How to concatenate column values from each group

I have a dataframe df

    df <- structure(list(GENE = c("TNFRSF4", "TNFRSF4", "VWA1", "VWA1", 
"PEX10", "CEP104"), KEY.varID = c("chr1:1213738:G:A", "chr1:1232280:T:C", 
"chr1:1435798:T:TGGCGCGGAGC", "chr1:1437401:C:G", "chr1:2406791:C:CT", 
"chr1:3844977:G:A")), row.names = c(NA, -6L), class = "data.frame")

Code I tried:

library(dplyr)
df %>%  group_by(GENE) %>%
  mutate(all_variants = paste(KEY.varID, collapse = ","))

Result I want:

GENE                  KEY.varID
TNFRSF4           chr1:1213738:G:A, chr1:1232280:T:C
VWA1              chr1:1435798:T:TGGCGCGGAGC, chr1:1437401:C:G
PEX10          chr1:2406791:C:CT
CEP104           chr1:3844977:G:A

>Solution :

Or using dplyr:

library(tidyverse)
library(data.table)

df %>% 
  group_by(GENE) %>% 
  summarise(KEY.varID = str_c(KEY.varID, collapse = ", ")) %>% 
  as.data.table

#>       GENE                                    KEY.varID
#> 1:  CEP104                             chr1:3844977:G:A
#> 2:   PEX10                            chr1:2406791:C:CT
#> 3: TNFRSF4           chr1:1213738:G:A, chr1:1232280:T:C
#> 4:    VWA1 chr1:1435798:T:TGGCGCGGAGC, chr1:1437401:C:G