How to do binding of cascading combo boxes?

<ComboBox Grid.Column="1" Grid.Row="1" Height="23" HorizontalAlignment="Center" Margin="2,2,0,0" Name="comboBoxServer"  VerticalAlignment="Top" Width="156" ItemsSource="{Binding ServerNameList}"    SelectedValuePath="key" SelectedValue="{Binding serverSelected, Mode=OneWayToSource,UpdateSourceTrigger=PropertyChanged}" >

            <i:Interaction.Triggers>
                <i:EventTrigger EventName="SelectionChanged">
                    <i:InvokeCommandAction Command="{Binding serverCommand}" CommandParameter="{Binding  ElementName=comboBoxServer,Path=SelectedItem}"/>
                </i:EventTrigger>
            </i:Interaction.Triggers>
        </ComboBox>

<ComboBox Grid.Column="1" Grid.Row="2" Height="23" HorizontalAlignment="Center" Margin="2,2,0,0" Name="comboBoxDBName" VerticalAlignment="Top" Width="156" ItemsSource="{Binding DBNameList}" SelectionChanged="comboBoxServer_SelectionChanged" SelectedValuePath="Key" SelectedValue="{Binding serverSelected, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" ></ComboBox>

 List<string> _ServerNameList;
        List<string> _DBNameList;

        public List<string> ServerNameList
        {
            get { return _ServerNameList; }
            set
            {
                _ServerNameList = value;
            }
        }

        public List<string> DBNameList
        {
            get { return _DBNameList; }
            set
            {
                if (_DBNameList != value)
                {
                    _DBNameList = value;
                    RaisePropertyChanged("DBNameList");
                }
            }
        }

        string _serverSelected;
        public string serverSelected
        {
            get { return _serverSelected; }
            set
            {
                if (_serverSelected != value)
                {
                    _serverSelected = value;
                    RaisePropertyChanged("serverSelected");
                }
            }
        }



        public BackUpViewModel()
        {

             BackUpContext servObj = new BackUpContext();

            _ServerNameList = servObj.GetServers();
            serverCommand = new RelayCommand(fetchServer);
            _DBNameList = servObj.GetDatabases(serverSelected);            
           }
public RelayCommand serverCommand { get; set; }

 public void fetchServer(object server)
        {            
            serverSelected =  server.ToString();
        }

public List<string> GetServers()
       {

           XmlDocument doc = new XmlDocument();
           doc.Load(@"C:\Users\477340\Desktop\newFile.xml");
           XmlNodeList nodes
               = doc.SelectNodes("/servers/server");           
           foreach (XmlNode node in nodes)
           {

               lstServerName.Add(node.Attributes["name"].InnerText);

           }
           return lstServerName;
       }

       public List<string> GetDatabases(string server)
       {
           XmlDocument doc = new XmlDocument();
           doc.Load(@"C:\Users\477340\Desktop\newFile.xml");
           XmlNodeList nodes
               = doc.SelectNodes("/servers/server");
           foreach (XmlNode node in nodes)
           {

               if (node.Attributes["name"].InnerText == server)
               {
                   XmlNodeList databases = node.SelectNodes("database");

                   foreach (XmlNode db in databases)
                   {
                       lstDBName.Add(db.InnerText);
                   }
               }


           }
           return lstDBName;

       }