Symfony doctrine: ManyToOne heritage doesn't work

Question

I am new to Symfony. I'm working on an application where I need a cascading heritage in Doctrine, but heritage doen't appear to work :

I have a mother class, "Content" :

/**
 * Content
 *
 * @ORM\MappedSuperclass
 */
class Content
{
    /**
     * @var User
     *
     * @ORM\ManyToOne(targetEntity="ApiBundle\Entity\User", cascade={"persist"})
     */
    private $publisher;

    /**
     * @var string
     *
     * @ORM\Column(name="title", type="string", length=255)
     */
    private $title;

    /**
     * @var text
     *
     * @ORM\Column(name="description", type="text", length=10000)
     */
    private $description;

    /**
     * @var Image
     *
     * @ORM\ManyToOne(targetEntity="ApiBundle\Entity\Image", cascade={"persist"})
     */
    private $img;


    // Getters and setters

A child class extending Content :

/**
 * News
 *
 * @ORM\MappedSuperclass
 * @ORM\Table(name="news")
 * @ORM\Entity(repositoryClass="ApiBundle\Repository\NewsRepository")
 */
class News extends Content
{
    /**
     * @var int
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var \DateTime
     *
     * @ORM\Column(name="publicationDate", type="datetime")
     */
    private $publicationDate;

    /**
     * @var Association
     *
     * @ORM\ManyToOne(targetEntity="ApiBundle\Entity\Association", cascade={"persist"})
     * @ORM\Column(name="association_id", type="integer")

     */
    private $association;


    // Getters and setters

And another child class that extends News:

/**
 * Events
 * @ORM\MappedSuperclass
 * @ORM\Table(name="event")
 * @ORM\Entity(repositoryClass="ApiBundle\Repository\EventRepository")
 */
class Event extends News
{
    /**
     * @var int
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;


    /**
     * @var DateTime
     *
     * @ORM\Column(name="eventStart", type="datetime")
     */
    private $eventStart;


    // Getters and setters

The problem is that in the generated SQL, News table has a "publisher_id" and "img" column, but Event doesn't. I can inherit regular Columns, but I can't inherit ManyToOne ones from Content.

Here is the generated SQL :

mysql> desc news;
+-----------------+--------------+------+-----+---------+----------------+
| Field           | Type         | Null | Key | Default | Extra          |
+-----------------+--------------+------+-----+---------+----------------+
| id              | int(11)      | NO   | PRI | NULL    | auto_increment |
| publisher_id    | int(11)      | YES  | MUL | NULL    |                |
| img_id          | varchar(64)  | YES  | MUL | NULL    |                |
| title           | varchar(255) | NO   |     | NULL    |                |
| description     | text         | NO   |     | NULL    |                |
| publicationDate | datetime     | NO   |     | NULL    |                |
| association_id  | int(11)      | NO   |     | NULL    |                |
+-----------------+--------------+------+-----+---------+----------------+


mysql> desc event;
+-----------------+--------------+------+-----+---------+----------------+
| Field           | Type         | Null | Key | Default | Extra          |
+-----------------+--------------+------+-----+---------+----------------+
| id              | int(11)      | NO   | PRI | NULL    | auto_increment |
| title           | varchar(255) | NO   |     | NULL    |                |
| description     | text         | NO   |     | NULL    |                |
| publicationDate | datetime     | NO   |     | NULL    |                |
| association_id  | int(11)      | NO   |     | NULL    |                |
| eventStart      | datetime     | NO   |     | NULL    |                |
+-----------------+--------------+------+-----+---------+----------------+

I'm using Symfony 3.2 and Doctrine 2.5.

What am I missing ? Thank you very much for your help and your time !