The HPRT1 gene encodes the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT), which is key in the purine salvage pathway. Below is a detailed overview of the HPRT1 gene, along with additional information requested:

### 1. Associated Genes
In addition to HPRT1, several other genes are associated with purine metabolism, metabolic disorders, and diseases linked with purine nucleotide synthesis. Some notable genes include:
- **APRT (adenine phosphoribosyltransferase)**: Involved in the salvage pathway of adenine nucleotides.
- **GPRT (guanine phosphoribosyltransferase)**: Involved in the metabolic conversion of guanine to guanosine monophosphate.
- **TP53**: Although primarily a tumor suppressor, TP53's function is influenced by nucleoside triphosphate pools, which are pertinent to the function of HPRT1.
- **PRPS1 (phosphoribosyl pyrophosphate synthetase 1)**: Involved in the synthesis of phosphoribosyl pyrophosphate, which is a substrate for HGPRT.

### 2. Aging and Age-Related Diseases
While there is limited direct association of HPRT1 with aging, defects in purine metabolism, as seen in Lesch-Nyhan syndrome, can lead to neurological and behavioral abnormalities, which may indirectly relate to age-associated decline in neurocognitive function. Additionally, elevated levels of uric acid due to faulty purine metabolism can influence age-related conditions such as gout and cardiovascular diseases.

### 3. Drug Interactions
Several contexts of drug interactions can be noted concerning the HPRT1 gene and its protein product:
- **Allopurinol**: A xanthine oxidase inhibitor used to lower uric acid levels in conditions like gout. It plays a role in the purine metabolism and/or may influence the activity of HPRT1 indirectly.
- **Febuxostat**: Another medication that inhibits uric acid production, relevant in treating chronic hyperuricemia.
- **Antimetabolites (e.g., Azathioprine)**: These drugs can impact purine metabolism, with therapeutic relevance to diseases like rheumatoid arthritis, where purine salvage pathways are crucial for proliferation.

### 4. Pathways and Biological Processes
HPRT1 is involved in multiple biological processes, most prominently:
- **Purine Salvage Pathway**: Converts purine bases into their nucleotide forms, contributing to nucleotide pool homeostasis.
- **Nucleotide Biosynthesis**: Particularly involving dNTPs and ATP, which are critical for DNA and RNA synthesis and repair.
- **Uric Acid Metabolism**: Contributes to the balance of purine metabolism, influencing uric acid levels in the body.

Moreover, dysfunction in HPRT1 can lead to:
- **Lesch-Nyhan Syndrome**: A rare genetic disorder characterized by neurological and behavioral issues, accompanied by hyperuricemia and self-harming behaviors.
- **Gout**: Resulting from the accumulation of uric acid due to impaired purine metabolism.

In conclusion, the HPRT1 gene is crucial for purine metabolism, and its dysfunction has significant implications for metabolic and neurological disorders. Understanding its associations, implications in aging, drug interactions, and involvement in biological pathways is key for comprehending both its basic biology and its clinical significance.