Which letter-cluster will replace the question mark (?) to complete the given series?

WGPR, UDLM, ?, QXDC, OUZX

SAHH

Explanation:

WGPR, UDLM, ?, QXDC, OUZX

The logic followed here is :

W → (-2) → U → (-2) → S → (-2) → Q → (-2) → O

G → (-3) → D → (-3) → A → (-3) → X → (-3) → U

P → (-4) → L → (-4) → H → (-4) → D → (-4) → Z

R → (-5) → M → (-5) → H → (-5) → C → (-5) → X

So, the letter-cluster that replaces the (?) to complete the series is "SAHH"