Entity Framework DB First: How to make a field optional

Question

I have some data class generated from the DB:

[Table("appdb.apptable")]

public partial class apptable
{
    [Key]
    public int Id { get; set; }

    public int Sum { get; set; }

    public string Comment { get; set; }
}

How, I need to support different table in a generic way. The table name the same, but it does not contain Comment field. I need somehow to make the Comment property optional.

How can I make an existent DB field optional in Entity Framework?