How to update a Doctrine Entity from a serialized JSON?

We are using Symfony2 to create an API. When updating a record, we expect the JSON input to represent a serialized updated entity. The JSON data will not contain some fields (for instance, CreatedAt should be set only once when the entity is created - and never updated). For instance, here is an example JSON PUT request:

{"id":"1","name":"anyname","description":"anydescription"}

Here is the PHP code on the Controller that should update the entity according to the JSON above (we are using JMS serializer Bundle):

$supplier = $serializer->deserialize(
    $this->get('request')->getContent(),
    'WhateverEntity',
    'json'
);

The EntityManger understands (correctly) that this is an update request (in fact, a SELECT query is implicitly triggered). The EntityManager also guess (not correctly) that CreatedAt property should be NULLified - it should instead keep the previous one.

How to fix this issue?