Index: class-wp-comments-list-table.php
===================================================================
--- class-wp-comments-list-table.php
+++ class-wp-comments-list-table.php	(working copy)
@@ -255,6 +255,8 @@
 		if ( !$post_id )
 			$columns['response'] = _x( 'In Response To', 'column name' );
 
+		$columns = apply_filters( 'manage_comments_custom_columns', $columns );
+		
 		return $columns;
 	}