  We consider the neutrino masses and mixing in the framework of SO(10) GUTs
with hidden sector consisting of fermionic and bosonic SO(10) singlets and
flavor symmetries. The framework allows to disentangle the CKM physics
responsible for the CKM mixing and different mass hierarchies of quarks and
leptons and the neutrino new physics which produces smallness of neutrino
masses and large lepton mixing. The framework leads naturally to the relation
$U_{PMNS} \sim V_{CKM}^{\dagger} U_0$, where structure of $U_0$ is determined
by the flavor symmetry. The key feature of the framework is that apart from the
Dirac mass matrices $m_D$, the portal mass matrix $M_D$ and the mass matrix of
singlets $M_S$ are also involved in generation of the lepton mixing. This opens
up new possibilities to realize the flavor symmetries and explain the data.
Using $A_4 \times Z_4$ as the flavor group, we systematically explore the
flavor structures which can be obtained in this framework depending on field
content and symmetry assignments. We formulate additional conditions which lead
to $U_0 \sim U_{TBM}$ or $U_{BM}$. They include (i) equality (in general,
proportionality) of the singlet flavons couplings, (ii) equality of their VEVs;
(iii) correlation between VEVs of singlets and triplet, (iv) certain VEV
alignment of flavon triplet(s). These features can follow from additional
symmetries or be remnants of further unification. Phenomenologically viable
schemes with minimal flavon content and minimal number of couplings are
constructed.
