Complex sentence: The decrease in miRNA expression caused by the haploinsufficieny of DGCR8 due to the deletion of the 22q11.2 chromosomal region accounts for the 22q11.2 deletion being the strongest genetic risk factor for schizophrenia.
Split into simpler sentences: The decrease in miRNA expression is caused by the haploinsufficieny of DGCR8 because of the deletion of the 22q11.2 chromosomal region. This is why the 22q11.2 deletion is the strongest genetic risk factor for schizophrenia.