A 50-year-old pulmonologist is diagnosed with pulmonary tuberculosis and staed on standard drug therapy. After 1.5 months, the patient has developed fatigue, low grade fever, muscle aches, bone pains and body aches. The patient also complains of pleuritic chest pain. His sputum tests negative for acid fast bacilli. On fuher investigations, the patient tests positive for anti-histone antibodies. Abnormality in which of the following metabolic processes is responsible for patient's current condition?