Smoking can increase the risk of developing heart disease.

动名词作介词的宾语，of介词短语是risk的定语，这是语法结构，语义上表示同位关系，但不能说动名词或介词短语作同位语。

There is still a risk that the whole deal will fall through.

that从句为同位语从句。

名词risk通常不能后接to do不定式。

但后接介词短语 risk to sth.

The group was considered to be a risk to national security.

对国家安全而言是个危险因素。