. The dashed line is the cut-off value for pathogenic GRN loss-of-function mutations based on the complete FTLD series and the black line indicates the minimum GRN expression identified in our control cohort (Fig. 1). Missense mutations p.R19W and p.C139R show GRN levels below the range detected in control individuals and may induce a partial loss of GRN function.

Figure 3

Plasma GRN levels in symptomatic and asymptomatic GRN mutation carriers in three GRN mutation families. Plasma GRN protein levels (ng/ml) in patients and relatives of three GRN mutation families ascertained in Mayo Jacksonville and Rochester: Symptomatic GRN mutation carriers (n = 9), asymptomatic GRN mutation carriers (n = 6) and relatives without GRN mutations (n = 12). Plasma levels are significantly decreased in symptomatic and asymptomatic GRN mutation carriers compared to relatives without GRN mutations (P < 0.001; Mann–Whitney test). Each data point represents an individual. For each group, the median plasma level is indicated with a wide horizontal line.