% Identify Control and Abnormal Cells
ctl =  {'C1', 'C2'};
abn =  {'A1', 'A2'};

% Fill NaN values with nearest value along  column  (dim = 2)
rna{:, ctl} = fillmissing(rna{:, ctl}, 'nearest', 2);
rna{:, abn} = fillmissing(rna{:, abn}, 'nearest', 2);

% Fill empty RNA elements names as 'unknown'
rna.RNAElement = fillmissing (rna.RNAElement, ...
                        'constant', 'unknown');