Can one say something about the determinat of $2n\times2n$ matrix
$$M =\begin{pmatrix} A & B\\ C& D\end{pmatrix},$$
if it is known that

1) $A,B,C,D$ are all $n\times n$ matrices

2) $A$ and $D$ are invertible

3) $\det(B) = \det(C) = 0$ and $B,C$ both have rank 1? 

It is well know that $\det(M)=\det(A)\cdot \det(D-CA^{-1}B)$, but what can we tell knowing $\det(B) = \det(C) = 0$?